User manual | RELATIONSHIPS BETWEEN RING -WIDTH VARIATION AND SOIL

TREE -RING RESEARCH, Vol. 57(1), 2001, pp. 105 -113
RELATIONSHIPS BETWEEN RING -WIDTH VARIATION AND SOIL
NUTRIENT AVAILABILITY AT THE TREE SCALE
PAUL R. SHEPPARD*
Departament d'Ecologia
Universitat de Barcelona
08028 Barcelona, España
PERE CASALS
Departament de Biologia Vegetal
Universitat de Barcelona
08028 Barcelona, España
and
EMILIA GUTIÉRREZ
Departament d'Ecologia
Universitat de Barcelona
08028 Barcelona, España
ABSTRACT
Within the framework of the linear aggregate model of dendrochronology, the potential role of soil
nutrient availability in explaining multi -decadal variation in radial growth at the tree level was studied in
the central Spanish Pyrenees. Increment cores were collected from 20 mature Pinus uncinata Ram. and
analyzed dendrochronologically. One ion -exchange resin capsule was buried within the root zone of each
sampled tree for just over eight months. The resins were chemically extracted and measured for NH4, NO
PO4, Ca, and K. Statistical relationships between indexed tree growth and soil nutrient availability were
determined with regression analysis and bivariate plots.
The single most important soil nutrient with respect to decadal -scale dendrochronological tree -growth
variables in this study was N in the form NO which explained 22% of variation of trend in growth since
1950. The 20 values of NO3 availability fell into two subgroups, one of trees with relatively higher NO3
availability and the other with lower NO3 availability. When the tree -growth data were grouped based on
NO3 availability, the two resultant index chronologies had different low- frequency features since 1950.
Trees with low NO3 availability have been growing as expected based on past growth, but trees with high
NO availability have been growing better than expected. Measuring and analyzing soil nutrient availability
at the tree level might enhance environmental applications of dendrochronological research. With soils
information at this spatial scale, it is possible to distinguish between subgroups of trees within a tree -ring
site and thereby construct subchronologies that differ significantly, especially for variation at the decadal
scale. Subsite- chronologies may then lead to different and presumably more informative environmental
interpretations relative to those based on a full -site chronology.
Keywords: dendrochronology, ion- exchange resins, soil nutrients, Spanish Pyrenees, Pinus uncinata.
INTRODUCTION
An underlying basis for environmental applications of dendrochronology is the linear aggregate
* Current address: Laboratory of Tree -Ring Research, University of Arizona, Tucson, AZ 85721, USA. office: (520) 6216474; fax: (520) 621 -8229; sheppard @Itrr.arizona.edu
Copyright © 2001 by the Tree -Ring Society
model (Cook 1987):
R = At + Cr + 8D1, + 8D2t + E
(1)
where t indicates time in calendar year and R is
an observed time series of ring widths of a tree
(or more broadly, any ring -growth variable), which
can be explained by some combination of variation
105
106
SHEPPARD, CASALS, and GUTIÉRREZ
related to age or size of the tree (A), climate (C),
endogenous or local disturbances (D1), and exogenous or stand -wide disturbances (D2). The error term (E) represents variation in R that cannot
otherwise be explained by the other terms. The 8
with each disturbance term is a binary indicator of
absence (8 = 0) or presence (8 = 1) of disturbance, while examples of endogenous disturbance
include gap creation (Bosch and Gutiérrez 1996)
and examples of exogenous disturbance include
insect epidemics (Swetnam et al. 1985).
A general strategy in dendrochronology is to
isolate the explainable variance of R, into just one
predictor term of the linear aggregate model by
reducing the effect of the other terms. For example, age- or size -related variation can be accounted
for by standardizing measured values with a tree -
specific growth curve of expected values empirically estimated from the measurement data (Fritts
1976). The effects of disturbance (D1, and D2,)
can be avoided by sampling trees with no outward
evidence of injury. The effects of climate (C,) can
be removed by quantitatively modeling out important climatic controls of tree growth.
One way of improving the environmental applications of the linear aggregate model is to add
explanatory terms that decrease the error term (E).
One such additional term could relate to the quality of soil, which provides moisture and nutrients
for tree growth and whose physical, chemical, and
biological properties vary at the tree spatial scale
or even less (Arnold and Wilding 1991). Indeed,
soil characteristics other than moisture availability,
which is accounted for by the climate term, were
specifically included as part of E, (Cook 1987).
Soil nutrient availability can vary dramatically in
soils across short distances (Beckett and Webster
1971; George et al. 1997) such that trees within a
typical dendrochronological site might be growing
in soils of different quality. Adding a soil nutrient
availability term to the linear aggregate model may
broaden the range of environmental applications of
dendrochronology as well as improve interpretations of radial growth patterns. The primary objective of this research was to assess the relationship between ring -width variation and soil nutrient
availability at the tree scale, with the potential goal
of adding a soil nutrient availability term to the
linear aggregate model.
METHODS
Study Site
The study site was located within the Aigüestortes and Sant Maurici Reservoir National Park
of Catalunya (42 °35'00 "N, 1 °0'00 "E, 2,000 m el-
evation) of the central Spanish Pyrenees (Figure
la). The site had a subsite with a 30° slope angle
and an adjacent flat subsite, which allowed for the
evaluation of topography and geomorphic position
on the relationship between ring -width trends and
soil nutrient availability. Because the study site
was small at only 0.2 -ha, trees within it have been
experiencing essentially the same climate through
time, thereby equalizing the effect of C, on R, for
all trees.
Weather records at the nearby town of Capdella
(42 °27'55 "N, 0 °59'28 "E, 1,270 m elevation, rec-
ords from 1945 to 1997, Figure la) show a mean
total annual precipitation of 1,261 mm evenly distributed across all months of the year and a mean
annual temperature of 9 °C with a range of 16 °C
between January and July average temperatures
(Figure 2). A field survey of the soil indicated that
it is generally shallow (<0.5 meters deep), dark
brown, and sandy loamy in texture with abundant
(-20 %) cobbles and weak granular structure; the
soil probably classifies as lithic Haplumbrept (Soil
Survey Staff 1990). The study site has a stem density of -200 trees/ha with Pinus uncinata Ram.
as the dominant overstory tree species and Vac -
cinium spp. and grasses as the understory and
ground cover species.
Field Sampling
Ten trees per subsite were sampled in October
1996 (Figure 1 b). To equalize the effects of A, on
Ri for all trees, mature, dominant trees of approximately the same age were selected. To avoid the
effects of D1, and D2, on R1, trees were selected
that did not have abrasion scars or other visible
evidence of injury. Two increment cores were col-
lected from each tree along opposing radii that
were parallel to the slope contour for trees of the
107
Ring -width Variation and Soil Nutrients
(a)
=4
410°
4°
42°
°
Aïguestortes and Sant
Maurici Reservoir
National Park
144°
i
42°
40°
38°
0
2
Madrid
i
200 km
36°
36°
10°
8
6°
4°
Barcelona
1 38 °\
SPAIN
0°
2°
4°
2°
\
100 km
Figure 1. (a) Federal and autonomy maps of Spain and Catalunya and (b) plan view of study site. Contour lines are relative to
the elevation of the flat subsite. Open circles denote trees with low NO3 availability while closed circles denote trees with high
NO3 availability.
20
150
J
FM A
M
J
J
A
S
O
N
D
Month
Figure 2. Climograph for Capdella (42 °27'55 "N, 0 °59'28 "E,
1270 m elevation, records from 1945 to 1997). Bars indicate
mean total monthly precipitation (mm) and solid line indicates
mean monthly temperature ( °C). MTAP is the mean total annual precipitation and MAT is the mean annual temperature.
sloped subsite and randomly oriented for trees of
the flat subsite. The location and topographic microsite conditions of each tree were recorded.
Measuring the total amount of a nutrient present
in soil would have quantified the potential nutrient
pool, but that may not relate reliably with what
actually becomes available in mineralized forms
(Binkley and Hart 1989). We measured potential
soil nutrient availability using ion -exchange resins
(IER), which approximate soil -root interactions
with respect to nutrient availability (Gibson 1986;
Skogley and Dobermann 1996). One IER capsule
( UNIBEST PST -1 capsules, Bozeman, Montana)
was buried within the root zone (1 -2 meters from
the trunk) for each sampled tree. Having more cap-
108
SHEPPARD, CASALS, and GUTIÉRREZ
suies per tree would have been preferable because
soil nutrient availability can vary at small spatial
scales (Beckett and Webster 1971). The capsules
were buried to a uniform depth of 10 -15 cm with
as little disturbance to the soil column as possible
(Carlyle and Malcolm 1986). The soil particles removed while digging with soil corers were placed
back into the hole over the capsule, and complete
contact between the capsules and the surrounding
mineral soil was attained (Gibson 1986; Skogley
et al. 1996). The IER capsules were retrieved in
May, 1997, after having resided in the soil for 247
days spanning autumn, winter, and the first half of
spring. The capsules were lightly rinsed with deionized water in the field (Giblin et al. 1994) and
stored individually in marked plastic bags (Skogley et al. 1997).
Laboratory and Quantitative Analysis
The tree cores were prepared and crossdated ac-
cording to standard dendrochronological procedures (Douglass 1941; Swetnam et al. 1985).
Width of all dated rings was measured to ±0.01
mm and checked for dating and measurement errors using cross -correlation testing (Holmes 1983).
Measured values were then averaged within each
tree for all years held in common by both cores
of each tree. To remove the effect of A, on RC for
all trees, measured values were converted to dimensionless indices by dividing them by curve fit
values. For this step, the cubic- smoothing spline
was selected whose flexibility retained 75% of the
variation at the 100 -year period in the resultant
index series (Cook and Peters 1981). This strategy
allowed for analysis of trends up to 50 years in
length in tree growth. All resultant index series
were averaged together into a standard chronology
(Fritts 1976).
Correlation functions between the standard
chronology and monthly precipitation and temperature variables were inspected to identify the important climatic controls of tree growth (Biasing
et al. 1984). A dendroclimatological year was tested, extending from September of the prior year to
September of the current year of growth. Regression analysis of the standard chronology and the
strongest climate variables was used to model cli-
mate with tree growth. Model residuals were
checked for the necessary assumptions of time -se-
ries regression analysis (Ostrom 1990). Once a
model was identified, it was re- evaluated for the
index series of each tree to remove the effects of
C, on Rr for all trees. This resulted in a time series
of residual tree growth for each tree.
Ions absorbed by the IER were extracted in
three steps using 20 ml of 2 M HC1 agitated for a
total of one hour (Dobermann et al. 1997; Skogley
et al. 1997). This resulted in a 60 -ml solution for
each tree. Solutions were then measured for NH4,
NO3, and PO4 using colorimetry (Clesceri et al.
1989), Ca using atomic absorption spectrometry
(Wright and Stuczynski 1996), and K using flame
emission spectrometry (Wright and Stuczynski
1996).
Relationships between temporal trends of residual tree growth and soil nutrient availability were
quantified using bivariate plots and regression
analysis. Because we were interested primarily in
analyzing relative tree growth of the last few decades, during which global deposition of N has
been increasing (Mayewski et al. 1986), trends in
growth indices since 1950 were tested as the de-
pendent tree- growth variable. Model residuals
were inspected for the necessary assumptions of
regression analysis (Sokal and Rohlf 1981).
RESULTS
The length of individual tree index series averaged 159 years and ranged from 128 to 184 years,
long enough for all trees to express departures of
50 years in length (Cook et al. 1995). The index
chronology did not show a significant trend since
1950 (Figure 3a).
Precipitation of fall and winter prior to the
growing season tended to correlate positively with
the standard chronology (Figure 4a). The strongest
multi -month season of precipitation was September of the prior year through January of the current
year of growth, with a correlation of +0.42.
Spring temperatures tended to correlate positively
with tree growth (Figure 4b). The strongest multi -
month season of temperature was April through
May of the current year, with a correlation of
+0.41. Neither of the seasonal climate variables
109
Ring -width Variation and Soil Nutrients
3
(a) Chronology from all trees
slope since 1950 = +0.006
(a) Precipitation
o.s
0.4
0.2
0
-0.2
-3
I
1950
2 3
pS pO pN
I
1970
1960
1980
pD
J
F
M
A
M
J
J
A
S
p
SepJan
1990
(b) Sep -Jan precipitation
r(b) Temperature
0.4
slope since 1950 = +0.009
f
0.2
ca
cc
0
0
-0.2
o
N
-0.4
-3
0.6
1950
1960
1970
1980
1990
pS
p0 pN pD
J
F
-3
1960
1970
M
J
J
A
S
1980
AprMay
Figure 4. Correlation functions for the index chronology using
all trees and (a) monthly total precipitation and (b) monthly
average temperature, recorded at Capdella. Dashed lines indi-
slope since 1950 = +0.006
0
1950
A
Month
(c) Apr -May temperature
3
M
1990
Year
Figure 3. (a) Index chronology derived from all trees of the
study site, (b) mean total precipitation at Capdella for the season from September to January, (c) mean temperature at Cap della for the season from April to May. All series were truncated to show the period 1950 to 1996. All original values have
been converted to z- scores to facilitate comparison between
series. Flat solid lines are the means while dashed lines are the
linear trends since 1950.
showed a significant trend since 1950 (Figures 3b
and 3c). The best dendroclimatic model used both
of the seasonal climate variables to explain 22%
of variation in the index chronology since 1950.
The model was significant (p < 0.01) and had residuals that were normally distributed, that showed
no relationship with predicted or predictor values,
and that were not significantly autocorrelated. The
model was re- evaluated for each tree to provide a
time series of residual tree -growth for each tree.
The best one -variable model of soil nutrients
and trends in residual tree growth used NO3 as an
independent variable to explain 22% of variation
of trends in growth since 1950 (Figure 5). This
model was significant (p < 0.05) and had normally
distributed residuals that did not relate with predictor or predicted values. No other single soil nutrient variable correlated significantly with the
tree -growth variables.
The 20 measured soil NO3 values happened to
cate significant r values (n = 53, a = 0.05, Rohlf and Sokal
1981). The "p" in front of months indicates a prior -year
month, otherwise months are for the current year of tree
growth. The pSep -Jan indicates the season of September
through January and the Apr -May indicates the season of April
through May.
fall into two clear groups, one of trees with more
than 10 µg /day /IER unit and the other of trees
with less than 5 µg /day/IER unit. The groups did
not correspond with the original flat or sloped sub sites, which had average soil NO3 values that did
not differ significantly from one another. Instead,
the majority of trees with high NO3 (seven of nine)
were in the transition zone between the two sub sites, either in the lower half of the sloped subsite
or in the part of the flat subsite that is adjacent to
the margin of the two subsites (Figure 1 b). Con.o
0.04
=-0.005+0.001 x
cr,
R2 = 0.22, p < 0.05
0.02
3
0.00
m
.g
F- -0.02
o
10
20
30
NO3 availability (pg /day /IER unit)
Figure 5. Regression results using NO3 availability to explain
trend in residual growth since 1950. Dashed lines are the 95%
confidence limits of the predicted values.
SHEPPARD, CASALS, and GUTIÉRREZ
110
3
(a) High nitrate
slope since 1950 = +0.029
Ai
m
r
-i-
-3
1960
1950
e
-o
(b) Low nitrate
It is impossible to know for certain from this
study if the spatial pattern of soil N availability
that exists now has existed for the last few decades. It would be best to answer this question by
studying a forest site for which soil nutrient avail-
1980
1970
1990
slope since 1950 = -0.018
ability for individual trees has been measured
more than once during the last few decades. Future
research using dendrochemical analysis to deter-
mine N concentrations in crossdated tree rings
Ñ
might also help in this regard (Sheppard and
-3
1950
1960
1970
1980
1990
Year
Figure 6. Residual chronologies for (a) trees with high NO3
availability ( >10 µg /day /IER unit), (b) trees with low NO,
availability ( <5 µg /day/IER unit). All original values have
been converted to z- scores to facilitate comparison between
series. Flat solid lines are the means while dashed lines are the
linear trends since 1950.
versely, the majority of trees with low NO3 (eight
of eleven) were either on the summit of the sloped
subsite or in the toeslope of the flat subsite.
After subdividing the residual index series of all
trees into two subsets based on NO3 availability,
the resultant subsite chronologies showed different
low -frequency features (Figure 6). The chronology
composed of trees with high NO3 availability had
a significantly positive slope since 1950 (p <
0.05). By contrast, the chronology of trees with
low NO3 availability had a slope since 1950 that
was actually negative though not significantly different from zero.
DISCUSSION
After removing or avoiding the effects of age or
size, climate, and disturbance on tree growth, there
was still variation in decadal -scale trends across
Thompson 2000). For now, we assume that there
is spatial persistence in soil properties regulating
nutrient availability such that current spatial variability in relative nutrient availability reflects that
of the recent past. We do not assume that current
absolute values of soil nutrient availability are the
same as those of the past, only that the spatial
patterns of relative availability have not changed
through time.
The two groups of trees with high- versus low NO3 availability transcended the original flat versus steep subsites, and instead they followed a pattern more related to geomorphic hillslope position.
Most of the high -NO3 trees were in or near the
backslope -footslope (Hall and Olson 1991) transition of the site. The hillslope of our site had a
convex contour and a weakly convex slope, which
may cause surface and subsurface water and nutrients to accumulate in the backslope -footslope
transition zone (Hall and Olson 1991). Trees of
this geomorphic position may tend to have more
available soil nutrients and therefore grow slightly
better than other trees (Hammer et al. 1991). However, this geomorphic association was not perfect
in this study: trees #6, #8 and #15 were growing
in the backslope -footslope zone but had low NO3
availability while trees #2 and #4 were in the toe slope zone but had high NO3 availability (Figure
lb). Thus, quantifying the topographic microsite
and geomorphic position of each tree may not suffice as a substitute for actually measuring soil nutrient availability for each sampled tree within a
trees in this study. Soil nutrient availability-most
notably N in the form of NO,-was useful in explaining some of that remaining variation between
trees. Trees with currently relatively high NO3 dendrochronological study.
The interpretability of the separate subsite -chroavailability have been growing better than expected since 1950 based on their own past growth pat- nologies was more informative than that of the
terns. In contrast, trees with less NO3 availability full -site chronology. The different decadal -scale
have been growing about as expected since 1950 patterns of the two subsite -chronologies in this
study were statistically related to current N availbased on their own past growth patterns.
111
Ring -width Variation and Soil Nutrients
1960s (personal communication with national park
across dendrochronological sites depending on the
spatial variability of soil nutrients within sites. The
SN term should play its strongest role in sites with
thin, poorly developed soils on complex geomorphic terrain, where soil nutrient availability is likely to vary at the individual tree spatial scale (Beas-
authorities), and this type of disturbance can
ley 1972). Conversely, the SN term should play
change below -ground competition for N (Pritchett
a weaker role in sites with well- developed soil on
uniform terrain, where soil nutrient availability is
likely to be relatively uniform across the individual tree spatial scale. In either case, accounting for
the SN term in the linear aggregate model of tree
growth could improve the interpretation of the other terms of the model, or it could be useful in its
own right in environmental studies of the effects
of spatial variation and temporal alterations in nutrient availability on tree growth.
ability at the tree level, and additional research
should focus on the mechanisms that could cause
this relationship. At least three possibilities exist.
First, light selection harvesting of trees took place
around the study site in the late 1950s and late
and Fisher 1987), perhaps more so for some remaining trees than for others. Second, grazing has
been allowed in the national park, and this activity
can redistribute nutrients in tree- specific ways
(Beckett and Webster 1971). Third, recent N deposition of the central Spanish Pyrenees has been
enhanced anthropogenically, though not to the level of central Europe (Camarero and Catalan 1993).
Nitrogen deposition is a low- concentration nutrient
input that might significantly affect tree growth
over decades, especially in cool, coniferous for-
CONCLUSIONS
ests, where N is often limiting to tree growth
(Pritchett and Fisher 1987).
If chronic atmospheric deposition is adding N
to the study site, then it is reasonable that soil and
slope position play a role at the tree level in mediating the availability of N from chronic pollution
to trees (Lammers and Johnson 1991) because
trees take in much of their N as ionic forms
through their roots (Kramer and Kozlowski 1979).
Trees growing in zones of natural accumulation of
soil moisture and nutrients might be fertilized
more effectively with N from atmospheric deposition than trees growing in zones of natural surface /subsurface runoff of water and nutrients.
The ability to distinguish between trees within
a site on the basis of environmental variables that
are independent of their growth data -NO3 avail-
ability in this case-is the essence of adding an
additional explanatory term to the linear aggregate
model of dendrochronology and decreasing the er-
ror term. Because the measurements of nutrient
availability in this study were for only a single
point in time, the time subscript is not applicable,
as follows:
Rr= Ar +Ct +8D1t +6D2t +E SN +E,
(2)
where / SN is the sum of effects of all n soil
nutrients that significantly affect tree growth.
The SN term undoubtedly plays a variable role
Measuring and analyzing soil nutrient availability at the tree level can enhance environmental ap-
plications of dendrochronological research. Although hillslope position can explain much of the
soil NO3 variability at the tree scale, hillslope position may not be a perfect surrogate for actual soil
availability measurements. Ion- exchange resins
appear to be a reasonable approach for measuring
soil nutrient availabilities in dendrochronological
studies.
With soils information at the tree spatial scale,
it is possible to distinguish between subgroups of
trees within a tree -ring site and thereby construct
subchronologies that differ significantly, especially
for variation at the decadal scale. Subsite- chronologies may then lead to different and presumably
more informative environmental interpretations
relative to those based on a full -site chronology.
Further testing of relationships between tree
growth and soil nutrient availability, across different forest types and levels of soil development, is
merited to determine the general utility of this concept in dendrochronological research. Tree growth
relationships with soil nutrients should be stronger
at sites with high topographic relief (and therefore
high soil variability) and weaker at sites with little
topographic relief. Furthermore, dendrochronolog ical studies of the role of increasing N deposition
SHEPPARD, CASALS, and GUTIÉRREZ
112
on tree growth might best be focused on sites with
sluggish soil nutrient cycling where N deposition
may be relatively important compared to natural
soil nutrient availability.
ACKNOWLEDGMENTS
We thank managers of the Aigüestortes and Sant
Maurici Reservoir National Park of Catalunya,
Dan Binkley, Jesus Julio Camarero, Richard
Holmes, Joan Romanyà, Neal Scott, Earl Skogley,
and Thomas Thompson for assisting this project.
This study was funded by a NATO Postdoctoral
Fellowship from the US National Science Foundation and by the Spanish Ministry of Education
and Culture (CICyT Project AMB95- 0160).
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Received 5/9/00; accepted 3/19/01.

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