Forests and Greenhouse gases Fluxes of CO2, CH and N2O from drained

Forests and Greenhouse gases Fluxes of CO2, CH and N2O from drained
Linköping Studies in Arts and Science no 302
Forests and Greenhouse gases
Fluxes of CO2, CH4 and N2O from drained
forests on organic soils
Karin von Arnold
Linköping Studies in Arts and Science
In the Faculty of Arts and Science at Linköping University research is
pursued and research training is given within seven broad problem areas
known as themes, in Swedish tema. These are: Child Studies, Communication
Studies, Cultural Inheritance and Cultural Production, Gender Studies, Health and
Society, Technology and Social Change, and Water and Environmental Studies. Each
tema publishes its own series of scientific reports, but they also publish
jointly the series Linköping Studies in Arts and Science.
Distributed by:
Department of Water and Environmental Studies
Linköping University
SE-581 83 Linköping
Sweden
Karin von Arnold
Forests and Greenhouse gases
Fluxes of CO2, CH4 and N2O from drained forests on organic soils
Cover illustration and layout by Tomas Eklund
ISBN: 91-85295-71-X
ISSN: 0282-9800
© 2004 Karin von Arnold
Department of Water and Environmental Studies
UniTryck, Linköping, 2004
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LIST OF PAPERS
(I)
von Arnold, K., Weslien, P., Nilsson, M., Svensson, B.H. and
Klemedtsson, L. 2004. Fluxes of CO2, CH4 and N2O from drained
coniferous forests on organic soils. Forest Ecology and Management
(Conditionally accepted).
(II)
von Arnold, K., Nilsson, M., Hånell, B., Weslien, P. and Klemedtsson,
L. 2004. Fluxes of CO2, CH4 and N2O from drained organic soils in
deciduous forests. Soil Biology and Biochemistry (Conditionally accepted).
(III)
von Arnold, K., Ivarsson, M., Öquist, M., Majdi, H., Björk, R.G.,
Weslien, P. and Klemedtsson, L. 2004. Can the distribution of trees
explain the spatial variation in N2O emissions from boreal forest soils?
Submitted to Plant and Soil.
(IV)
Klemedtsson, L., von Arnold, K., Weslien, P. and Gundersen, P.
2004. Soil CN ratio as a scalar parameter to predict nitrous oxide
emissions. Manuscript.
(V)
von Arnold, K., Hånell, B. and Klemedtsson, L. 2004. Net fluxes of
greenhouse gases between drained Swedish organic forestland and the
atmosphere. Manuscript.
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TABLE OF CONTENTS
INTRODUCTION ............................................................................................................. 7
AIMS OF THIS THESIS ...................................................................................................... 8
BACKGROUND................................................................................................................. 8
Forest drainage..................................................................................................................... 8
Pre-drainage conditions .......................................................................................................................... 8
Impact of drainage................................................................................................................................... 9
Drainage intensity .................................................................................................................................... 9
Problems associated with scaling fluxes .............................................................................. 11
Complexities in production and consumption.................................................................................... 11
Temporal and spatial variation in fluxes .............................................................................................. 12
Temporal variation ................................................................................................................................. 12
Spatial variation..................................................................................................................................... 13
Within-site spatial variation ........................................................................................................ 13
Among-site spatial variation....................................................................................................... 13
Net GHG fluxes between the atmosphere and poorly drained forests ................................ 14
Management of peatlands ................................................................................................... 15
Up-scaling ........................................................................................................................... 16
STRATEGIES FOR DEVELOPING EMISSION FACTORS FOR POORLY DRAINED FORESTS ..... 17
Study sites ............................................................................................................................ 17
Measurements of GHG ....................................................................................................... 22
Biotic and abiotic variables measured ................................................................................. 23
RESULTS AND DISCUSSION ............................................................................................. 24
Do groundwater level and air temperature explain temporal variation in GHG fluxes? .... 24
Does distance to trees affect the emissions of GHG? ......................................................... 26
Do the soil emissions of GHG differ among sites differing in fertility and tree species?.... 27
Are poorly drained forests sources or sinks for GHG? ........................................................ 30
Net emissions of GHG at poorly drained forest sites....................................................................... 31
Sensitivity analysis.................................................................................................................................... 32
Reallocation of carbon ............................................................................................................................ 33
Are poorly drained forest sites larger net sinks than well-drained and virgin sites?............ 35
Comparison between poorly drained and virgin sites........................................................................ 35
Comparison between poorly drained and well-drained sites ........................................................... 37
Do drained sites contribute significantly to the Swedish GHG budget? ............................. 38
MAIN CONCLUSIONS AND FUTURE RESEARCH ................................................................ 41
REFERENCES .................................................................................................................. 42
ACKNOWLEDGEMENTS ................................................................................................... 47
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INTRODUCTION
In the atmosphere there are gases, referred to as greenhouse gases (GHG),
which restrict the outward flow of infrared radiation. These gases cause a net
warming of the Earth’s surface called the greenhouse effect. If no GHG were
present in the atmosphere, the global temperature would be 33oC lower, i.e.
the mean global temperature would be –18oC instead of the current 15oC
(IPCC, 1990). Thus, the greenhouse effect is essential for most of the life
forms that have developed on Earth. At present, however, the concentrations
of GHG in the atmosphere are increasing, promoting further global warming.
The exact effects these changes will have are not known, but according to the
IPCC (2001a) it is possible that both ecological and socio-economic systems
may be irreparably damaged. In addition, the probability of extreme weather
events, such as periods with very high or very low temperatures, extreme
floods, droughts, tropical cyclones, and storms will increase, as will the
probability of large-scale singular events, such as the collapse of the West
Antarctic ice sheet or shutdown of the Gulf Stream. In Sweden, modelling
suggests that by the year 2050 the temperature will be on average 2.5-4.5°C
higher and precipitation 8-23% greater than today (Räisänen et al., 2003).
Furthermore, although the potential productivity of forests and agricultural
crops is likely to be higher in a warmer and rainier climate, any such gains
could be undermined by conditions becoming more favourable for harmful
insects, diseases and changes in soil moisture (Mattsson and Rummukainen,
1998).
Carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) are regarded
as the most important greenhouse gases, accounting for an estimated 80% of
the total global warming (IPCC, 2001b). The global warming potential
(GWP), which describes the cumulative warming over time caused by the
emission of a gas, differs among the gases. The two basic factors governing
each gas’s GWP value are its radiative forcing, i.e. the infrared absorption of
an incremental amount of the gas in the atmosphere, and its rate of decay in
the atmosphere. The GWP of CO2 is set to 1, and the corresponding figures
for CH4 and N2O are 23 and 296, respectively (IPCC, 2001b), i.e. it takes 23
or 296 CO2 molecules to cause the same warming as one molecule of CH4 or
N2O, respectively. There are two ways of reducing the concentrations of
GHG in the atmosphere: to increase the strength of the sinks or to decrease
the strength of the sources. Forestry can help reduce national emissions in
either of two ways. Firstly, forests can accumulate carbon in their biomass or
soil and, secondly, the produced biomass can be used as substitutes for other
products, most notably fossil fuels, but also materials that are produced by
energy-consuming processes, e.g. cement and plastics.
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The major part of Sweden’s land surface (52% or 23.5 Mha) is covered with
productive forestland (SCB, 2004). Forestry activities are, therefore, of major
importance when assessing Sweden’s national GHG budget. However, not all
forests are sinks for CO2. For example, Lindroth et al. (1998) and Lohila et al.
(2004) found that drained forests on organic soil could act as sources for
atmospheric CO2. In addition, these soils may be major sources of N2O
(Martikainen et al., 1993; Maljanen et al., 2003a).
AIMS OF THIS THESIS
The general aim of the work underlying this thesis was to elucidate how
forests on drained organic soils function in the context of GHG exchange.
Specific aims were to:
• determine the most important factors regulating the emissions of CO2,
CH4 and N2O in drained forests on organic soil both temporally
(Papers I and II) and spatially (Papers I, II, III and IV)
• determine the net fluxes of greenhouse gases from poorly drained
forests on organic soil (Papers I and II)
• establish management strategies for poorly drained soils in order to
minimize their GHG source strength or maximize their GHG sink
strength (Papers I and II)
• estimate total net GHG exchange between the area of drained forest on
organic soil in Sweden and the atmosphere (Paper V)
BACKGROUND
In this section the state-of-the-art knowledge about the fields addressed in this
thesis are presented. At the end of each subsection a hypothesis is formulated,
which relates to the specific aims.
Forest drainage
Land used for forestry has commonly been drained in various areas of the
world, especially Fenno-Scandia and the former USSR (Paavilainen and
Päivänen, 1995). To date, about 15 Mha of peatlands and wetlands have been
drained for forestry in boreal and temperate regions (Paavilainen and
Päivänen, 1995).
Pre-drainage conditions
The sites that have been drained for forestry had high groundwater levels
before drainage. Decomposition in anaerobic environments occurs through
8
the cooperative action of several microbial populations and results in the
production of CH4 (Guijer and Zehnder, 1983; Conrad, 1989). Anaerobic
decomposition is less effective than aerobic decomposition, resulting in
incomplete degradation of litter from the vegetation and an accumulation of
organic matter in the soil (Swift et al., 1979; Clymo, 1984). In Sweden, there
are about 10 Mha of peat-covered land, of which about 15% has been drained
(Hånell, 1990). Sixty percent of the peat-covered land area is classified as
peatland (Hånell, 1990), i.e. has a peat layer thicker than 30 cm. The remaining
40% of the peat-covered land area has a shallow peat layer (Hånell, 1990), i.e.
thinner than 30 cm, and is thus not classified as peat soil. Nevertheless, these
soils may have a high organic content and be classified as organic (which
applies to any soil with a proportion of organic matter exceeding 20%
according to the FAO, 1998). Of the total area drained for forestry in Sweden,
approximately 50% is situated on peat soil, while the rest has a peat layer
thinner than 30 cm, according to data from the Swedish National Forest
Inventory (S-NFI).
Impact of drainage
Drainage for forestry generally results in sites with high forest productivity
(Holmen, 1978), and consequently the CO2 accumulation in tree biomass may
be high on drained sites. However, the accumulated organic matter in the soil
becomes available for aerobic decomposition after drainage, which promotes
high soil CO2 release rates, as shown, for example, by Silvola et al. (1996a).
Furthermore, the nitrogen contained in the organic matter becomes available
for N2O-producing microbes after drainage. Consequently, drained organic
forest soils have been found to be significant sources of both CO2 and N2O
(Martikainen et al., 1993; Laine et al., 1996; Silvola et al., 1996a; Regina et al.,
1998; Widén, 2001; Maljanen et al., 2003a; Weslien et al., XXXX). In addition,
CH4 is exchanged between the atmosphere and drained organic forests (see,
for instance, Nykänen et al., 1998; Maljanen et al., 2003b; Weslien et al.,
XXXX). It has also been shown that the size of all the fluxes depends on the
type of land that is drained (e.g. Minkkinen et al., 2002).
Drainage intensity
In Finland GHG fluxes at drained forestland have been measured extensively
(see Table 2 in Paper V). Swedish forestland differs from Finnish, as the
forests are more productive due to the warmer climate. Thus, emissions
derived from measurements at Finnish drained forests cannot be uncritically
used for Swedish areas. In Sweden, only two drained sites have been studied,
one dominated by spruce and pine (Lindroth et al., 1998; Widén, 2001) and
one dominated by birch (Weslien et al., XXXX). The mean annual position of
the groundwater tables were between 40 and 100 cm (Lundblad and Lindroth,
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2002) and 53 cm (Weslien et al., XXXX) below the soil surface for the two
sites, respectively.
The optimal groundwater table position after drainage is >35 cm below the
soil surface for weakly decomposed nutrient poor peat and >55 cm for well
decomposed nutrient rich peat (Paavilainen and Päivänen, 1995). When a tree
stand has developed, the transpiration of the trees further lowers the
groundwater table (Paavilainen and Päivänen, 1995). However, subsidence,
through physical compaction of the soil and decomposition of the soil organic
matter, result in a successive rising of the groundwater table (Eggelsmann,
1986). A large survey of drained peatlands in Finland showed that the average
subsidence was 22 cm, approximately 60 years after drainage (Minkkinen and
Laine, 1998). At logging, the transpiration by the trees is decreased, and thus
the water table is likely to rise further (Roy et al., 1996). Consequently,
complementary or remedial drainage is needed in order to maintain high
productivity in drained forests.
In Sweden, the most extensive drainage period was between 1920 and World
War II (Hånell, 1990) and the most intensive remedial drainage period was
during the late 1970s and 1980s (Hånell, 1990). Thus, the peak in remedial
drainage activity followed approximately 50 to 60 years after the peak in
activities associated with dewatering land for forestry. Recently, the average
area annually subjected to remedial drainage has been small, equivalent, on
average, to less than 0.3% of the drained forestland or 2600 hectares per year
in 1992 to 2002 (National Board of Forestry, 2003). Assuming that there is a
50 to 60 year period before drained land needs remedial drainage, the areas
drained during World War II need remedial drainage now, but drainage
activity during the World Wars was low (Hånell, 1990). If this is the reason for
the currently low level of remedial drainage, a new peak in remedial drainage is
likely to occur in the fairly near future, since the area subjected to drainage
increased again after the end of World War II. However, the low remedial
drainage activity may also reflect the present recognition of swamp forests and
wetlands as valuable biotopes that are worth protecting (see, for instance,
Rubec, 1997) and should not, therefore, be remedially drained. This view is
also reflected in the fact that since 1986, drainage of wetlands has been
prohibited, for environmental reasons, without a special permit. Nevertheless,
it is very likely that the area of moist drained forestland in Sweden will
increase in the near future. Hence, there is an urgent need to enhance our
understanding of these systems in order to develop sustainable management
strategies. Therefore, this thesis focuses on GHG exchange between the
atmosphere and Swedish moist drained forests. Moist drained forests will be
referred to as poorly drained in the following text due to their drainage depth
being below the optimal.
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Problems associated with scaling fluxes
Complexities in production and consumption
When calculating the net GHG balance of a drained forest, several fluxes
must be included (Fig. 1). The net CO2 exchange of a system is the sum of
CO2 uptake via plant photosynthesis, CO2 respired from below- and aboveground parts of plants and CO2 released from decomposition of soil organic
matter, i.e. both recently added litter and organic matter accumulated before
drainage (Fig. 1). If the amount of CO2 incorporated into plant biomass
exceeds the CO2 released via the decomposition of soil organic matter the site
is a net sink for CO2 and if the CO2 released from decomposition exceeds
CO2 incorporation into biomass the site is a net source for CO2. CH4 is
produced in the anaerobic fraction of the soil and consumed in the aerobic
fraction (Fig. 1) (Sundh et al., 1994; 1995). Consequently, soil fluxes of CH4
are a result of the balance between CH4 production and consumption, and
drained forests can be either sources or sinks for CH4 (Nykänen et al., 1998;
Maljanen et al., 2003b; Weslien et al., XXXX). Nitrification and denitrification
are the two most important processes involved in soil N2O-production
(Firestone and Davidson, 1989) and the N2O flux is a result of the N2O
released from both nitrification and denitrification. These two processes are
tightly coupled to each other and to mineralization, since nitrifiers use NH4+
derived from mineralization and denitrifiers use NO3- produced by
nitrification (Fig.1). Some denitrifiers can also gain energy by using
atmospheric N2O as a substrate and, therefore, water-saturated soils can be
sinks for atmospheric N2O (Blackmer and Bremner, 1976; Regina et al., 1996;
Johansson et al., 2003).
CO2
Respiration
CO2
Photosynthesis
CO2
Litter input
Aerobic decay
OM
CO2 CH4 CO2
N2O
N2O
Oxidation
Mineralization
NH4+
Nitrification
NO3-
CH4
Anaerobic decay
OM
NO3-
Denitrification
Figure 1. Schematic diagram of GHG production, consumption and fluxes in terrestrial
systems.
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The temporal and spatial variation of these fluxes is complex since they are
the net results of diverse processes (Fig. 1), all of which are regulated by
multiple biotic and abiotic factors (e.g. Swift et al., 1979; Conrad, 1989;
Robertson, 1989; Paavilainen and Päivänen, 1995).
Temporal and spatial variation in fluxes
A number of authors have published formulas for calculating the carbon
accumulation in tree biomass (e.g. Marklund, 1988; Fridman, 1995; Peterson,
1999). These formulas have input parameters that are easily measured, e.g.
diameter at breast height, age of trees and altitude. Thus, the carbon
accumulation in tree biomass in forested areas can be determined quite
accurately over large temporal and spatial scales based on variables that can be
measured at one visit to a site. On the other hand, the spatial and temporal
variation in soil fluxes of GHG is high (e.g. Matson et al., 1989). In this
context the soil emissions of CO2 represent the sum of CO2 released from
roots and decomposition. One of the aims of this thesis was to identify easily
measurable variables that could explain a major part of the temporal and
spatial variation in soil GHG fluxes. If possible, it would be very useful to
base the up-scaling of emissions on parameters available in national databases,
such as the S-NFI. Therefore, the scope for coupling the spatial variation in
GHG fluxes to some of the S-NFI variables was studied.
Temporal variation
As shown by various authors (e.g. Swift et al., 1979; Conrad, 1989; Robertson,
1989) the activity of microbes producing and consuming GHG in terrestrial
systems is heavily influenced by soil moisture and temperature. Total soil CO2
release has been found to correlate positively with both soil temperature and
groundwater table depth in drained, as well as undrained, peat soils (Silvola et
al., 1996a; Wickland et al., 2001). Positive correlations between soil
temperature and CH4 emission rates (Frolking and Crill, 1994; Nykänen et al.,
1998; Wickland et al., 2001) and negative temporal relationships between CH4
emissions and the groundwater table position have been found at both
drained and undrained peatlands (e.g. Nykänen et al., 1998). Similarly,
temporal variations in N2O fluxes have been found to be positively correlated
with air temperature and groundwater table in a drained peat forest soil
(Maljanen et al., 2003a).
Hypothesis I: Thus, I hypothesized that the variation in groundwater table and
air temperature could explain the temporal variation in GHG soil fluxes in
poorly drained forests.
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Spatial variation
In this thesis the spatial variations in soil GHG emissions are considered at
two different scales: within sites and among sites.
Within-site spatial variation
Forest soils are, naturally, heavily affected by trees, but the impact of the trees
is not evenly distributed within the forested area. For example, higher
concentrations of organic matter have been found in areas close to stems
(Liski, 1995), and due to the uneven distribution of fine roots (Olsthoorn, et
al., 1999), root exudates, competition for nutrients, oxygen demand and
content, pH and soil moisture are all very likely to differ spatially within a
forest stand. Furthermore, the amount of throughfall increases with distance
from stems, and the concentrations and loads of NO3- and NH4+ decrease
with distance from stems (Hansen 1996; Whelan et al., 1998). All of these
factors are known to affect the production and consumption of GHG (see,
for instance, Swift et al., 1979; Conrad, 1989; Robertson, 1989).
The impact of distance to stems on soil fluxes has been investigated in studies
of forests on mineral soil. Scott-Denton et al. (2003) found that rates of soil
CO2 release decreased with distance from trees. Similarly, Brumme (1995)
found that soil CO2 release rates are lower in gaps than under canopies in
forests, which was attributed to the likelihood that the amount of CO2
released via root activity will be higher in areas closer to stems. ButterbachBahl et al. (2002) have reported that the net consumption of CH4 is lower and
net emissions of N2O higher in the soil within a 1 m radius of Norway spruce
stems compared to soil more distant from tree stems. They suggested that this
was an effect of the significantly higher soil nitrogen content found close to
stems.
Hypothesis II: Given the above considerations, I hypothesized that the
distance to stems and soil fluxes of GHG are also related in organic, poorly
drained forests. If so, the number of stems per hectare, a parameter that is
recorded in the S-NFI database, could be used for extrapolation purposes.
Among-site spatial variation
Drainage intensity, tree species and soil fertility could be the most important
factors regulating the soil emissions of GHG in drained forests since they
affect many GHG-regulating factors, for example soil oxygen content, litter
quantity (Bray and Gorham, 1964) and quality (Gosz, 1981; Staaf and Berg,
1981; Johansson, 1995, Wedderburn and Carter, 1999), and thus soil nutrient
conditions (Menyailo et al., 2002, Smolander and Kitunen, 2002) and pH
(Menyailo et al., 2002; Smolander and Kitunen, 2002).
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Silvola et al. (1996a) found that the soil CO2 release rates increased with water
table depth at a number of drained and undrained peat sites. CH4 fluxes in
drained areas have been found to decrease with increasing groundwater table
depth (Nykänen et al., 1998), and although N2O emissions are affected by
many factors other than groundwater table there is, at least, a tendency for
emissions to be higher at drained areas with relatively low groundwater tables
compared to wetter drained areas (Regina et al., 1996). Silvola et al. (1996a)
compared CO2 fluxes from drained peat areas differing in fertility and found
that differences in fluxes among sites could be partly explained by differences
in fertility. Soil fertility has also been reported to affect CH4 emissions in
undrained peatlands (see, for instance, Martikainen et al., 1995; Nilsson et al.,
2001) and in drained peatlands higher N2O emissions have been found from
nutrient rich than from nutrient poor, drained peat soils (Martikainen et al.,
1993; Regina et al., 1996). To my knowledge no studies have compared GHG
fluxes from drained organic soils dominated by different tree species.
However, studies on mineral soils have shown that soil CO2 release is higher
in areas dominated by deciduous species than in areas dominated by
coniferous species (Hudgens and Yavitt, 1997; Janssen et al., 1999; Longdoz et
al., 2000). Hudgens and Yavitt (1997) reported that mineral soils dominated by
deciduous tree species had higher net CH4 consumption rates than mineral
soils dominated by coniferous species. Borken and Brumme (1997) found
similar results and attributed them to the coniferous litter having lower
diffusivity for CH4. Soil collected in stands of different tree species grown on
the same mineral soil showed that N2O emissions correlated with litter CN
ratios, and increased in the order larch < pine < spruce < cedar < aspen <
birch (Menyailo and Huwe, 1999).
Hypothesis III: Thus, I hypothesized that the GHG fluxes in poorly drained
forest soils would differ significantly from the fluxes in well-drained forest
soils and that soil fertility and dominating tree species are important regulators
of system GHG exchange at poorly drained forests.
Net GHG fluxes between the atmosphere and poorly drained forests
Forests can decrease the national emissions of GHG by accumulating CO2 in
biomass or soil, and by producing biomass, which can be substituted for fossil
fuels. Consequently, a drained forest could be regarded as a net sink for GHG
if the CO2 uptake by the vegetation can compensate for the decomposition of
organic matter in the soil and soil emissions of CH4 and N2O. If the tree
biomass on drained forestland was used to substitute for fossil fuels then the
impact would be more complex, at least from a political perspective, because
drainage causes a shift in the allocation of carbon from peat to tree biomass.
In the same way that the GHG emissions from inputs of external energy in
forest operations have to be included when estimating the environmental
14
impacts of using forest biomass for energy production (as done, for example,
by Berg and Karjalainen, 2003), the decomposition of peat has to be included
in estimates of the impact of using wooden material on drained forestlands for
energy production. The climatic impact of using peat as an energy source has
been discussed extensively in recent years, and a number of studies have
shown that the impact of peat utilization is comparable to that of fossil fuels
(e.g. Savolainen et al., 1994; Rodhe and Svensson, 1995), while other reports
have claimed that the utilization of peat should be compared to use of forest
residuals (Åstrand et al., 1997). Tree stands, which represent a renewable fuel,
are ready for harvest after approximately 100 years while fossil fuels, such as
coal and oil, have been embedded in the Earth’s crust for maybe 100 million
years. As it takes thousands of years for peat deposits to be harvestable, peat
can neither be classified as a renewable nor a fossil fuel. It has been suggested
that peat should be treated separately and classified as a slowly renewable fuel
(Crill et al. 2000; SOU, 2002). Thus, to use peat as a substitute for fossil fuel is
more controversial than use of biofuel. Taking the allocation of carbon into
consideration, some of the carbon accumulated in tree biomass in drained
forests could be regarded as peat carbon. Burning tree biomass on drained
forestland for energy production could therefore be viewed to some degree as
burning peat.
Hypothesis IV: I hypothesized that tree accumulation of CO2 more than
compensates for the soil emissions of GHG at poorly drained organic forest
areas, making the areas net sinks, but the major part of the carbon in the trees
should be considered as peat carbon.
Management of peatlands
Assuming that poorly drained soils will increase in abundance it is of interest
to know how these areas should be managed in order to keep them as large
sinks (or as small sources) as possible. There are three easy options: (i) to
rewet the area by closing ditch systems or merely neglecting them, and thus
allow a return to a paludified state through gradual subsidence; (ii) to further
lower the water table, i.e. to use complementary or remedial drainage; or (iii)
to keep the areas poorly drained but prevent them returning to a paludified
state.
The impact of drainage and drainage intensity has been discussed above.
Complementary or remedial drainage is very likely to improve the forest
growth conditions (Paavilainen and Päivänen, 1995) and decrease CH4
emissions (Nykänen et al., 1998), but increase soil emissions of both CO2
(Silvola et al., 1996a) and N2O (Regina et al., 1996). A rewetting of the site
would result in decreased carbon uptake by tree vegetation (Paavilainen and
Päivänen, 1995) and, in addition, CH4 emissions are likely to be increased
15
(Nykänen et al., 1998). On the other hand, decomposition rates of soil organic
matter (Silvola et al., 1996a) and emissions of N2O would decrease (Regina et
al., 1996).
Hypothesis V: I hypothesized that the increased CO2 uptake by trees and the
decreased soil emissions of CH4 could not compensate for the increased rates
of soil CO2 and N2O release resulting from complementary or remedial
drainage. Similarly, I hypothesized that the decreased soil CO2 and N2O
releases could not compensate for the decreased CO2 uptake by trees and the
increased soil emissions of CH4 resulting from a rewetting. Consequently, I
hypothesized that both well-drained and rewetted organic soils are larger
sources (or smaller sinks) of GHG than poorly drained soils.
Up-scaling
I hypothesised that poorly drained areas are net sinks of GHG. Observations
have shown that well-drained areas, on the other hand, may be sources of
GHG, as net releases of CO2 have been found from them (Lindroth et al,
1998; Lohila et al., 2004). As only 20% of the drained area is classified as wet
or moist at present in Sweden (according to S-NFI data), the major part of the
land is well-drained and may be a source of GHG. In order to determine the
impact of Swedish drained forest ecosystems on the national GHG budget,
accurate up-scaling and evaluation of the net emissions from drained forests
on organic soils is essential, and requires high quality data on fluxes from
different types of drained soils. There is, however, a paucity of flux data,
making attempts to scale up the fluxes highly uncertain. The IPCC has
developed guidelines to be applied when countries calculate and report their
national emissions and removals of GHG. In the Good Practice Guidance for
Land Use, Land-Use Change and Forestry (GPG-LULUCF) default emission
factors for drained forests on organic soils are available which could be used
for estimating the net fluxes from drained organic forest soils. However, these
data are rough and it is suggested that country-specific data on fluxes from
drained organic soils should be used if available (Penman et al., 2003).
Swedish data on GHG emissions from drained organic forest soils are scarce.
Therefore, more flux measurements are needed in order to scale up the
emissions. However, the aim of an up-scaling may not only be to provide an
exact value of the GHG exchange between the Swedish area of drained forest
on organic soil and the atmosphere, but also to obtain an estimate based on
present knowledge. Such estimates are needed by the decision makers.
Furthermore, they are useful as they may highlight sectors where more
research is needed.
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Hypothesis VI: I hypothesized that the GHG contribution from drained
forests on organic soils would have a significant impact on the national GHG
budget.
STRATEGIES FOR DEVELOPING EMISSION FACTORS FOR POORLY
DRAINED FORESTS
This section contains a presentation of the study sites as well as a discussion
about the methods used for measuring GHG emission in poorly drained
forests.
Study sites
To test the hypothesis stated above five drained sites differing in fertility and
tree species were studied. All five sites were classified as poorly drained, with
mean annual groundwater tables in the upper 30 cm of the soil (Table 1). Four
of the sites were classified as having peat soils, while the fourth had a peat
layer thinner than 30 cm (Table 1). The organic content in the soils was over
20% at all sites (Table 1), and thus they were classified as organic according to
FAO criteria (FAO, 1998). One of the poorly drained sites was dominated by
Scots pine (Pinus sylvestris (L.)), one by downy birch (Betula pubescens Ehrh)),
two by Norway spruce (Picea abies (L.) Karst.) and one by black alder (Alnus
glutinosa (L.) Gaertn.). The soil fertilities of the sites were based on the
classification by Hånell (1991). At the poorly drained pine site the forest floor
vegetation was dominated by Vaccinium uliginosum, and the site was,
consequently, classified as dwarf shrub type, i.e. of low fertility (Fig. 2). At the
poorly drained birch site herbs were abundant. However, most of these herbs
were not indicator species listed in the classification scheme. Therefore, based
on the large amounts of Trientalis europea, the site was classified as bilberryhorsetail type, i.e. of medium fertility (Fig. 3). One of the spruce sites was
dominated by 40-year-old trees. It was sparsely vegetated, but the present
forest floor vegetation was mainly Vaccinium myrtillus and the site was,
therefore, classified as billberry-horsetail type, i.e. of medium fertility (Fig. 4).
The other spruce site had older trees, about 80 years old, and was also quite
sparsely vegetated. Maianthemum bifolia and Oxalis acetocella were present and
the site was, therefore, classified as low herb type, i.e. highly fertile (Fig. 5).
Tall herbs, such as Dryopteris species and Filipendula ulmaria, dominated the
poorly drained alder site (Fig. 6). For comparison two undrained sites, one in a
fen and one in an alder swamp, were chosen. These systems represent two
different site types which were commonly drained for forestry. The fen was
classified as impediment and dominated by tall Carex species and therefore
classified as tall sedge type, i.e. of low fertility (Fig. 7).
17
Figure 2. The photo to the left shows the
vegetation at the drained pine site. The diagram to
the left shows groundwater table (blue), peat depth
(brown) and position of chambers (bars), while the
diagram to the right shows the position of ditches
(lines, the thickness of the line corresponding to
the width of the ditch) and chambers (dots).
Figure 3. The drained birch site
Figure 4. The drained spruce site with young trees
18
Figure 5. The drained spruce site with old trees
Figure 6. The drained alder site
Figure 7. The undrained fen
19
Figure 8. The undrained swamp
The swamp, dominated by black alder, was classified as low herb type, i.e.
highly fertile, based on the presence of Filipendula ulmaria and Viola species
(Fig. 8).
The sites were chosen to represent different types of poorly drained areas
(Fig. 9). The classification in Fig. 9 is based on potential productivity (data
from Table 1, Paper V), which is not directly convertible to soil fertility.
While the fertility classification has been developed for determining potential
productivity after drainage (Hånell, 1991), and thus focuses on the nutrient
content in the soil, the potential productivity is based on both soil nutrient
conditions and other factors such as groundwater table. However, the
productivity of the trees is strongly influenced by the soil fertility and it is
therefore assumed that a comparison is justifiable.
DP
20
DB DSy
DSo
Figure 9. Relative areas of poorly drained types of
productive forestland in Sweden, as represented
by areas of boxes. The total area is divided into
three fertility classes, based on potential forest
productivity, i.e. the box to the left represents low
productivity (<4 m3 ha-1 y-1), the box in the middle
medium productivity (4-8 m3 ha-1 y-1) and the box
to the right high productivity (>8 m3 ha-1 y-1)
areas. The percentage of different kinds of poorly
drained forest types in Sweden is then shown by
the different coloured areas within the three
boxes, i.e. peat soils (white), mineral soils (gray),
coniferous (unstriped white and gray areas) and
DA deciduous (white and gray striped areas). The
types of area that the sites discussed in this thesis
(i.e. the sites dominated by pine (DP), birch (DB),
young spruce trees (DSy), old spruce trees (DSo)
and alder (DA)) represent, are indicated by
arrows.
The most common poorly drained forested site type consists of peat soils
dominated by coniferous tree species (Fig. 9). The areas classified as being of
low, medium and high productivity were represented by the pine site (DP),
the spruce site with young trees (DSy) and the spruce site with old trees
(DSo), respectively (Fig. 9). However, the spruce site, which was classified as
highly fertile, had approximately the same carbon accumulation in tree
biomass as the medium fertility spruce site (Table 2), indicating that its fertility
may be medium rather than high. Nitrogen is one of the most important
nutrients, and the finding that the nitrogen content of the organic matter was
similar at the two spruce sites (Table 1) further strengthens the possibility that
their fertility was similar. Similarly, the birch site (DB) was chosen in order to
represent the medium productivity peat forests dominated by deciduous tree
species, but the growth rate at the birch site was lower than expected from the
soil fertility (Tables 1 and 2). Therefore, the fertility classification may not
reflect actual conditions sufficiently well, and should be treated with care.
About 40% of the poorly drained forest soils in Sweden have a peat layer
thinner than 30 cm (Fig. 9). About 15% of these sites are classified as highly
productive (Fig. 9), and can thus be represented by the poorly drained alder
site (DA).
Table 1. Soil parameters for all sites, both poorly drained, i.e. pine (DP), birch (DB),
spruce, young trees (DSy), spruce, old trees (DSo) and alder (DA), and undrained, i.e. fen
(UF) and alder swamp (US).
Annual mean
groundwater table (cm)
Probable time since
drainage (years)
Peat depth a
Organic matter (%)
Dry bulk density
(g/cm3)
Porosity (%)
Tot N 0-10 cm (%) b
Tot C 0-10 cm (%) b
CN ratio 0 -10 cm b
pH 0 -10 cm
Productivity class
Humification degree c
DP
17
DB
15
DSy
27
DSo
22
DA
18
40
60
>30
40-50
20
53 - >120
92
0.17
114 - >120 52 - >120
94
73
0.10
0.32
93
84
86
1.3
2.2
1.9
56
52
54
44
25
29
2.7
3.4
3.2
5
3
3
low low low medium
medium
medium
a measured down to a maximum depth of 120 cm
b of organic matter
c classification based on von Post and Granlund (1926).
UF
7
US
-1
7 - >120 5 – 49
86
40
0.13
0.63
70 - >120
90
0.03
41 - 81
92
0.10
87
1.9
54
29
3.3
2-3
medium
- high
93
1.2
49
48
3.9
4
low
91
2.5
54
22
4.2
1-2
low medium
72
2.8
47
16
4.5
1-2
medium
- high
However, the sites also differed in respects other than tree species and fertility
(Table 1). For example, the thickness of the peat and the soil content of
organic matter differed amongst them (Table 1).
21
Table 2. Tree parameters for all treed sites, both poorly drained, i.e. pine (DP), birch (DB),
spruce, young trees (DSy), spruce, old trees (DSo) and alder (DA), and the undrained alder
swamp (US).
DP
DB
Age (years)
70
60
Height (m)
16
16
Diameter, breast height (mm) 200
150
Diameter increment (mm y-1) 1.8
1.9
Number of stems of the
1100 850
dominating tree species (ha-1)
Calculated biomass growth
3.6
3.2
(tonnes DW ha-1 y-1)
a all trees taller than 1.3 m were counted
DSy
50
18
180
2.8
1350
DSo
90
24
290
2.2
750
DA
40
19
220
3.5
1750
US
80
18
220
1.8
500
7.6
7.7
20.5
3.3
Measurements of GHG
The net GHG exchange was determined from dark static chamber
measurements of soil GHG release and CO2 accumulation in biomass. With
this method the net exchange of CH4 and N2O can be estimated quite well,
although transport of CH4 and N2O through plants may be inhibited
(Sebacher et al., 1985; Chang et al., 1998). However, the measuring technique
has several limitations with respect to determination of the net ecosystem
exchange of CO2. Firstly, the carbon accumulation in certain fractions, mainly
fine roots, is difficult to estimate. The formulas used for estimating the carbon
accumulation in coniferous trees (Paper I) do not account well for the
contribution of fine roots, since they were primarily designed for estimating
the amount of dry weight in different fractions of the standing biomass
(Peterson, 1999). In the formulas used for deciduous tree species the fine root
fraction was not included (Paper II). Consequently, the carbon accumulation
in tree biomass was underestimated using the formulas. Secondly, the soil CO2
release as measured in chambers is the sum of both root and decomposition
activity. The contribution of root activity to measured soil CO2 release has
been shown to be on average 50% in forests (e.g. Hanson et al., 2000) and
around 10% in a virgin bog (Silvola et al., 1996b). Due to the nature of the
techniques used for measuring the CO2 originating from roots, the 50% value
includes not only direct root activity, but also decomposing activity associated
with root exudates and recently dead root tissues (collectively called rootderived activity). As fine roots have a life-time of about a year (Majdi and
Andersson, 2004), the decomposition of fine roots was assumed to be
included in the measurements of root-derived activity. Consequently, the CO2
allocated to fine roots is not considered in the estimate of tree carbon
accumulation, but is instead subtracted from the soil CO2 release. Thus, the
estimate of the net ecosystem exchange was assumed to be accurate, in spite
of the somewhat back-calculation involved.
Measurement with dark chambers also leads to the exclusion of
photosynthetic activity of the forest floor vegetation. Furthermore, the forest
22
floor vegetation was left intact, so the measured forest floor CO2 release also
included CO2 respired by the above-ground parts of the understory.
Removing plant tissues from the chambers would have solved the problems
associated with subtracting the fraction of the CO2 release originating from
forest floor vegetation. Furthermore, the cut-away vegetation could have been
used to estimate the carbon accumulation in forest floor biomass. On the
other hand, cutting the forest floor might have resulted in overestimation of
the decomposition rates as the roots would have died off and provided the
microorganisms with easily decomposable organic matter. Therefore, the
forest floor vegetation was disturbed as little as possible. At the spruce sites
there were only small amounts of forest floor vegetation, mostly a thin layer
of mosses, so it is not likely that the forest floor respiration contributed to the
CO2 release to any great extent (Figs. 4 and 5). Compared to the carbon
accumulation in trees it is also likely that the CO2 uptake by the forest floor
vegetation is negligible. At the deciduous sites, both poorly drained and
undrained, the forest floor vegetation was denser (Figs. 3, 6 and 8), but still
the CO2 release from forest floor vegetation and the carbon accumulation by
the forest floor vegetation was assumed to be negligible. On the other hand,
both the pine site and the virgin fen had thick Sphagnum layers (Figs. 2 and 7),
which might have significantly contributed both to the annual forest floor
CO2 release and to the annual carbon accumulation by vegetation. At these
sites estimates of the growth of the forest floor vegetation were based on
literature data. The carbon use efficiency was assumed to be 0.5 (based on
Choudhury, 2000, 2001). Consequently, the same amount of CO2 that was
estimated to be annually accumulated in forest floor vegetation was also
assumed to be released by forest floor vegetation, and thus subtracted from
the forest floor respiration in order to obtain an estimate of its contribution to
soil respiration (Paper I).
The ditches were not evenly distributed within the sites, but chambers were
placed so that as much as possible of the differences in groundwater level and
peat depth within the sites was covered (Figs. 2-8). For more information
about the measuring techniques see Papers I-III.
Biotic and abiotic variables measured
The air temperature and groundwater level were measured concurrently with
the gas sampling. Other variables were measured occasionally for three
different purposes: to check the differences among the studied sites (Papers I
and II), to test within-site variability (Paper II) and to examine the effect of
distance to tree stems (Paper III). At the site level the age, height, diameter
and diameter increase of trees were measured and the number of stems per
hectare calculated (Table 2). Furthermore, dry bulk density and porosity of the
soil were determined at site level (Table 1, Papers I and II). Mean annual
23
groundwater levels and peat depth were used to characterize each chamber
and soil samples were collected once at each chamber within the sites and
used to measure degree of humification, organic matter content, total nitrogen
and carbon content in the organic matter, CN ratio and pH (Table 1, Papers
I and II). The parameters measured in the transect between two trees were
peat depth, leaf area index, dry weight of different species, pH, soil content of
water, organic matter, nitrogen and carbon, CN ratio and potential
denitrification (Paper III).
RESULTS AND DISCUSSION
The results of the studies are structured around the six hypotheses that were
formulated in the introduction.
Do groundwater level and air temperature explain temporal variation in
GHG fluxes?
I hypothesized that the variation in groundwater table and air temperature
could explain the variation in soil fluxes of GHG in poorly drained forests
(hypothesis I).
Between 47 and 68% of the temporal variations in forest floor CO2 release at
the poorly drained sites were explained by differences in air temperature and
sometimes also groundwater level (Tables 6 and 5 in Papers I and II,
respectively). In Papers I and II it was suggested that the response at high
temperatures might have been underestimated as the days when the
groundwater level was below the depth which could be measured, were
usually warm (Papers I and II). However, even at the site where the largest
number of samples (32%) was excluded from the regressions (Paper I), there
was no significant difference between the shapes of the curves obtained by (i)
including all of the samples, and (ii) excluding samples collected on occasions
800
Forest floor CO2 release
600
400
200
0
0
10
20
Temperature
24
30
Figure
10.
Relationships
between mean temperature
and mean site flux obtained
when including (() and
excluding ()) chambers at
which the groundwater level
could not be measured. The
correlation found when all
data
are
included
is
represented by the full line,
and the correlation found
when chambers with a
groundwater table below the
depth that could be measured
was exluded is represented by
the dotted line.
when the groundwater level could not be measured (Fig. 10). Thus, this
possibility is unlikely to have caused a significant error in the estimates.
The temporal variation in CH4 fluxes could be explained by groundwater level
and air temperature to a smaller extent than the variation in CO2, i.e. 0 to 26%
(Tables 6 and 5 in Papers I and II, respectively). Air temperature was more
important than groundwater level, which only significantly affected CH4
emissions at the poorly drained pine site (Table 6 in Paper I). CH4 fluxes
were not related to groundwater level in any easily predictable non-linear way
either (Fig. 11).
1800
Figure 11. Relationship
between temporal variations
in CH4 fluxes and mean site
groundwater table at the
poorly drained sites, i.e. pine
()), birch (+), spruce with
young trees (i), spruce with
old trees (") and alder (h).
1600
1400
1200
CH4 flux
1000
800
600
400
200
0
-200
-400
0
20
40
60
80
Groundwater level
In two of the poorly drained sites, i.e. the pine and alder sites, virtually none
of the temporal variation in N2O fluxes could be explained by differences in
groundwater level and air temperature (Tables 6 and 5 in Papers I and II,
respectively). For the other three sites, i.e. the spruce sites and birch site,
between 19 to 27% of the temporal variance could be explained by these two
factors.
These results show that the hypothesis - that groundwater level and air
temperature are the most important temporal regulating factors of GHG
emissions at poorly drained organic sites - was only supported for CO2.
Consequently, at poorly drained forest sites, factors other than groundwater
level and air temperature, or at least a more complex function describing the
relationship between these two factors, are needed to model temporal
variations in the emissions of CH4 and N2O. Since the years in which the
measurements were performed were warmer than the 30-year mean (Fig. 1 in
Papers I and II) the mean annual forest floor CO2 release may have been
higher than usual and due to anticipated climatic changes, which are expected
to raise temperatures in Sweden (Räisänen et al., 2003) it is very likely that the
future forest floor release rates will be even higher. That the temporal
25
variation in forest floor CO2 release was so strongly correlated with air
temperature and groundwater table has important implications for attempts to
scale up emissions as it implies that estimates of annual CO2 emissions at
poorly drained sites could be based on a limited number of flux
measurements.
Does distance to trees affect the emissions of GHG?
I hypothesized that there is a relationship between distance to stems and soil
fluxes of GHG in forests on poorly drained organic soils (hypothesis II).
There were no consistent patterns in the
variations in CO2 and CH4 fluxes in
transects between two trees. On two
occasions, however, the emissions in
transects between trees (Paper III) and the
emissions in the rest of the poorly drained
spruce site with old trees (Paper I) were
measured at the same time. Using all data
from these two occasions, both CO2 and
CH4 were linearly correlated (p<0.05) with
distance to stems (n=51 and 28,
respectively): CO2 during the second
sampling occasion and CH4 during the first
(Fig. 12). However, only 8 and 13% of the
spatial variation in forest floor CO2 release
and CH4 fluxes, respectively, was explained
by distance to stems. For N2O there was no
linear correlation (Fig. 12). On the other
hand, N2O fluxes showed large spatial
variations within transects with peaks
(attributed to root dynamics) occurring
during spring and autumn (Fig. 3 in Paper
Figure 12. Correlation between III). The emissions in these peaks were
distance to the closest stem and soil
much higher than the emissions measured
fluxes of GHG.
at the rest of the site (Fig 3 in Paper I).
Thus, there also seems to be a tree effect on
N2O emissions, but it is not easily
predictable in time and space.
220
200
-2
mg CO2 m h
-1
180
160
140
120
100
80
60
20
-2
µg CH4 m h
-1
0
-20
-40
250
-2
µg N2O m h
-1
200
150
100
50
0
-50
0
50
100
150
200
250
300
350
Distance to closest stem (cm)
The hypothesis that there is a relation between distance to stems and soil
GHG fluxes in poorly drained forest sites was partly supported for all gases.
The distance to trees should, therefore, be taken into consideration when
planning sampling schemes for poorly drained organic forest soils.
26
Consequently, the GHG fluxes from the poorly drained sites in this study may
have been over- or under- estimated as distance to stems was not taken into
account when the positions of the chambers were chosen. However, the
results show that the number of stems per hectare is not a useful parameter
for scaling up site emissions as linear correlations were weak or non-existing.
Do the soil emissions of GHG differ among sites differing in fertility
and tree species?
I hypothesized that soil fertility and dominating tree species are important
regulators of system GHG exchange in poorly drained forest sites (hypothesis
III).
The differences in soil fertility among the poorly drained sites, as determined
by forest floor vegetation, were partly reflected in forest productivity (the
birch and the highly fertile spruce sites being exceptions) and the CN ratio of
the organic matter, except for the highly fertile spruce site. Similarly, the
expected effects of tree species on the soil CN ratio and pH in the upper 10
cm of the soil were found, i.e. CN ratio decreased in the order pine > spruce
> birch > alder (Johansson, 1995; Wedderburn and Carter, 1999; Menyailo et
al., 2002, Smolander and Kitunen, 2002) and pH decreased in the order alder
> birch >spruce > pine (Menyilo et al., 2002; Smolander and Kitunen, 2002)
(Table 1). Thus, there were differences among the sites caused by differences
in soil fertility and tree species.
Despite the differences in soil fertility and tree species, the forest floor CO2
release did not differ significantly among the poorly drained sites (Fig. 13;
Papers I and II), except that forest floor CO2 release rates were significantly
(p<0.05) lower in the highly fertile spruce site dominated by old trees than in
the deciduous sites (Paper II). The CO2 release from root activity is likely to
be dependent in some way on the amount of stems per hectare, as the amount
of roots is likely to be dependent on the amount of stems, and this variable
differed among the sites (Table 2). For example, the number of stems per
hectare was lower at the spruce site with old trees compared to the pine site
(Table 2). Consequently, the CO2 release originating from root activity is
probably higher at the pine site. As the forest floor CO2 release did not differ
significantly between the two sites the results indicate that the CO2 release
from decomposition was higher at the more fertile spruce site.
27
2500
CO2
CH4
N2 O
-2
-1
CO2 eqvivalents (g m y )
2000
1500
1000
500
0
DP
DB
DSy
DSo
DA
UF
US
Figure 13. Mean annual soil emissions at the studied sites, both poorly drained, i.e. pine
(DP), birch (DB), spruce, young trees (DSy), spruce, old trees (DSo) and alder (DA), and
undrained, i.e. fen (UF) and alder swamp (US).
Hence, there may be an effect of soil fertility and tree species on the CO2
release originating from decomposition, which is masked by the respiration of
the roots and forest floor vegetation. Furthermore, the sites do not differ only
in terms of tree species and fertility, which weakens the conclusion that
neither tree species nor fertility influence forest floor CO2 release. However, it
is possible that the high groundwater tables at these sites (Table 1) limited the
decomposition rates. Oxygen status is recognized as one of the most
important factors affecting decomposition rates in terrestrial systems (Swift et
al., 1979). In this case the other factors, known to vary amongst the sites are
unlikely to affect the decomposition rates. For example, a load of 100 kg
NH4NO3-N ha-1 y-1, which should have had a major effect on soil fertility, did
not significantly increase the forest floor CO2 release at a Finnish pine bog
with a mean annual groundwater table at approximately 20-30 cm below the
soil surface (Nykänen et al., 2002).
The CH4 emissions decreased significantly (p<0.05) in the order pine > birch
and spruce site with old trees > spruce site with young trees and alder (Fig. 13;
Papers I and II). There was no correlation with soil fertility, CN ratio or tree
species. Instead, the differences in CH4 emissions among the sites were, most
likely, governed by the mean annual groundwater level (Fig. 14). Although the
groundwater levels differed between the wettest and driest sites by only 10
cm, the effects of these differences may have masked the effects of tree
species and fertility, which have previously been found to cause differences in
CH4 fluxes between mineral forests (Borken and Brumme, 1997; Hudgens and
Yavitt, 1997) and undrained peat soils (Martikainen et al., 1995; Nilsson et al.,
2001).
28
1,2
Figure 14. Soil CH4 fluxes
and their correlation with
mean annual groundwater
level at the poorly drained
sites, i.e. sites dominated by
pine (DP), birch (DB),
young spruce trees (DSy),
old spruce trees (DSo) and
alder (DA).
-2
-1
Mean annual CH4 emission (g m y )
DP
1,0
DB
DA
0,8
0,6
0,4
DSo
0,2
0,0
DSy
-0,2
14
16
18
20
22
24
26
28
Mean annual groundwater level (cm)
The N2O emissions at the poorly drained sites increased significantly (p<0.05)
in the order pine < spruce with young trees < birch < alder, while the N2O
emissions at the highly fertile spruce with old trees did not differ significantly
from either the pine or the other spruce site (Fig. 13; Papers I and II).
Consequently, higher emissions were found for the sites dominated by
deciduous sites and the emission pattern is similar to trends Menyailo and
Huwe (1999) found for tree species planted on mineral soils. Furthermore,
they are highly correlated with the CN ratio in the upper 10 cm of the soil
(Fig. 15). Under conditions of low nitrogen availability larger fractions of the
nitrogen in leaves and needles are withdrawn before the litter falls (Gosz,
1981; Staaf and Berg, 1981). Thus, both the soil fertility and tree species affect
the nitrogen concentration in the litter.
As the relationship between CN ratio and N2O emissions at the poorly
drained sites was so strong, data from other studies were also included (Paper
IV), and the relationship remained strong after their inclusion (Fig. 15). The
N2O emission rates at CN ratios >25 are low, i.e. in the range 0.005 to 0.08 g
m2 y-1. Below this level, the emissions increase with further reductions in the
CN ratio. The threshold value at a CN ratio of 25 agrees well with
observations that net nitrification (i.e. accumulation of nitrate) only occurs at
low CN ratios (Gundersen et al., 1998a). Net nitrification has been found to
increase exponentially with reductions in CN below the threshold (Ollinger et
al., 2002), as found for N2O emissions in this study (Fig. 15). The findings
also have analogues with observations of significant N losses by nitrate
leaching in forests on mineral soils, which again occur at soil organic matter
CN ratios below 25 (Gundersen et al., 1998b; MacDonald et al., 2002). This
indicates that nitrification may be the rate-limiting process for N2O emissions
in drained organic forest soils.
29
Figure 15. Soil N2O fluxes
and their correlation with
CN ratios in the upper 10
cm of the soils at the poorly
drained sites, i.e. sites
dominated by pine (DP),
birch (DB), young spruce
trees (DSy), old spruce trees
(DSo) and alder (DA). Data
points for other sites (see
Paper IV) are included as
dots.
3,0
-2
-1
mean annual N2O emission (g m y )
3,5
2,5
2,0
1,5
1,0
DA
0,5
DB
0,0
0
20
DSy
DSo
DP
40
60
80
100
CN ratio in the upper 10 cm of the soil
There is a problem associated with the curve form for CN ratios below 15-20,
which is linked to the hierarchical control of the emissions (Brumme et al.,
1999). Once the rate-limiting parameter loses importance, other factors (e.g.
pH, soil moisture and temperature (Robertson, 1989)) start to act as
moderators of the emissions. Consequently, more data are needed to improve
the curve form for the emissions at CN ratios below 15-20.
The hypothesis that soil fertility and tree species would be the most important
factors causing differences in GHG emissions at poorly drained sites was only
supported for N2O. The CN ratio in the top-soil seems to be a good predictor
for mean annual N2O emissions, as shown by the strong correlation between
these two variables. Attempts to scale up N2O emissions should therefore be
based on CN ratios. For CO2 and CH4 the groundwater table seems to be of
major importance. Consequently, the groundwater table needs to be
considered in attempts to scale up CO2 and CH4 fluxes. The results show that
the forest floor CO2 release rates from a poorly drained forest with a peat
layer thinner than 30 cm were not significantly lower than those from poorly
drained sites with a peat layer thicker than 30 cm. This indicates that drained
sites with a peat layer thinner than 30 cm should also be included in estimates
of GHG emissions from drained areas. As about 50% of the drained organic
forest soils in Sweden have a peat layer thinner than 30 cm (according to data
from S-NFI), the inclusion of this area would have a significant impact on
estimates of GHG emissions.
Are poorly drained forests sources or sinks for GHG?
I hypothesized that tree accumulation of CO2 more than compensates for the
soil emissions of GHG at poorly drained organic forest areas, making the
30
areas net sinks, but that the major part of the carbon in the trees should be
considered as peat carbon (hypothesis IV).
Net emissions of GHG at poorly drained forest sites
The contribution of CH4 and N2O to the soil fluxes of GHG was small at all
sites (Fig. 13). Neither the forest floor CO2 release nor the estimated CO2
release originating from decomposing activity differed significantly among the
poorly drained sites, since the fraction originating from decomposing activity
was assumed to be 50% of the forest floor release at all sites except the pine
site (Paper I). Consequently, the differences in net GHG fluxes among the
sites were largely due to the differences in calculated CO2 accumulation in the
trees.
The poorly drained sites were very different in many respects (Tables 1 and 2).
However, most sites were net sinks of -0.2 to -2.7 kg CO2 equivalents m-2 y-1,
showing that the forest production at poorly drained sites, in most cases,
compensated for soil emissions (Fig. 16; Papers I and II). Only the poorly
drained birch site was a net source of GHG (0.4 kg equivalents m-2 y-1; Fig. 16;
Papers I and II). This was mainly due to the very low level of carbon
accumulation in the trees. The calculated carbon accumulation in tree biomass
of 600 g CO2 m-2 y-1 is equivalent to between 40 and 60% of the average
calculated carbon uptake by trees in deciduous moist drained forests in the
area (Paper V). Even the average growth in wet areas on peat soils, although
almost half the average for moist areas, is higher than the growth at the poorly
drained birch site. Accordingly, the calculated net GHG flux is probably not
representative for larger areas, and most of the poorly drained forest area is
most probably a sink for GHG.
Figure 16. Calculated net
GHG fluxes at all sites.
The drained sites, i.e.
pine (DP), birch (DB),
spruce
young
trees
(DSy), spruce old trees
(DSo) and alder (DA),
are represented by black
bars and the undrained
sites, i.e. fen (UF) and
swamp (US), by white
bars.
1.0
0.0
-2
-1
CO2 equivalents (kg m y )
0.5
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
DP
DB
DSy
DSo
DA
UF
US
31
Sensitivity analysis
There are large uncertainties in the estimates of net GHG fluxes at the poorly
drained sites. The major uncertainty is coupled to the CO2 fluxes. For
example, no biomass functions were available for estimating the below ground
carbon accumulation for birch. Such formulas were only available for spruce
and pine (Peterson, 1999), so the formula derived for spruce was used to
determine the birch parameter since birch, like spruce, has a flat root system,
while pine, in contrast, can have a taproot system. The above-ground growth
of alder was calculated using formulas derived for birch, and although alder
has a taproot system, identical formulas were used for birch and alder, i.e. the
formula derived for spruce roots was also used to estimate the fraction
allocated below-ground for alder. The estimates resulted in a below-ground
allocation of 25-27% of the total annual biomass increment for the deciduous
species (Paper II). This is similar to the 24 to 26% recommended by the
Good Practice Guidance for deciduous species in temperate regions (Penman
et al., 2003). Thus, the growth estimates would have been similar if default
values had been used. This does not, of course, guarantee their accuracy, and
more studies are need to resolve the biomass distribution, but the
assumptions regarding below-ground carbon accumulation were assumed to
be correct in this sensitivity analysis.
The carbon accumulation in forest floor vegetation was assumed to be
negligible for all sites except the poorly drained pine site. This may not,
however, be the case. Between 0 and 30% of the total carbon annually
assimilated has been found to be taken up by forest floor vegetation at
productive forested sites (see, for instance, Widén, 2001). Therefore, in this
sensitivity analysis, a range for carbon accumulation in biomass was used,
between the values presented in Table 2 and 1.3 times these values. For the
poorly drained pine site, 30% of the CO2 annually accumulated was estimated
to be taken up by Sphagnum, based on literature data on the production of the
species present at the site (Paper I), and in the sensitivity analysis values of 20
and 40% were used.
To get a range for the forest floor CO2 release, the differences among years,
although seldom statistically significant (Papers I and II), and standard errors
associated with the forest floor CO2 release, were used. The standard errors
were multiplied by two in order to determine the theoretical 95% confidence
intervals. The part of the CO2 release originating from roots is also uncertain.
Estimates of 50% of the root-derived CO2 release for the forested areas were
used for this parameter. To check the sensitivity of the net CO2 exchange
data, values of 40 and 60% were used. The assumption that there is significant
carbon accumulation in the forest floor vegetation also implies that the
respiration of the forest floor vegetation contributes to the forest floor CO2
32
release. However, due to the large ranges used for the other fluxes this was
not included in the calculations.
When calculating the net ecosystem exchange of GHG, emissions of CH4 and
N2O were also included. As for CO2 the differences among years and
standard errors (times two) were used to determine the respective ranges
(Papers I and II).
Table 3. Results of the sensitivity analysis of the CO2 exchange (in kg CO2 m-2 y-1 ) and
GHG exchange (in kg CO2 equivalents m-2 y-1 ) at the drained sites, i.e. pine (DP), birch
(DB), spruce, young trees (DSy), spruce, old trees (DSo) and alder (DA), and undrained, i.e.
fen (UF) and alder swamp (US). The ranges arise from differences in assumptions
regarding carbon accumulation in the forest floor vegetation and soil fluxes of GHG.
DP
DB
DSy
DSo
DA
UF
US
Range in annual CO2 exchange
-0.9 to 0.8
-0.5 to 1.2
-1.5 to -0.2
-1.7 to -0.1
-4.2 to -2.2
-0.6 to 1.1
-0.1 to 0.5
Range in annual GHG exchange
-0.9 to 0.9
-0.5 to 1.4
-1.6 to -0.2
-1.7 to 0.0
-4.2 to -1.5
-0.6 to 1.6
0.0 to 1.0
These calculations showed that the two poorly drained sites with the lowest
carbon accumulation in tree biomass, i.e. the pine and birch sites, might be in
equilibrium rather than being sinks or sources, while the more productive
poorly drained sites are almost certainly net sinks for GHG (Table 3).
Reallocation of carbon
The carbon release estimated to originate from decomposition is due not only
to the decomposition of organic matter stored before drainage. Some of it is
also due to the decomposition of organic matter deposited from the growing
vegetation. Consequently, the CO2 originating from the decomposition of
new plant material has to be subtracted to estimate the fraction released as a
result of drainage. Organic matter is mainly deposited in the form of aboveand below- ground litter from trees and vegetation. As the annual litter input
was not measured, these values had to be derived from the literature.
Decomposition of fine roots has already been considered above, as it is
included in the CO2 from root-derived activity. Assuming that the
contributions of coarse roots is relatively small they will be excluded from the
estimates. The annual litter input in a part of the poorly drained spruce site
with young trees was measured as part of the LUSTRA project (LUSTRA,
2004). Preliminary data indicate that above-ground litter inputs from the trees
amount to 86 g C m-2 y-1 and corresponding inputs from the field and bottom
layers amount to 20 g C m-2 y-1 (Berggren et al., 2002). The above-ground litter
input from trees corresponds to 20% of the total calculated carbon uptake by
33
trees (Table 2). The proportion of the above-ground litter originating from the
forest floor vegetation was about 20%.
Given the lack of better data, 20% of the annual carbon accumulation in trees
at the coniferous sites was assumed to be deposited as litter and the litter
input from forest floor vegetation was assumed to be equivalent to 20% of
the litter input from trees at the spruce sites. For the pine site, the previously
applied figure for carbon accumulation in forest floor vegetation, i.e. 30% of
the carbon accumulated in tree biomass (Paper I), was used and all that is
produced during the course of a year was assumed to be added to the soil as
litter. The annual tree litter input at deciduous sites has been found to be
about 35% of the annual tree biomass production (Mar-Möller et al., 1954;
Duvigneaud and Denaeyer-De Smet, 1970). As for coniferous forests, about
20% of the litter originates from forest floor vegetation (M. Johansson pers.
comm.). These values were used in the calculations. The estimated CO2
contribution from the decomposition of recently added litter was divided by
the estimated soil CO2 release, which gave the proportion originating from
recently added litter. The values derived from the calculations of litter input
from above-ground parts of the trees were subtracted from the carbon
accumulations in tree biomass, to obtain an estimate of the fraction of the
CO2 annually taken up by the trees that is allocated to the standing vegetation.
The fraction of the tree carbon that should be considered as peat carbon was
calculated by dividing the amount of CO2 estimated to originate from peat
decomposition by the annual CO2 increment in the standing vegetation.
Table 4. Estimates of different CO2 fluxes (g C m-2 y-1) at all drained sites, i.e. the pine
(DP), birch (DB), spruce young trees (DSy), spruce old trees (DSo) and alder (DA) sites,
are represented by black bars and the undrained sites, i.e. fen (UF) and swamp (US), by
white bars.
Tree
Annual
litter
carbon
accumulation
in trees
DP 180
40
DB 160
60
DSy 380
80
DSo 385
80
DA 1025
360
Annual
carbon
accumulation
minus litter
140
100
300
305
665
Forest floor CO2 release from Proportion of
litter
decomposition of total estimated
recently added litter decomposition
(%)
50
90
50
10
70
30
20
100
50
20
100
70
70
430
100
Proportion of tree
carbon considered
peat carbon (%)
100
100
70
50
0
Based on these assumptions, between 30 and 100% of the estimated total CO2
release from decomposition originated from decomposition of new litter at
the sites (Table 4). Consequently, between 0 and 100% of the carbon in the
trees could be considered as peat carbon (Table 4). Of course, there are major
uncertainties associated with the estimates for each site. Nevertheless, this
gives an indication of an important problem that should be addressed when
assessing drained forestland. The calculations show that the low productivity
sites are the most crucial, as 100% of the tree carbon could be regarded as
34
peat carbon at the pine and birch sites. The finding that 100% of the
estimated soil CO2 release originates from the decomposition of new litter at
the alder site implies that poorly drained forested sites on soils with a peat
layer thinner than 30 cm may not release additional soil CO2 from the
decomposition of organic matter stored before drainage. However, in another
study a forested drained mineral soil has been found to be a net emitter of
CO2 (Lindroth et al., 1998).
Thus, the results support the hypothesis that tree accumulation of CO2 more
than compensates for the soil emissions of GHG at poorly drained organic
forest areas, making the areas net sinks. The exceptions are sites with low
forest growth, which may be either sources or in equilibrium. Consequently,
the total area of poorly drained forestland in Sweden is likely to be a net sink
of GHG. The results also support the hypothesis that most of the carbon in
the trees should be considered as peat carbon. This allocation also has to be
considered in attempts to scale up emissions from drained forest soils.
Are poorly drained forest sites larger net sinks than well-drained and
virgin sites?
I hypothesized that the GHG fluxes in poorly drained forest soils would differ
significantly from the fluxes in well-drained forest soils (hypothesis III) and
that both well-drained and rewetted organic soils are larger sources (or smaller
sinks) of GHG than poorly drained soils (hypothesis V).
Comparison between poorly drained and virgin sites
To estimate the impact of a management strategy involving rewetting and the
restoration of previously drained areas, the emissions from the poorly drained
sites were compared with emissions from undrained sites. As climatic
variables, e.g. groundwater table and temperature, are known to affect the
emissions to a large extent (Silvola et al., 1996a; Nykänen et al., 1998; Maljanen
et al., 2003a) measurements from both poorly drained and undrained sites in
the same region during the same measuring period were needed for the
comparison. To obtain a possible span for undrained conditions two sites
were chosen, one fen and one alder swamp, which may be representative for
the pre-drainage conditions of the poorly drained sites. These two peatcovered sites differ in many respects. The pH and nitrogen content is higher
for alder swamps and the CN ratio lower compared to the corresponding
figures for fens and bogs (Mäkinen, 1979; Urvas et al., 1979). There is also a
significant limit to the peat thickness of alder swamps, of around 50 cm,
because roots of alder trees cannot extend much deeper into the mineral soil
(Mäkinen, 1979). Similar differences were also found between the two drained
sites in this study (Table 1). The soil emissions of GHG at the sites were
found to be representative for their respective wetland types (Papers I and
35
II). However, the fen may not be representative for all untreed peat-covered
soils. The carbon accumulation in biomass is larger in bogs than in fens (see,
for instance, Tolonen and Turunen, 1996) while CH4 emissions are larger in
fens than in bogs (Martikainen et al., 1995; Nilsson et al., 2001). Likewise, the
alder swamp may not be considered to be representative for other swamps
and treed mires. The growth at the alder swamp was associated with an uptake
of CO2 by the trees of around 600 g m-2 h-1 (Paper II). The average carbon
uptake by the trees in coniferous drained productive wet forests is in the order
of 200 to 400 g m-2 h-1 (based on S-NFI data, using the same method for
calculating as in Paper V). Consequently, the carbon accumulation in tree
biomass at coniferous undrained swamps and treed mires is most likely lower
than the carbon accumulation in the alder swamp. Emissions of CH4 have
been found to be lower from treed compared to untreed mires (Bubier et al,
1995; Granberg et al., 1997). However, the CH4 emissions did not differ
significantly between the fen and swamp, indicating that the CH4 emissions
were high at the alder swamp compared to other swamps and treed mires.
Furthermore, N2O emissions of most undrained mires have been found to be
low (Martikainen et al., 1993, Laine et al., 1996; Regina et al., 1996) while the
emissions from the alder swamp were significant. Thus, the net fluxes of
GHG may be either smaller or larger from coniferous swamps and treed
mires compared to the alder swamp. Consequently, the net GHG fluxes in the
poorly drained sites in a virgin condition may have differed from the net
GHG fluxes at the fen and swamp in this study. Nevertheless, the fen and the
alder swamp will be used for comparison.
Comparing the mean net emissions, all poorly drained sites had smaller net
emissions than the fen, and all but the poorly drained birch site had smaller
net emissions than the swamp (Fig. 16). However, as for the poorly drained
sites, a sensitivity analysis of the net annual fluxes had to be performed. For
the fen substantial uncertainty is associated with the carbon accumulation in
the forest floor vegetation. The forest floor carbon accumulation was
estimated to be 200 g DW m-2 y-1 (Paper I). The highest Sphagnum production
rate reported by Lindholm and Vasander (1990) was 380 g DW m-2 y-1 and
Moore et al. (2002) reported net primary production for the above-ground
forest floor of 360 g DW m-2 y-1 at a bog. In the sensitivity analysis, therefore,
a range of carbon accumulation of 100 to 400 g DW m-2 y-1 was used.
Furthermore, the contribution to the soil CO2 release from roots was assumed
to be 10% of the total, based on Silvola et al. (1996b). In the sensitivity
analysis root-derived contributions of 0 to 20% were used. As for the poorly
drained sites, the contribution of forest floor respiration to forest floor CO2
release was not changed in the different calculations. The sensitivity analysis
for the alder swamp was based on the same assumptions as for the poorly
drained sites (see the Sensitivity analysis section above).
36
The sensitivity analysis shows that both the fen and the swamp may have been
in equilibrium, or the fen even a sink, instead of sources during the sampling
period (Table 3). The poorly drained birch and spruce sites may have net
emissions similar to the undrained sites. However, although the differences
between the poorly drained and undrained sites were not significant (Table 3),
it is still likely that the poorly drained spruce sites were larger sinks for GHG,
during the measuring period, than both the fen and the swamp. Furthermore,
it is almost certain that the poorly drained alder site was a larger sink for
GHG than the undrained sites. This is in accordance with Minkkinen et al.
(2002) who claimed that forestry drainage had decreased the radiative forcing
of Finnish peatlands.
It is not certain that the emissions from a rewetted forest are similar to those
in virgin conditions. After a disturbance of an agricultural soil, 10-15 years or
more of constant management is required for soil organic carbon to reach a
new balance (Batjes, 1998), in which it is possible that either smaller or larger
amounts of carbon can be stored in the soil. Therefore, it is likely that it will
take some time for rewetted sites to return to a state similar to the virgin
conditions, and in some cases the rewetted conditions may never be similar to
them. However, the comparison gives an indication that the net emissions of
GHG fluxes from rewetted sites are likely to be larger than those from poorly
drained sites, i.e. the results support the hypothesis.
Comparison between poorly drained and well-drained sites
To estimate the impact of a remedial or complementary drainage, the
emissions from the poorly drained sites were compared with emissions from
well-drained sites. The average annual forest floor CO2 release in a welldrained site dominated by pine, with a mean annual groundwater table at
about 70 cm (M. Lundblad pers. comm.), was 2.8 kg CO2 m-2 (Widén, 2001).
Average annual forest floor CO2 release rates of 2.4 kg m-2 y-1 have been
reported for a well-drained Swedish birch site with an average annual
groundwater level at 53 cm (Weslien et al., XXXX). The average annual forest
floor CO2 release rates in the pine and birch sites in the present study were 1.5
and 1.9 kg m-2 y-1, respectively (Fig. 14). Consequently, the forest floor CO2
release rates were 1.3 and 0.5 kg m-2 y-1 higher at the well-drained sites
compared to the poorly drained sites. This might, however, be partly due to
differences in root activity as well as in decomposition rates between the
investigated areas. Nevertheless, it seems very likely that the soil CO2 release
increases with drainage depth, as reported by Silvola et al. (1996a).
However, the CO2 accumulation in trees is also likely to be increased by a
lowering of the groundwater table. The pine site studied by Widén (2001) was
37
situated in a heterogeneous area with patches of both mineral and peat soils.
In this area measurements were also performed with micrometeorological
techniques. The site was found to be a net source of between 0.2 and 0.8 kg
CO2 m-2 y-1 (Lindroth et al., 1998). Thus, the carbon assimilation by trees could
not compensate for the soil emissions. Our poorly drained pine site was, in
contrast, a net sink of 0.2 kg CO2 m-2 y-1. In the site studied by Weslien et al.
(XXXX) the carbon uptake by trees was estimated to amount to 1.0 kg CO2
m-2 y-1. Using the same assumptions for root contributions as those used for
the poorly drained birch site, the site was an estimated net source of 0.2 kg
CO2 m-2 y-1, i.e. approximately equal to the poorly drained birch site (0.3 kg
CO2 m-2 y-1). Soil fluxes of CH4 and N2O were reported to be on average -0.1
g CH4 m-2 y-1 and 3.2 g N2O m-2 y-1, respectively (Weslien et al., XXXX).
Summing these fluxes makes the site a net source of 1.1 kg CO2 equivalents
m-2 y-1, while the poorly drained birch site was a net source of 0.4 kg CO2
equivalents m-2 y-1.
Consequently, it is very likely that net emissions of GHG from the welldrained forest areas and undrained areas are higher than from poorly drained
areas. This supports the hypothesis that the best management strategy for
poorly drained soils is to keep them moist. However, the virgin mires are only
weaker estimated sources of GHG than poorly drained forests if the
reallocation of carbon is not considered (see section Reallocation of carbon).
Overall, the results indicate that the poorly drained sites should at least not be
subjected to complementary or remedial drainage, if the GHG perspective is
considered. The results also support the hypothesis that the GHG fluxes in
poorly drained forest soils differ significantly from the fluxes in well-drained
forest soils. As the net emissions of GHG differ among sites that differ in
drainage intensity the groundwater table has to be considered in attempts to
scale them up.
Do drained sites contribute significantly to the Swedish GHG budget?
Based on the results of the study an attempt was made to scale up the
emissions from drained organic forestland in Sweden to a national level. I
hypothesized that the contribution of GHG from drained forests on organic
soils would have a significant impact on the national GHG budget (hypothesis
VI).
For valid up-scaling high quality emission data are required for different site
types. My experimental work was focused on poorly drained sites. As less than
20% of the drained organic forestland in Sweden is classified as wet or moist
(i.e. poorly drained, see Paper V), the up-scaling had to be complemented by
literature data. Only Finnish and Swedish sites were included. Finnish
38
emission data may not accurately describe the conditions in southern Sweden,
but were still used due to the paucity of Swedish data.
The emissions reported in Finnish and Swedish studies were divided into site
property groups based on the climatic conditions in the areas in which the
respective sites were situated, mean annual groundwater table, tree species and
soil fertility (Paper V). The soil CO2 release was higher in sites with an
average annual groundwater level below 40 cm compared to sites with a
groundwater level above 40 cm. Mean annual groundwater table also
significantly separated the CH4 emissions: the poorly drained sites having
higher emissions. CH4 fluxes were also significantly divided by soil fertility,
the less fertile sites having higher emissions. The N2O emissions only differed
significantly among sites dominated by different tree species. Sites dominated
by deciduous species had higher emissions than sites dominated by coniferous
species. Consequently, the data currently available support the finding that
groundwater table is the most important factor affecting CO2 and CH4, while
tree species were of importance for the N2O emissions.
All drained forestland on organic soils, rather than merely forestland on peat
soils, was included in the up-scaling. Based on the emission factors the
drained forestland in Sweden was estimated to be a net source of 2.2 Mtonnes
CO2 equivalents y-1 (Paper V). The N2O emissions may have been
underestimated since accurate up-scaling cannot be based on tree species
alone since N2O emissions from forest sites with organic soils that have
previously been used for agriculture have been found to be high, i.e. 8-33 kg
N2O ha-1 y-1, regardless of tree species (Maljanen et al., 2004; Weslien et al.,
XXXX). Between 30 and 45% of the organic drained forestland in Sweden
may have been used for agriculture previously. As this area is not accounted
for separately the national N2O emissions from drained organic forestland
may have been underestimated by a factor of up to four (Paper V). These
sites would most probably be recognized as highly emitting if the CN ratio
was considered in the up-scaling. Such data are, however, not available in SNFI but in the Swedish Survey of Forest Soils and Vegetation. The number of
sampling plots in the latter database is smaller than for S-NFI and,
consequently, the coupling between these two databases would result in a
larger uncertainty in the areal estimates. Therefore, a larger survey is needed
before an up-scaling can be based on CN data.
In the estimate the CO2 exchange was of most importance (Paper V).
Therefore, only the CO2 exchange will be discussed in the following sections.
The uptake of CO2 by the vegetation was estimated to amount to 8.9 and the
soil CO2 release from decomposition to 10.4 M tonnes CO2 y-1 (Paper V). As
previously discussed, the decomposition of peat should be treated separately.
39
The estimates were based on the same assumptions as previously used (see
section Reallocation of carbon), i.e. that 20 and 35% of the carbon accumulated in
trees annually in coniferous and deciduous areas, respectively, is deposited as
litter, that the same amount of carbon accumulated in forest floor vegetation
during the course of a year becomes litter, and that the root litter has already
been accounted for by subtracting the estimated root-derived activity from the
soil respiration. Of the 2.2 Mtonnes y-1 of carbon (or 8.0 Mtonnes CO2 y-1)
incorporated into tree biomass annually about 70% was attributed to growth
in areas dominated by coniferous species (Paper V). Consequently, the total
litter input from the above-ground tree vegetation was estimated to be 0.5
Mtonnes C y-1 (0.3 Mtonnes C y-1 in coniferous and 0.2 Mtonnes C y-1 in
deciduous forests). The annual carbon incorporation into forest floor biomass
was estimated to be 0.2 Mtonnes C y-1 (or 0.9 Mtonnes CO2 y-1 Paper V). If all
of the 0.7 Mtonnes C produced forest floor biomass is added to the soil as
litter annually and all added litter is decomposed, 2.6 Mtonnes CO2 y-1 of the
soil release originates from the decomposition of litter, resulting in an
estimated contribution of organic matter stored before drainage of 7.8
Mtonnes CO2 y-1. The total CO2 emissions from the consumption of fossil
fuels are somewhere around 50 Mtonnes CO2 y-1 (SNV, 2004). Consequently,
the release of CO2 from soil organic matter corresponds to approximately
15% of the total amount of CO2 released from the consumption of fossil
fuels.
The trees growing on drained organic forests soils could compensate for most
of the soil CO2 release. Still the drained forested area in Sweden was a net
source of CO2, and therefore a major part of the carbon in tree biomass
should be regarded as peat carbon. Consequently, if peat should not be used
for energy production neither should the trees growing on drained organic
soils. This has profound political implications for the suitability of using the
tree biomass from drained forestland for fuel.
If the main goal is to reduce the radiative impact of drained forestland the
results suggest that the best alternative is to let all sites become poorly drained
by not subjecting them to complementary or remedial drainage. Based on the
emission factor for poorly drained soils (Paper V) this should reduce the
amount of CO2 released from the soil via the decomposition of soil organic
matter by 30%. The tree growth would decrease, but probably not to the same
extent. Assessing the average potential productivities for moist drained forests
(based on S-NFI data) for the total area of organic drained forestland in
Sweden, the carbon uptake by the trees would only decrease by 6%.
Consequently, if the total area of drained organic forestland in Sweden
becomes moister in the future, the net CO2 emissions are likely to decrease
significantly.
40
The hypothesis – that the contribution of GHG from drained forests on
organic soils would have a significant impact on the national GHG budget –
was not supported by the results of the up-scaling. The net emissions were
small and will probably become even smaller in the future if the drained
forested area is not complementary or remedially drained.
Savolainen et al. (1994) argued that it was better to use peat from cultivated
soils than peat from virgin peatlands or forest-drained peatlands from a GHG
perspective and that it was better to use peat from cultivated peatlands than
coal for energy production. Their conclusion was based on the assumption
that cultivated peatlands were net sources of GHG, while forest-drained
peatlands were net sinks. This is not supported by our findings that the total
area of drained forest on organic soil is a net source of GHG. Consequently, it
may be better to use peat from drained forestland for energy production than
using coal. In addition, I argue that if the wood on the drained peatlands in
Sweden is to be used to substitute for fossil fuels it is better to use the peat
directly as most of the tree carbon is to be defined as peat carbon. This would
also reduce the CO2 emissions from decomposition of organic matter stored
before drainage, which corresponded to 15% of the national consumption of
fossil fuels.
MAIN CONCLUSIONS AND FUTURE RESEARCH
This thesis contributes to the knowledge of how forests on drained organic
soils function in the context of GHG exchange. Results are presented on
GHG exchange in a peat-covered forest with a peat layer thinner than 30 cm
and at sites dominated by deciduous trees, i.e. systems not previously well
studied, and provides an estimate of the total emissions of GHG from the
drained forestland in Sweden.
The most important conclusions were:
• Temporal variations in forest floor CO2 release from poorly drained
forest soils can to a large extent be explained by air temperature and
groundwater table.
• All soil fluxes of GHG were to some extent affected by distance to
trees. Although, distance to tree stems is not a good predictor for
spatial variations in soil GHG fluxes, distance to trees should be taken
into consideration when planning sampling schemes for poorly drained
organic soils.
• The CN ratio is the most important factor affecting between-site spatial
variation in N2O fluxes, both for poorly and well-drained forests on
organic soil. Groundwater table may be the most important factor
determining the size of soil CO2 and CH4 fluxes.
41
• Most poorly drained sites in Sweden are probably net sinks for GHG.
A large part of the tree biomass carbon could be regarded as peat
carbon, which has profound political implications for the suitability of
using the wood for fuel.
• If the main objective is to reduce the concentrations of GHG in the
atmosphere, the results indicate that the poorly drained forested area
should not be subjected to remedial drainage.
• The net impact of drained forestland in Sweden on the national GHG
budget is small at present. However, the CO2 release from
decomposition of soil organic matter is significant.
In my opinion more research is needed in several fields. One of the most
important concerns is the GHG fluxes from drained soils with a peat layer
thinner than 30 cm. The different fluxes of CO2 also need further resolution.
The estimates in this thesis are based on several assumptions about the
different sources and sinks of CO2 (e.g. the carbon accumulation in fine roots
and forest floor vegetation), which may not be accurate. Furthermore, the
contribution to soil CO2 release of different sources needs much more
attention. Finally, I think that the strong correlation between soil CN ratios
and N2O fluxes needs to be studied more thoroughly, especially at sites with
low CN ratios.
REFERENCES
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46
ACKNOWLEDGEMENTS
Detta arbete skulle inte ha varit möjligt att utföra utan hjälp från en mängd personer. Jag
vill speciellt tacka:
Min handledare, Leif Klemedtsson, som alltid varit inspirerande och full av idéer och som
brytt sig om mig som person och inte bara som doktorand;
Mina båda bihandledare Mats Nilsson, som kommit med kloka synpunkter på manuskript,
och Björn Hånell, som kommit med uppmuntrade tillrop och självförtroendehöjande
kommentarer;
Bo Svensson, som alltid ställt upp och funnits tillgänglig under doktorandutbildningens
mindre roliga skeden;
All personal på Asa försökspark som bidragit till att fältarbetet blivit avsevärt mycket
enklare och roligare. Speciellt vill jag tacka Stefan Eriksson som med aldrig sinande energi
och kompetens utfört alla konstiga uppgifter som dykt upp under projektets gång. Jag vill
även tacka Pia Rickardsson och Karin Magnusson som under perioder hjälpt till med
fältarbetet och Magnus Ripström, ägaren av Asa vandrarhem, som varit mycket flexibel
med bokningar;
All teknisk och administrativ personal på Tema Vatten som tålmodigt hjälpt mig lösa alla
möjliga olika problem;
Alla andra på Tema Vatten som bidragit till att skapa en miljö i vilken det varit roligt att
arbeta. Flera av er har även kommenterat manuskript och på andra sätt hjälpt mig att få
min avhandling bättre. Jag vill speciellt tacka David Bastviken, Andreas Berg, Gunnar
Börjesson, Åsa Danielsson, Jenny Grönwall, Elisabeth Johansson, Lena Lundman, Malin
Mobjörk, Peter Wihlborg, Julie Wilk och Mats Öquist;
Per Weslien, Maria Gustafsson, Josefin Norman och Robert Björk som analyserat prover
och kommenterat manuskript;
Hans Toet, som gjort alla kartor;
John Blackwell som språkgranskat artiklar och kappa;
Mina föräldrar, mor- och farföräldrar och alla vänner utanför universitetet som stöttat mig,
låtit mig invadera sina hem när jag haft möten i olika delar av landet samt, framför allt,
påmint mig om att det finns viktigare saker i livet än att bli doktor;
Mattias som ställt upp för mig under denna period både genom att läsa, kommentera och
diskutera texter och ta hand om allt hushållsarbete. Jag lovar att försöka återgälda dig!
47
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