1
RELATIONSHIPS BETWEEN RING-WIDTH TRENDS
2
AND
3
SOIL NUTRIENT AVAILABILITY
4
AT THE TREE SCALE
5
6
PAUL R. SHEPPARD1,3
7
PERE CASALS2
8
EMILIA GUTIÉRREZ1
9
1
10
11
2
Departament d'Ecologia
Departament de Biologia Vegetal
12
Universitat de Barcelona
13
08028 Barcelona España
14
15
3
Current address:
16
Laboratory of Tree-Ring Research
17
The University of Arizona
18
Tucson, AZ 85721 USA
19
office: (520) 621-6474
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fax: (520) 621-8229
21
sheppard@ltrr.arizona.edu
22
Sheppard, Casals, Gutiérrez: Soil nutrients and tree growth.
2
1
ABSTRACT
2
The objective of this research was to evaluate the potential role of soil nutrient availability in
3
explaining dendrochronological variation at the tree level. From a 0.2-ha study site in the central
4
Spanish Pyrenees, two increment cores were collected from 20 mature Pinus uncinata Ram. and
5
analyzed dendrochronologically. One ion-exchange resin capsule was buried within the root
6
zone of each sampled tree for just over eight months. The resins were chemically extracted and
7
measured for ammonium, nitrate, phosphate, calcium, and potassium. Statistical relationships
8
between indexed tree growth and soil nutrient availability were determined with regression anal-
9
ysis and bivariate plots.
10
The single most important soil nutrient with respect to decadal-scale dendrochronological
11
tree-growth variables in this study was nitrogen, as nitrate availability explained 32% of varia-
12
tion of average growth since 1970 and 22% of variation of trend in growth since 1950. The 20
13
values of nitrate availability fell into two subgroups, one of trees with high nitrate availability
14
and the other of trees with low nitrate availability. When the tree-growth data were grouped
15
based on nitrate availability, the two resultant index chronologies had different low-frequency
16
features since 1950, that is, trees with low nitrate availability have been growing normally but
17
trees with high nitrate availability have been growing better than expected. Measuring and ana-
18
lyzing soil nutrient availability at the tree level can substantially enhance environmental applica-
19
tions of dendrochronological research.
20
21
Keywords: dendrochronology, soil nutrient availability, ion-exchange resins, environmental sci-
22
ence
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Sheppard, Casals, Gutiérrez: Soil nutrients and tree growth.
1
INTRODUCTION
2
An underlying basis for paleoenvironmental applications of dendrochronology is the linear
3
3
aggregate model (Cook 1987):
[1]
Rt = At + Ct + D1t + D2t + Et
4
where t indicates time in calendar year and R is the observed time series of ring width (or more
5
broadly, any ring-growth variable), which can be explained by some combination of variation
6
related to age or size of the tree (A), climate (C), endogenous or local disturbances (D1, such as
7
gap-phase dynamics, Spurr and Barnes 1980), exogenous or stand-wide disturbances (D2, such
8
as insect epidemics, Fritts and Swetnam 1989), and an error term (E) for variation in R that can-
9
not otherwise be explained with available observations or data.
10
A general strategy in dendrochronology is to optimize the explained variance in Rt with the
11
predictor terms of the linear aggregate model and thereby maximize the paleoenvironmental in-
12
terpretability of ring-growth data. For example, age- or size-related variation is accounted for by
13
standardizing measured values with a tree-specific growth curve of expected values empirically
14
estimated from the measurement data (Fritts 1976). The effects of disturbance (D1t and D2t) are
15
reduced in studies of past climate by sampling only trees with no outward evidence of disturb-
16
ance. The effects of climate (Ct) are removed in studies of past forest disturbance to isolate ring-
17
growth responses to the disturbance under study.
18
One way of improving the linear aggregate model is to add additional explanatory terms and
19
thereby decrease the error term (Et). One such additional term could relate to the quality of soil,
20
which provides moisture and nutrients for tree growth and which varies at the tree scale (Arnold
21
and Wilding 1991). Indeed, soil characteristics other than moisture, which is accounted for by
22
the climate term, were specifically included as part of Et (Cook 1987, p. 43). Soil nutrient avail-
23
ability can vary dramatically in soils across short distances (Beckett and Webster 1971; George
24
et al. 1997) so that trees within even a stand might be growing in soils of different quality. Add-
25
ing soil nutrient availability to the linear aggregate model could significantly increase its predic-
26
tive power and consequently improve the interpretability of a dendrochronology. Accordingly,
Sheppard, Casals, Gutiérrez: Soil nutrients and tree growth.
4
1
the primary objective of this research was to evaluate the potential of soil nutrient availability in
2
explaining dendrochronological variation at the tree level.
3
4
METHODS
Study Site
5
The study site was located within the Aigüestortas and Sant Maurici Reservoir National Park
6
of Catalunya (42° 30" N, 1° 30" E, 2000 m elevation) of the central Spanish Pyrenees. The 0.2-
7
ha study site had a moderately sloped (up to 30° slope angle) subsite and an adjacent flat subsite
8
(Figure 1), which allowed for evaluating the influence of topography and geomorphic position on
9
the relationship between ring-width trends and soil nutrient availability.
10
The forest type was moderately open (~200 trees/ha) upper montane to subalpine conifer with
11
Pinus uncinata Ram. as the dominant overstory tree species and Vaccinium spp. and grasses as
12
the understory and ground cover species. A cursory field survey of the soils indicated that they
13
are generally shallow (<0.5 meters deep), dark brown, and sandy loamy in texture with abundant
14
(~20%) cobbles and weak granular structure. Weather records at the nearby town of Esterri indi-
15
cate that climate of this site is mesic (mean annual precipitation of 614 mm, evenly distributed
16
across all months of the year) and cool (mean annual temperature of 5° C).
17
Field Sampling
18
Ten mature trees per subsite were subjectively selected for field sampling in October 1996
19
(Figure 1). Two increment cores were collected along opposing radii of each tree. The opposing
20
radii were parallel to the slope contour for trees of the steep subsite. The location and topograph-
21
ical microsite conditions of each tree were also recorded.
22
Measuring the total amount of a nutrient present in soil would quantify the potential nutrient
23
pool, but that may not relate reliably to what actually becomes available in mineralized forms
24
(Binkley and Hart 1989). We opted instead to measure actual soil nutrient availability using ion-
25
exchange resins (IER), which closely approximate soil-root interactions with respect to nutrient
26
availability (Gibson 1986; Skogley and Dobermann 1996). One IER capsule (UNIBEST PST-1
27
capsules, Bozeman, Montana) was buried within the root zone (1-2 meters from the trunk) for
Sheppard, Casals, Gutiérrez: Soil nutrients and tree growth.
5
1
each sampled tree. Having more capsules per tree would have been preferable because soil nu-
2
trient availability can vary at small spatial scales (Beckett and Webster 1971), but we were lim-
3
ited financially to one capsule per tree. The capsules were buried to a uniform depth of 10-15
4
cm with as little disturbance to the soil column as possible (Carlyle and Malcolm 1986). The soil
5
particles removed while digging with a hand trowel were placed back into the hole over the cap-
6
sule, and complete contact between the capsules and the surrounding mineral soil was attained
7
(Gibson 1986; Skogley et al. 1996). The IER capsules were retrieved in May 1997, after having
8
resided in the soil for 247 days spanning autumn, winter, and the first half of spring. The cap-
9
sules were lightly rinsed with de-ionized water in the field (Giblin et al. 1994) and stored indi-
10
vidually in marked plastic bags (Skogley et al. 1997).
11
Laboratory and Quantitative Analysis
12
The tree increment cores were prepared and crossdated according to standard dendrochrono-
13
logical protocols (Douglass 1941; Phipps 1985). Width of all dated rings was measured to ±0.01
14
mm and checked for dating and measurement errors using cross-correlation testing (Holmes
15
1983). Measured values were then averaged within each tree for all years held in common by
16
both cores of each tree. Measured values were converted to dimensionless indices (with a mean
17
of 1.0) by dividing them by curve fit values to remove the age- and/or size-related growth trends
18
from these time series. For this step, the cubic-smoothing spline was selected whose flexibility
19
left 75% of the variation at the 100-year period in the resultant index series (Cook and Peters
20
1981). This strategy allowed for analysis of trends in tree growth up to 50 years in length.
21
Ions absorbed by the IER capsules were extracted in three steps using 20 ml of 2 M HCl agi-
22
tated for a total of one hour (Dobermann et al. 1997; Skogley et al. 1997). This resulted in a 60-
23
ml solution for each tree. Solutions were then measured for ammonium, nitrate, and phosphate
24
using colorimetry (Clesceri et al. 1989), calcium using atomic absorption spectrometry (Wright
25
and Stuczynski 1996), and potassium using flame emission spectrometry (Wright and Stuczynski
26
1996).
27
Relationships between averages or trends of indexed tree growth and soil nutrient availability
Sheppard, Casals, Gutiérrez: Soil nutrients and tree growth.
6
1
were quantified using regression tests as well as bivariate plots and time-series. Because we
2
were interested primarily in analyzing relative tree growth of the last few decades, we tested av-
3
erage growth index since 1970 and trend in growth indices since 1950 as dependent tree-growth
4
variables. These two variables were related (r = +0.88), though not identical.
5
RESULTS
6
The best one-variable models used nitrate as an independent variable to explain 32% of varia-
7
tion of average growth since 1970 and 22% of variation of trend in growth since 1950 (Figure 2).
8
Both models were significant and had normally distributed residuals that had no pattern related
9
to predictor or predicted values. No other single soil nutrient correlated significantly with the
10
11
tree-growth variables.
The 20 measured nitrate values fell into two clear subgroups, one of trees with more than 10
12
µg/day/capsule and the other of trees with less than 5 µg/day/capsule (Figure 2). The trees of
13
each nitrate subgroup fell almost equally across both subsites, but the majority of trees with high
14
nitrate (seven of nine) were growing either in the lower half of the steep subsite or in the part of
15
the flat subsite that is adjacent to the margin of the two subsites (Figure 1).
16
After subdividing the dendrochronological data into two subsets based on nitrate availability,
17
the two resultant index chronologies (average of all trees within each site or subsite, Fritts 1976)
18
since 1950 correlated strongly with one another (r = +0.78) but had different low-frequency fea-
19
tures (Figure 3). The chronology composed of trees with high nitrate availability had an average
20
index value since 1970 that was significantly above the mean of 1.0 as well as a significantly
21
positive slope since 1950 (Figure 3a). By contrast, the chronology of trees with low nitrate
22
availability had an average index value since 1970 that was not significantly different from 1.0 as
23
well as a slope since 1950 that was not significantly different from zero (Figure 3b). The full-
24
site chronology using all 20 trees also had an average index value since 1950 that was not signif-
25
icantly different from 1.0 and a slope since 1950 that was not significantly different from zero
26
(Figure 3c).
Sheppard, Casals, Gutiérrez: Soil nutrients and tree growth.
1
7
DISCUSSION
2
The single most important soil nutrient with respect to decadal-scale dendrochronological
3
tree-growth variables in this study was nitrogen. Regardless of the origin of the current spatial
4
variation in nitrate availability, trees with currently relatively high nitrate availability have been
5
growing better since 1950 than trees with less nitrate availability. This conforms to extensive
6
forest soils research that has shown nitrogen to be the macronutrient most limiting to tree growth
7
(Binkley and Hart 1989). In particular, nitrogen is often deficient in coniferous forests with cool
8
climates (Pritchett and Fisher 1987), such as our study site.
9
It is impossible to know for certain from this study if the spatial pattern of soil nitrogen avail-
10
ability that exists now has existed for the last few decades. Perhaps future research using den-
11
drochemical analysis to determine nitrogen concentrations in crossdated tree rings could help in
12
this regard (Sheppard and Thompson, in review). For now, it is necessary to depend on the geo-
13
logical Principal of Uniformitarianism, which asserts that environmental processes (e.g., spatial
14
variation in relative soil nutrient availability) operate the same now as they did in the past (Bates
15
and Jackson 1984). This does not imply that current absolute quantities of nutrient availability
16
were the same as those decades ago. Rather, it implies that soil factors regulating nutrient avail-
17
ability across space are operating now as they have in the past such that current spatial variability
18
in relative nutrient availability reflects that of the recent past.
19
The two groups of trees with high- versus low-nitrate availability transcended the original
20
subgroups of flat versus steep subsites, and instead they followed a pattern more related to geo-
21
morphic hillslope position. Most of the high-nitrate trees were in or near the backslope-footslope
22
(Hall and Olson 1991) transition of the site. The hillslope of our site had a convex contour and a
23
weakly convex slope, which may cause surface and subsurface water and nutrients to flow
24
through and/or accumulate in the backslope-footslope transition zone (Hall and Olson 1991).
25
Thus, trees of this geomorphic position may tend to have more available soil nutrients and there-
26
fore grow slightly better than other trees (Hammer et al. 1991). However, this geomorphic asso-
27
ciation was not perfect in this study: trees #6, #8 and #15 were growing in the backslope-
Sheppard, Casals, Gutiérrez: Soil nutrients and tree growth.
8
1
footslope zone but had low nitrate availability while trees #2 and #4 were in the toeslope zone
2
but had high nitrate availability (Figure 1). Thus, quantifying the topographic microsite and ge-
3
omorphic position of each tree may not suffice as a substitute for actually measuring soil nutrient
4
availability for each sampled tree within a dendrochronological study.
5
The interpretability of the separate sub-chronologies was enhanced relative to that of the full-
6
site chronology. The different decadal-scale patterns of the two sub-chronologies in this study
7
were obviously related to current nitrogen availability at the tree level, and additional research
8
should focus on the mechanisms that may be causing this statistical relationship. At least three
9
possibilities exist. One, light selection harvesting of trees took place around the study site in the
10
late 1950s and late 1960s, and this disturbance may have changed below-ground competition for
11
nitrogen (Pritchett and Fisher 1987), perhaps more so for some remaining trees than for others.
12
Two, grazing has been allowed in the National Park, and perhaps this has redistributed nutrients
13
in tree-specific ways (Beckett and Webster 1971). Three, recent N deposition of the central
14
Spanish Pyrenees has been enhanced anthropogenically slightly, though not to the level of cen-
15
tral Europe (Camarero and Catalan 1993). Nitrogen deposition is a low-grade nutrient input that
16
can significantly affect tree growth throughout decades of time (Pritchett and Fisher 1987).
17
Since trees take in much of their nitrogen as ionic forms through their roots (Kramer and Ko-
18
zlowski 1979), it is reasonable that soil plays a central role at the tree level in mediating the
19
availability of nitrogen from chronic pollution (Lammers and Johnson 1991).
20
The ability to distinguish among trees within a site on the basis of environmental variables
21
that are independent of the trees–nitrate availability in this case–is the essence of adding an addi-
22
tional explanatory term to the linear aggregate model of dendrochronology. Because the meas-
23
urements of nutrient availability in this study were for only a single point in time, the additional
24
explanatory term must have a different subscript than that of other terms, as follows:
[2]
25
Rt = At + Ct + D1t + D2t + SNn + Et
where SNn is the sum of effects of all n soil nutrients that significantly affect tree growth.
Sheppard, Casals, Gutiérrez: Soil nutrients and tree growth.
1
2
9
CONCLUSION
Measuring and analyzing soil nutrient availability at the tree level can enhance environmental
3
applications of dendrochronological research. With soils information at this spatial scale, it is
4
possible to distinguish between subgroups of trees within a tree-ring site and thereby construct
5
subchronologies that differ significantly, especially for variation at the decadal scale. Subchro-
6
nologies may then lead to different–and presumably improved–environmental interpretations rel-
7
ative to those based on a full-site chronology. Further testing of relationships between tree
8
growth and soil nutrient availability, across different forest types and levels of soil development,
9
is merited to determine the general utility of this concept in dendrochronological research.
10
ACKNOWLEDGEMENTS
11
We thank managers of the Aigüestortas and Sant Maurici Reservoir National Park of Catalu-
12
nya, Dan Binkley, Jesus Julio Camarero, Richard Holmes, Joan Romanyà, Neal Scott, Earl
13
Skogley, and Thomas Thompson for assisting this project. This study was funded by a NATO
14
Postdoctoral Fellowship from the U.S. National Science Foundation and by the Spanish Ministry
15
of Education and Culture (CICyT Project AMB95-0160).
16
REFERENCES CITED
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Dobermann, A., Pampolino, M.F., and Adviento, M.A.A. 1997. Resin capsules for on-site as-
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Fritts, H.C. 1976. Tree Rings and Climate. Academic Press, London.
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Fritts, H.C., and Swetnam, T.W. 1989. Dendroecology: a tool for evaluating variations in past
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and present forest environments. Adv. Ecol. Res. 19: 111-188.
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Pseudotsuga roots to heterogeneous nutrient distribution in soil. Tree Physiol. 17: 39-45.
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Giblin, A.E., Laundre, J.A., Nadelhoffer, K.J., and Shaver, G.R. 1994. Measuring nutrient avail-
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Sheppard, Casals, Gutiérrez: Soil nutrients and tree growth.
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1
FIGURE CAPTIONS
2
Figure 1. (a) Plan view and (b) longitudinal profile of study site. Contour lines of (a) and verti-
3
cal reference scale of (b) are relative to the elevation of the flat subsite. Lines for trees
4
in (b) are not drawn to scale. Open circles (a) and lines (b) denote trees with low ni-
5
trate availability while closed circles (a) and lines (b) denote trees with high nitrate
6
availability.
7
Figure 2. Single-variable regression results using nitrate availability to explain (a) average
8
growth index since 1970 and (b) trend in growth indices since 1950. Dashed lines are
9
the 95% confidence limits.
10
Figure 3. Time-series plots of index chronologies from 1950 to 1996 for (a) trees with high ni-
11
trate availability, (b) trees with low nitrate availability, and (c) all trees of the site. For
12
each chronology, the dashed line is the 1.0 mean, the light gray line is the slope since
13
1950, and the dark gray line is the average since 1970.
Sheppard, Casals, Gutiérrez: Soil nutrients and tree growth.
Figure 1
(a)
0
0
16
15
+15
8
13
9
0
-5
(b)
17
14
13
19
16
20
12
15
10
+5
5 4
3
1
SSE
14
Sheppard, Casals, Gutiérrez: Soil nutrients and tree growth.
Figure 2
Average grow th index since 1970
(a)
1.40
y = 1.0 + .007 • x
R ² = 32% , p = 0.005
1.20
1.00
0.80
0
10
20
30
20
30
Trend in grow th indices since 1950
(b)
0.04
y = -0.004 + 0.0003 • x
R ² = 22% , p = 0.022
0.02
0.00
-0.02
0
10
Nitrate availability (µg/day/capsule)
Sheppard, Casals, Gutiérrez: Soil nutrients and tree growth.
(a)
Figure 3
High Nitrate (9 trees) :
average since 1970 = 1.10
slope since 1950 = + 0.005
1.5
1.0
0.5
1950
1960
Index value (dimensionless)
(b)
1970
1980
1990
Low Nitrate (11 trees):
average since 1970 = 1.01
slope since 1950 = -0.002
1.5
1.0
0.5
1950
1960
(c)
1970
1980
1990
Full Site (20 trees):
average since 1970 = 1.05
slope since 1950 = + 0.001
1.5
1.0
0.5
1950
1960
1970
Y ear
1980
1990