forests Consequences of More Intensive Forestry for the Sustainable

forests Consequences of More Intensive Forestry for the Sustainable
Forests 2011, 2, 243-260; doi:10.3390/f2010243
OPEN ACCESS
forests
ISSN 1999-4907
www.mdpi.com/journal/forests
Article
Consequences of More Intensive Forestry for the Sustainable
Management of Forest Soils and Waters
Hjalmar Laudon 1,*, Ryan A. Sponseller 1 , Richard W. Lucas 1, Martyn N. Futter 2,
Gustaf Egnell 1, Kevin Bishop 2,3, Anneli Ågren 1, Eva Ring 4 and Peter Högberg 1
1
2
3
4
Department of Forest Ecology and Management, Swedish University of Agricultural Sciences
(SLU), Umeå, SE-901 83 Umeå, Sweden; E-Mails: [email protected] (R.A.S.);
[email protected] (R.W.L.); [email protected] (G.E.); [email protected] (A.A.);
[email protected] (P.H.)
Department of Aquatic Sciences and Assessment, SLU, SE-750 07 Uppsala, Sweden;
E-Mails: [email protected] (M.N.F.); [email protected] (K.B.)
Department of Earth Sciences, Uppsala University, SE-752 36 Uppsala, Sweden
The Forestry Research Institute of Sweden, Uppsala Science Park, SE-751 83 Uppsala, Sweden;
E-Mail: [email protected]
* Author to whom correspondence should be addressed; E-Mail: [email protected];
Tel.: +46-90-786-8584.
Received: 19 November 2010; in revised form: 17 January 2011 / Accepted: 8 February 2011 /
Published: 16 February 2011
Abstract: Additions of nutrients, faster growing tree varieties, more intense harvest
practices, and a changing climate all have the potential to increase forest production in
Sweden, thereby mitigating climate change through carbon sequestration and fossil fuel
substitution. However, the effects of management strategies for increased biomass
production on soil resources and water quality at landscape scales are inadequately
understood. Key knowledge gaps also remain regarding the sustainability of shorter
rotation periods and more intensive biomass harvests. This includes effects of fertilization
on the long-term weathering and supply of base cations and the consequences of changing
mineral availability for future forest production. Furthermore, because soils and surface
waters are closely connected, management efforts in the terrestrial landscape will
potentially have consequences for water quality and the ecology of streams, rivers, and
lakes. Here, we review and discuss some of the most pertinent questions related to how
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increased forest biomass production in Sweden could affect soils and surface waters, and
how contemporary forestry goals can be met while minimizing the loss of other ecosystem
services. We suggest that the development of management plans to promote the sustainable
use of soil resources and water quality, while maximizing biomass production, will require
a holistic ecosystem approach that is placed within a broader landscape perspective.
Keywords: soils; streams and rivers; sustainability; forestry; biomass production;
nitrogen fertilization
1. Introduction
Developing management strategies for the sustainable use of soil and water resources is recognized
as a major environmental challenge for future generations [1,2]. Both soil and water resources are
likely to become increasingly exploited and threatened by a growing human population, changes in
biogeochemical cycles, and major land-use transitions at global scales [3]. Even in sparsely populated
boreal regions, soils and waters are being affected by long-range transport and atmospheric deposition
of pollutants, climate change, and land use activities, including forest management [4,5]. In order to
manage forests in a sustainable way, decision making must be based on a sound understanding of the
relationships between environmental change and ecosystem function. To develop this understanding,
we need to improve our basic knowledge of the underlying mechanisms behind forest ecosystem
dynamics and land-water interactions across the range of spatial and temporal scales relevant to
land management.
Greater pressure is being placed on forests as the demand for wood products increases, and forest
biomass becomes increasingly popular as an alternative to fossil fuels [6,7]. For example, the use of
forest products in district heating in Sweden has dramatically increased from 1 PJ (~0.15 million m3
tree biomass) in 1980 to 92 PJ (~13.8 million m3 tree biomass) in 2008 [8]. In an effort to meet the
rising demand for forest products, it has been suggested that forestry should be intensified by
increasing the use of fertilizers and by using genetically improved seedling stocks to increase biomass
production, and thereby decrease rotation periods. Whole-tree harvesting and full-tree harvesting
(defined respectively as the removal of all above-stump tree biomass and the removal of all tree
biomass including the stump) are two other forestry techniques that could potentially be used to meet
the increased demands for forest biomass [9].
Intensified harvesting, in combination with other forest management practices, could have large
ecological consequences for both terrestrial and aquatic ecosystems, including effects on the storage
and availability of carbon (C), nitrogen (N), calcium (Ca), potassium (K) and other nutrients [10-12].
Because forestry operations typically affect the biological, physical and chemical properties of
soils [9,13], it has been suggested that preserving forests in reserves is the best way to protect several
ecosystem services, including the sequestration of soil C [14]. However, Berg et al. [15] used 41 years
of soil survey data across all forested areas of Sweden and showed an average accumulation in the
humus layer of 250 kgC·
ha−1·
yr−1. The corresponding accumulation in predominately managed forests
in the northern part of the country estimated by Berg et al. [15] was actually higher than that estimated
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to accumulate in soils of the same region of undisturbed, old-growth forests during periods with no
human or fire disturbance [16]. Thus, for Swedish forest soils, forestry may lead to an increased C
accumulation, presumably by maintaining a lower average stand age, which is associated with greater
rates of growth and detrital inputs to soil, when compared to old-growth forests. While conventional
forest management may lead to increased carbon sequestration above and below ground, it is critical to
understand how more intensive forestry approaches may influence the entire suite of ecosystem
services that these landscapes provide.
The potential combined effect of new forest management approaches and climate change on the
long-term sustainability of soil and water resources on soil and water resources also needs
consideration. Of particular interest is whether and how the environmental impact of intensified
biomass production and harvest will be affected by predicted climate changes that may alter both
temperature and precipitation in Sweden. Soils and surface waters are closely linked by groundwater
and runoff that transport dissolved nutrients and other solutes from terrestrial to aquatic ecosystems.
As a consequence, climate-induced changes in plant productivity, soil biogeochemistry, and catchment
hydrology will likely also affect the water quality and ecology of streams and lakes [17]. The potential
downstream consequences of the combined effects of intensified biomass production and climate
change will ultimately depend on how individual landscape elements or small catchments are affected,
the distribution and arrangement of these affected subsystems, and how biogeochemical changes are
propagated in time and space among hydrologically linked areas.
One critical management goal is to develop and implement strategies that increase tree biomass
yield while maintaining the long-term sustainability of forested landscapes, which is defined here as
the ability of the soil ecosystem to support desired rates of biomass production over several rotation
periods without significantly affecting water chemistry and/or aquatic biodiversity. To better assess
and predict the impacts of more intensive forestry in a changing climate on soils and streams, a more
holistic ecosystem approach needs to be taken that includes perspectives from silviculture, ecology,
hydrology, and biogeochemistry. Here we review and discuss some of the most pertinent questions,
from a Swedish perspective, relevant to the future of both forest soils and waters. Additionally, we
suggest approaches that can be used to provide much needed answers to questions about the long-term
management of soil and surface water resources in the Swedish forested landscape. Our long-term
objective is to move towards an enhanced mechanistic understanding that will lead the way to
improved predictive models of ecosystem functioning under changing land use and climate scenarios.
The more immediate goal is to provide knowledge that can direct future research efforts and help
improve the decision support for forestry and land management communities in order to minimize
environmental impacts on soils and waters associated with future biomass production.
2. Impacts of Nutrient Additions
Critical to the development of plans that promote the sustainable use of forest soil and water
resources is an understanding of the controls over nutrient availability and retention within terrestrial
ecosystems. One nutrient of particular importance is nitrogen (N), which is necessary for plant growth,
and in many forests of boreal and temperate regions is the primary limiting factor for net primary
production [18,19]. Because most N in soils is bound in relatively recalcitrant soil organic matter not
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easily accessed by the trees [20], the decomposition of large macromolecules to peptides, amino acids
and inorganic N-forms, and the relationships between trees and their symbiotic mycorrhizal fungi are
particularly important to the supply of N to primary producers [21,22]. Given the role of N as the
primary limiting nutrient in boreal forests, one plan for intensified forestry in the future involves more
frequent application of N-based fertilizers to specified production areas [23]. This includes frequent
nutrient additions with a mineral balance based on the nutritional requirements of the forest stand [24].
This type of fertilization approach in Norway spruce stands of Sweden can increase stem wood
production by more than 200% [25]. It remains unclear, however, whether achieving this increase in
production is possible at broad spatial scales, and how such a management approach would affect other
ecosystem services that these landscapes provide.
Over the past century, the N economy of forests in many parts of the globe has changed
dramatically following increases in atmospheric N deposition linked to the combustion of fossil fuels
and intensive agriculture [26]. These increases in N inputs have had major consequences for the
ecological and biogeochemical dynamics of forest ecosystems. In general, for N-limited forests, small
to moderate N additions (e.g., between 3 and 20 kgN·
ha−1·
yr−1) stimulate photosynthesis per unit
foliage and increase the biomass of leaves and needles, as well as elevate above-ground biomass
production relative to that below-ground [27]. Thus, moderate levels of N addition can positively
stimulate tree growth, which could represent an important ecosystem C sink. Moreover, N addition
may further increase ecosystem C sequestration as these inputs in some cases can have a retarding
effect on the decomposition of soil organic matter [28,29].
Data from Swedish forests typically show that N fertilization can increase C sequestration [4]. This
relationship is non-linear, however, and at higher levels of fertilizer addition, forest ecosystems
become saturated as the availability of mineral N exceeds the demand of plants and microbes [30,31].
Under these circumstances, forests lose retentive capacity and begin to leach N from soils to
groundwater and streams (Figure 1), mainly in the form of nitrate (NO3−) as this is the most mobile
form [32-34]. From a forest owner perspective, added N that is not captured by trees or retained in soil,
and which is instead lost from the system, is a costly and unwanted effect of fertilization. Thus, finding
a management formula for forest fertilization that will optimize biomass production, while minimizing
N leaching, represents a major research priority and will be essential for plans to increase biomass
yield and carbon sequestration without seriously affecting water quality (Figure 1).
Because NO3− can act as a pollutant in surface waters [35], an increase in stream N would be an
undesirable consequence of more intensive forest fertilization. The potential loading of N to streams
following forest fertilization will likely depend on how fertilizer is added (i.e., the timing and
application rate), tree nutrient demand (phase of the growth cycle or rotation period), and soil
processes that act to retain N in the terrestrial ecosystem [23]. Most terrestrial ecosystems have a large
capacity to retain N, but the threshold at which forests begin to lose N, and the mechanisms by which
N can be retained within landscapes are not fully understood. If fertilizers are added at low to moderate
rates, at times of high plant demand, it is possible that long-term fertilization would have few
consequences for nutrient loading in Swedish streams and downstream in the Baltic Sea. Forestry
operations, specifically final felling, is presently a minor contributor to NO3− pollution in Swedish
surface waters [36,37]. However, escalating the use of fertilizers may cause greater N pollution if this
practice is not based on a solid, fundamental understanding of the factors that regulate nutrient
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retention in forest ecosystems. On the other hand, increased N retention in young, managed and
productive stands may help reduce N leakage, compared to old-growth forests at ―steady state‖ [38].
Figure 1. A conceptual model to describe how additions of N increase tree biomass
production (black line) and nitrogen leakage (red line). Increased rates of fertilization result
in greater biomass production until tree growth levels off as other factors become limiting.
Up to a point, this added N is retained within forest ecosystems, but eventually these
systems becomes saturated, as the availability of mineral N exceeds the demand of plants
and microbes, and begin to leak N from soils to groundwater and streams. Nitrogen
fertilization schemes should aim for the grey, shaded area, where a high forest production
is combined with low leakage of N.
Nitrate leakage with increased fertilization intensity could contribute to the eutrophication and
reduction of species diversity in surface waters [39]. Once in the stream, the fate of NO3− depends
upon the strength and nature of nutrient limitation, the types and rates of biological processes that
occur locally (e.g., assimilation vs. denitrification), and the balance between biological demand and
hydrologic transport. There is evidence that in-stream processing can reduce the downstream losses of
N from some landscapes [40], but understanding the role of streams and rivers in this regard remains
an active area of research. Nutrient enrichment in streams is often associated with a suite of other land
use changes (e.g., transitions to agriculture) that also alter habitat structure and the light regime, which
together influence local communities and ecosystem processes. The effects of nitrogen enrichment in
streams draining intact forests, however, are less clear. Evidence from a multi-year enrichment
experiment (of N and P) in a temperate forest stream suggests that nutrient loading can increase
invertebrate secondary production [41], alter detritus processing and the overall stream carbon
balance [42], and result in unexpected, long-term changes in macroinvertebrate community
composition [43]. It is not clear how similar enrichment would influence small, humic-rich boreal
streams. There is, however, some evidence for N limitation of dissolved organic carbon (DOC)
processing by microbes in streams and lakes in northern Sweden [44], but the broader potential
consequences of enrichment at community and ecosystem levels are largely unknown. Of particular
interest is whether and how changing inorganic nutrient dynamics in forests will alter the delivery,
form and processing of organic nutrients in adjacent and downstream aquatic ecosystems.
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Elevated levels of NO3− in streams and lakes also pose additional environmental concerns. For
example, such increases in concentration are known to promote denitrification and associated
production of N2O, a known greenhouse gas [45]. This risk is expected to be lower in forest, than
agricultural and urban streams, which commonly are more enriched with anthropogenic NO3− [40];
however, these microbial processes remain to be evaluated in boreal forest streams subject to elevated
N loading. Additionally, NO3− leaching may cause soil acidification. As NO3− ions percolate through
the soil into the stream water, accompanying base cations (i.e., Ca2+, Mg2+ and K+) or other cations
from forest soils move out of the soil into stream water as well. As more base cations are leached, the
capacity of forest soils to buffer changes in pH will be reduced. This could lead to more acidification
of susceptible soils and undesirable cation movements from the soil to stream water (i.e., H+, Al3+). An
increasing proportion of the acidity in surface waters in North America and Europe is related to NO 3−
export [46], which can be especially problematic in areas where large amounts of N fertilizer have
been added such as intensively managed forests or agricultural land [9,47]. Thus, increased N inputs
over long time-scales could actually reduce the fertility of forest soils [47]. Although not an issue in
Sweden so far, base cation depletion has been a major problem in parts of central Europe, where levels
of atmospheric N deposition have been up to 30–40 kgN·
ha−1·
yr−1 [48,49], which is several times
higher than anywhere in Sweden.
3. Impacts of Tree Harvesting
Tree harvesting has obvious and dramatic effects on the structure and function of terrestrial
ecosystems [9]. With the introduction of whole-tree and full-tree harvesting, greater attention has been
devoted to understanding the effects of felling operations on forest soils, including effects on
biodiversity [50], and the potential for long-term sustainability of tree production owing to increased
nutrient losses [51,52]. In addition to these effects on forest and soil properties, it is also long
recognized that forest harvesting can influence the ecology and biogeochemistry of adjacent headwater
streams [53,54]. Indeed, the general recognition of key linkages between terrestrial landscapes and
their associated streams has a long history in ecology [55,56], and a rich scientific literature exists
investigating the relationships between forest management practices, water chemistry, and stream
discharge, illustrating how closely linked small streams are to their surrounding forests [57,58]. From
this work, it is well established that clear-felling results in altered hydrology: decreased
evapotranspiration, increased groundwater tables, and therefore greater runoff [59,60].
Given the central role of the hydrologic regime in the ecology and biogeochemistry of catchments,
changes to the hydrologic cycle following forest harvesting are likely to have consequences for both
terrestrial and aquatic ecosystems. The effects of altered flow may include changes in the rates of
biogeochemical processes in both terrestrial and aquatic habitats (e.g., decomposition, nutrient cycling),
and the timing and magnitude of downstream hydrologic transport of dissolved and particulate
materials. For example, recent research has shown that clear-cutting can increase both the
concentration and total flux of DOC in streams of boreal regions [61,62]. Such an increase is
particularly important in boreal landscapes, not only because DOC represents a potentially important
flux in the regional C cycle [63], but also because DOC acts as an important transport-vector for
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contaminants such as mercury [64] and persistent organic pollutants [65], and because it represents a
major source of energy to foodwebs in downstream lakes [66].
In addition to these effects on DOC, forest harvesting also can result in losses of nutrients and other
elements from forest soils with corresponding increases in the concentration of these materials in
adjacent streams [54]. There is some evidence, however, that whole-tree harvesting may actually
reduce short term NO3− losses [67]. According to this hypothesis, removing tops, branches, and
needles substantially reduces logging residues and the corresponding detrital N pool, which could
potentially reduce NO3− leaching immediately following harvesting [68-70]. However, base cations are
also lost from sites during whole-tree harvesting [70]. Given the role of base cations for controlling pH
in surface waters [71], the long-term consequences of whole-tree harvesting needs further evaluation
before it is used as a means to reduce short-term N losses from catchments, particularly as an
intensification of forestry in the future could have substantial effects on weakly-buffered streams
in Sweden [72].
Forestry activities, including clear-felling and off-road transports, also have the potential to increase
toxic methyl mercury concentrations in surface waters [73,74]. Further research is needed on the link
between forestry and elevated levels of methyl mercury in surface waters. It is possible that the
elevated levels of methyl mercury are related to the wetter ground conditions often seen after final
harvest, or to soil disturbance associated with harvest and site preparation. More recent results have
also suggested a large degree of variation in the sensitivity of different catchments to harvest
operations with respect to mercury response [60]. Understanding the causes of the varying sensitivity
may hold the key to more effective measures to mitigate the effects of forestry operations on
mercury outputs.
Forest harvesting can also influence streams and rivers through alterations of key habitat features,
which may persist for variable lengths of time [53]. For example, removal of streamside vegetation can
temporarily increase incident light, which may elevate stream temperatures [75], and stimulate
in-stream primary production and ecosystem respiration [76]. These effects of increasing light on
benthic metabolism may be exacerbated when coincident with elevated nutrient concentrations [77].
Consistent with this, studies of clear-cutting in boreal streams show increases in benthic algal and
bacterial productivity that correspond to both elevated incident light, and increases in stream nutrient
availability [78,79]. Tree harvesting and near-stream management can also greatly impact sediment
delivery and inputs of woody debris, both of which can have long-term effects on channel structure [80],
and play a key role in the ecology and biogeochemistry of forest streams [81]. This effect of forestry
on channel form has received little attention in boreal regions; however, a study of streams in Finland
and Russia has demonstrated that forest management can affect the structure of stream channels,
including a reduction in both the abundance of coarse woody debris and fine detritus [82].
As suggested above, the effects of forestry on aquatic resources at multiple time scales could be
exaggerated by expected future increases in precipitation [83]. In Scandinavia, global climate models
predict there will be shorter winters and precipitation events will likely be more variable, both in
intensity and frequency [84]. Consequently, the length of the snow-covered season, the timing and
magnitude of spring runoff, and the number of extreme events could be altered in the future [85].
Shorter periods with frozen ground could have profound influences on the physical effects of
harvesting on forest soils. For example, forest machinery operating on unfrozen, wet ground will
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potentially have much larger and longer-lasting impacts on soil physical structure and surface water
quality, when compared to these same activities on frozen ground. This could be further exaggerated
when logging residues are harvested as part of a whole-tree or full-tree harvesting strategy instead of
the conventional use as bedding on the skid road to avoid soil damages [86]. Overall, understanding
the interactive effects of potential climate change and land management on aquatic resources
represents an important research goal, and progress along these lines will require close collaboration
among foresters and scientists.
4. Future Research Needs
To address the above problems and incorporate them into practical use, forest management has to
be based on sound scientific understanding, and an improved dialogue between foresters and multiple
scientific communities. Reaching this goal will require the integration of scientific approaches that
span traditional disciplinary boundaries. This integration should include compiling available data from
long-term descriptive and experimental studies, new innovative experimental manipulations, and both
analytical and simulation modeling to evaluate questions across a range of scales, using multiple
methods of inference. These research efforts need to be designed a priori to address questions relevant
to forest management, and should also include interaction and feedback from stakeholders and
managers according to an adaptive forest management approach. Such an approach may help to
provide answers to questions required for the transition into a new era of environmental stewardship
that focuses on maintaining, or even increasing, biomass production without jeopardizing soils
or waters.
Historically, a main limitation to answering questions that integrate large spatial and temporal
scales of forest, soils, and streams has been the availability of well organized, high-quality field data.
There is, however, a unique opportunity emerging in Sweden to use recently compiled field data from
hundreds of forest and soil experiments across the country, several established in the first half of the
20th century, and others more newly established within the Long-term Ecological Research (LTER)
platform (Figure 2). The integration of research efforts among these sites would serve to establish an
extensive network of geographically distributed experiments that also includes well-studied
ecosystems within the network. One example of such an LTER platform is the Svartberget LTER site
that constitutes the base for several large-scale, long-term research programs. Field research began in
this area in the early 1920s and includes ecological studies on the effects of different forest
management practices [27,58,87,88], climate change [89,90], and long-range transport of air pollutants
on soils and water quality [65,91]. This area has also been used to develop a complete carbon budget of
boreal mires and forests [92], and is home to long-term experiments on tree-soil interactions following
conventional fertilization and nutrient optimization [27,93], and whole-tree harvesting [11,94-96]. As a
result of these research activities, the Svartberget LTER infrastructure provides outstanding
opportunities to study how different silvicultural practices influence soils and water quality in
boreal forests.
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Figure 2. Location of long-term forest experiments in Sweden, as well as sites for
integrated monitoring (IM), experimental forests, and forest monitoring (FutMon). The
insert shows the Svartberget long-term ecological research (Svartberget LTER) site that
includes several unique long-term field research programs that combine process-based
research with long-term environmental assessment and modeling. The Svartberget LTER
includes the 6900 ha Krycklan Catchment Study which is one the most ambitious projects
integrating water quality, hydrology, and aquatic ecology in running waters of the boreal
region. The Balsjö Clear-Cut Catchment Experiment is a site where the effect of final
felling on water quality has been studied since 2004. Strömsjöliden Production Park is a
2900 ha research facility for inter-disciplinary experimental research on the environmental
and socio-economic consequences of increased biomass production on a landscape scale.
Norrliden was started in 1971 and is home to one of the most comprehensive tree nutrition
studies in the world, comprising more than 100 plots. Flakaliden is a large scale nutrient
optimization experiment commenced in 1987. Rosinedal is an experimental forest for
studying atmospheric fluxes of CO2, H2O and energy using three eddy-correlation towers.
Degerö Stormyr is currently one of the most intensively instrumented and studied mire
ecosystems for biogeochemical research, including the third longest carbon balance record
in the world for a mire ecosystem.
Future forest management that includes an increased use of fertilizers will need to be based on a
clear understanding of the underlying mechanisms of N cycling and retention in boreal forest
ecosystems. A key aspect of this cycle involves symbiotic relationships between plants and
mycorrhizal fungi, and determining the effects of N addition on this relationship remains an important
research frontier. One testable hypothesis in this context is that the mycorrhizal fungi play a key role in
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N retention in boreal forests and that these groups are negatively affected by increased N supply [97].
An important question is whether functional populations of these fungi can be maintained under
conditions of high N supply and if not, what happens to the capacity of forest ecosystems to retain
nutrients? To move from merely speculation, based on individual studies, to an understanding of the
long-term consequences of N fertilization there is a need to combine controlled, experimental
case-studies with larger meta-analyses that involve in-depth reviews of published data. Together, these
approaches can help deliver predictive models to forest managers that describe the effects of
atmospheric N deposition and forest fertilization on mycorrhizal populations, and provide insight into
how altering the activity of these soil fungi will influence broad-scale biogeochemical cycles.
To better understand how both conventional and future tree harvesting strategies will influence soil
and water quality, we need a better mechanistic understanding of the processes controlling the delivery
of materials from terrestrial to aquatic ecosystems, and the associated ecological and biogeochemical
consequences of these transfers. One important question in this context centers on the availability of
base cations in the soil. While there may not be an immediate threat to the long-term productivity in
Sweden [98], little is known about how intensified forestry, in combination with potentially longer
growing seasons, will affect soil base cations. Results from weathering models indicate that forest
growth will be negatively affected by deficiencies in the supply of base cations in the soil following
both conventional and whole-tree harvesting [99]. Furthermore these models predict that forest soils
will take decades or centuries to recover from such losses [100]. Conversely, large-scale experimental
manipulations suggest that decreases in the concentration of exchangeable base cations in soils do not
necessarily limit forest growth in Sweden [98]. Recent results also suggest that the rate of base cation
recovery following the termination of N addition is much more rapid than predicted by weathering
models [93]. In a recent study from the Svartberget LTER, Klaminder et al. [101] showed that seven
different weathering models produced contrasting results with a precision far from what is needed in
order to predict any forestry related effects on the base cation pool. This type of result demonstrates
the need for further research on this topic, the potential pitfalls associated with the over-reliance on
individual models, and the benefits of using model ensembles to produce more realistic results and
estimates of uncertainty [102].
Another research priority is to understand how proposed management practices will affect aquatic
ecosystems, and how strategies can be developed that may mitigate these effects. For example, how
might changes in water chemistry and habitat quality resulting from forest management be propagated
through aquatic foodwebs, and downstream through drainage networks that also include wetlands and
lakes? In this context, what is the role of landscape heterogeneity, and what are the ranges of spatial
and temporal scales at which these management effects are manifested in aquatic ecosystems? Finally,
to what degree are these land-water linkages buffered by riparian vegetation and organic soils? Indeed,
refining our understanding of the role of riparian zones seems particularly important [103]. There is
evidence that certain aspects of water quality may be maintained by riparian processes, which can
include the retention of nutrients and sediments from upslope terrestrial habitats [104]. Given the many
functions of the riparian zone, leaving buffer zones at harvesting has become a standard procedure
along streams and rivers in Sweden. More could be done, however, to improve the functionality of
these areas by better incorporating aspects related to riparian ecology, erosion control, biogeochemical
hotspots, and groundwater discharge into the management decision.
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Other management approaches that are promising from a water quality perspective involve
organizing landscapes into areas that are highly sensitive to both traditional and intensive forestry and
those locations that are less so. By applying this ‗landscape sensitivity‘ approach, greater care can be
devoted to protecting certain areas from (for example) off-road transport or intensive forest harvesting
practices. One example of this is that areas at the interface of mineral and organic soils are hotspots of
methyl mercury production. If these locations are also in close connection to streams, rutting by forest
machinery could cause not only increased methylisation, but also rapid hydrological connection to
adjacent surface waters. This general management approach will require more input from scientists
related to how intrinsic properties of forest ecosystems (e.g., landscape position, slope, underlying
geology, soil texture, etc.) influence this degree of sensitivity to various harvesting practices. Another
way forward will include the use of planning tools designed to help managers develop strategies for
maximizing forest yield in the long term, while maintaining particular water quality parameters below
specific thresholds [105]. Furthermore, optimized use of fertilizers, differential management of
sensitive forest stands, a better understanding of the functionality of riparian areas, and implementation
of improved planning tools could lead the way towards more productive and sustainable forestry in
the future.
5. Summary
Trees, soils and water are at the center for understanding how Swedish forests will be affected by
intensified biomass production and climate change. While increased production of forest biomass will
potentially help mitigate predicted climate changes, the negative effects on soils and/or the
deterioration of water quality that may arise could influence other ecosystem services such as future
forest productivity, biodiversity, and recreation. However, some of these potential negative impacts
can be reduced by prudent forest management. While we are beginning to better understand how
forestry affects forest soils and waters, the synergistic effects of climate change and land management
are almost entirely unknown. Reliance on inadequate knowledge of the relationships among climate
change, tree harvesting, and soil and water quality represents a profound risk of mismanaging two of
the most abundant, but also most precious natural resources in Sweden. Future research therefore
needs to provide applicable information about plant–soil and soil–water interactions that is valuable to
society and forest managers and that ensures intensified forestry is performed without jeopardizing
forest soils and water quality.
Acknowledgements
The research was funded through Future Forests, a multi-disciplinary research program supported
by the Foundation for Strategic Environmental Research (MISTRA), the Swedish Forestry Industry,
the Swedish University of Agricultural Sciences (SLU), UmeåUniversity, and the Forestry Research
Institute of Sweden.
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