Introduction
Water availability strongly constrains the distribution of plants on Earth's
land surface (Holdridge, 1947; Major, 1963) and the resulting
structure and function of terrestrial ecosystems (Churkina and Running,
1998; Law et al., 2002; Schuur, 2003). For instance, desert (Whittaker
and Niering, 1975), grassland (Yang et al., 2008), and forest
productivity (Law et al., 2002; Schuur, 2003; Berner and Law, 2015)
differ widely among sites with contrasting water availability. Water
availability is shaped by regional climate (e.g., precipitation, atmospheric
evaporative demand), as well as by local topography and soils (Webb et
al., 1983). Water availability is projected to change in many parts of the
world over the coming century in response to continued atmospheric warming
from sustained anthropogenic greenhouse gas emissions (Collins et al.,
2013; Dai, 2013; Walsh et al., 2014). Societies depend on the goods and
services provided by terrestrial ecosystems (e.g., forests; Williams,
2006) and thus it is imperative to elucidate climatic controls over
ecosystem structure and function to help anticipate and mitigate potential
impacts of ongoing climatic change.
The western US is a region where pronounced spatial variation in
water availability exerts a strong influence over forest structure and
function. For instance, average annual precipitation varies over 500 cm yr-1
across this region, with particularly steep hydrologic gradients
in the Pacific Northwest (Daly et al., 2008). Differences in water
availability give rise to forest communities that range from dry,
low-productivity woodlands to high-productivity coastal temperate
rainforests where live-tree biomass (BIO) attains levels thought to be
exceeded only by primary Eucalyptus regnans forests in southern Australia (Waring and
Franklin, 1979; Keith et al., 2009).
Prior studies drew on small networks of field sites (n < 20) to
investigate how tree net primary productivity (NPP) and BIO varied among
mature stands spread along hydrologic gradients in parts of this region
(Whittaker and Niering, 1975; Gholz, 1982; Webb et al., 1983; Berner and
Law, 2015). Tree BIO and NPP can vary widely with stand age
(Hudiburg et al., 2009) and thus these studies focused on mature
stands (stand age generally > 100 years) where BIO and NPP had
somewhat stabilized after reaching their “climatic potential.” These studies
showed that BIO and NPP tended to increase linearly or curvilinearly across
sites as average water availability increased (Whittaker and Niering,
1975; Gholz, 1982; Webb et al., 1983; Berner and Law, 2015). These spatial
relationships are thought to reflect long-term climatic constraints on
ecosystem structure (e.g., BIO) and function (e.g., NPP) that are shaped by
gradual shifts in community composition and population size (Jin and
Goulden, 2014). The field studies mentioned above make a compelling case
that water availability is an important determinant of BIO and NPP in mature
stands, yet these studies were based on a small number of field sites
selected using a set of criteria (e.g., mature stands near a road) rather
than on a large sample of mature stands in the region.
Several of these earlier field studies also indicated that plant communities
accumulated more BIO per unit of NPP in progressively wetter areas,
suggesting slower turnover of plant BIO as climate became wetter
(Whittaker and Niering, 1975; Webb et al., 1983). Mean carbon
residence time (CRT) describes the average duration that a carbon molecule
will remain in a specific pool (Waring and Running, 2007), and for CRT in
live biomass it can be computed as BIO / NPP, assuming that BIO remains constant
over time (Whittaker, 1961; Friend et al., 2014). Carbon residence time
in live biomass is also known as the biomass accumulation ratio (Whittaker, 1961) and ranged, for
instance, from ∼ 2 years in a hot desert shrubland to
∼ 75 years in a wet, old-growth Douglas-fir forest (Webb
et al., 1983). Differences in CRT among plant communities with contrasting
climate are potentially associated with shifts in carbon allocation (e.g.,
short-lived fine roots and foliage vs. long-lived stem wood) and disturbance
regimes (Girardin et al., 2010). Together, these field studies
illustrate that forest structure and function are constrained by water
availability in parts of the western US; however, additional efforts are
needed to assess these relationships on larger scales across the region,
particularly given that climate models project a pronounced shift towards
hotter, drier conditions over much of the region during the coming century
(Collins et al., 2013; Walsh et al., 2014; Cook et al., 2015).
Our objective in this study was to explore how forest structure and function
change along spatial gradients in water availability across the western US.
We used the average water-year climate moisture index
(CMIwy‾; 1985–2014) as an indicator of
long-term water availability (Webb et al., 1983; Hogg and Hurdle, 1995),
which we computed as the cumulative difference between precipitation (P) and
reference evapotranspiration (ET0) over the approximate seasonal cycle
of soil water recharge and draw-down (October–September). Furthermore, we
focused on forest stands that were at least 100 years old because field
surveys from the region indicated that BIO and NPP reached much of their
climatic potential after a century. However, we acknowledge that BIO tends to
gradually increase and NPP remains stable or gradually declines during
subsequent centuries (Hudiburg et al., 2009). Building on prior
field studies (e.g., Gholz, 1982; Webb et al., 1983; Berner and Law,
2015), we hypothesized that long-term water availability limits tree NPP,
BIO, and CRT in mature forest stands across the region. We thus predicted
that tree NPP, BIO, and CRT in mature forests would increase with increasing
CMIwy‾. Tree NPP, BIO, and CRT were
based on above and belowground components. We tested these hypotheses
first across Washington, Oregon, and California (WAORCA) using forest inventory measurements from 1953 sites
and then across 18 Mha of mature forest in the western US using satellite
remote sensing data sets. These data sets included three national biomass
maps, along with NPP derived from the Moderate Resolution Imaging
Spectroradiometer (MODIS). Forest inventories provide rigorous, though
spatially limited, field measurements of forest structure and function, while
satellite remote sensing provides spatially continuous, albeit modeled,
estimates of forest structure and function across large domains.
Results
Average annual water availability varied widely across both WAORCA and the
broader western US from 1985 to 2014 (Fig. 1a, b). The
CMIwy‾ ranged from a minimum of
-370 cm yr-1
in southern California and Arizona to a maximum of 490 cm yr-1 in the Olympic Mountains in northwestern Washington. Forests
mapped by MODIS occurred in areas where CMIwy‾ was between -340 and 490 cm yr-1, though 98 % of
forest area occurred between -200 and 200 cm yr-1, and 72 % occurred
between -100 and 100 cm yr-1. Average (±1 SD)
CMIwy‾ in forested areas was -40 ± 80 cm yr-1.
The Coast Range and Cascade Mountains in Washington
and Oregon were the wettest areas, with CMIwy‾ generally > 100 cm yr-1. Water availability
decreased rapidly in the rain shadows east of the Cascades and Sierra
Nevada, giving rise to very steep CMIwy‾ gradients. For instance, annual CMIwy‾
in northern Oregon decreased nearly 350 cm over ∼ 30 km
between high-elevation forests in the Cascades and low-elevation woodlands
in the eastern foothills of the Cascades. The range in
CMIwy‾ encountered along this gradient
in the Cascades almost spanned the full range in CMIwy‾ that supported 98 % of forest area in the western US. Dry
forests occurred along the low-elevation margins of mountain ranges
throughout continental areas, though the largest tract of dry forest was
found in Arizona and New Mexico.
Tree net primary productivity (NPP, Mg C ha-1 yr-1), live biomass (BIO, Mg C ha-1), and carbon residence time
(CRT, years) increased with increasing water availability across both WAORCA
(a–c) and the broader western US (d–f). Forest characteristics were derived
from field measurements at 1953 inventory plots in WAORCA (a–c) and from
satellite remote sensing data sets across 18 Mha of mature forest in the
western US (d–f). Tree NPPsat was characterized using MODIS data averaged
annually from 2000 to 2014. Tree BIOsat was quantified based on an ensemble
of aboveground biomass maps plus estimates of coarse-root, fine-root, and
foliage biomass. Tree CRT was computed for each field plot and pixel as BIO / NPP.
Water availability was quantified using a climate moisture index
(CMI = P-ET0) summed over the water year (October–September) and then
averaged from 1985 to 2014 (CMIwy‾). The
region was partitioned into 10 cm yr-1 (nonoverlapping)
CMIwy‾ bins, pixels and plots were
allocated to bins, and then forest characteristics were summarized within
each bin. In each panel, the bold line denotes the median, the dark gray band
the 25–75th percentiles, and the light gray band the 10–90th
percentiles. Note the different y axis scales between (b) and (e), as well
as (c) and (f).
Changes in tree net primary productivity (NPP, Mg C ha-1 yr-1), live biomass (BIO, Mg C ha-1), and carbon
residence time (CRT, year) for stands over 100 years old along gradients in
a climate moisture index (CMIwy‾, cm yr-1) in both WAORCA and the broader western US. Forest characteristics
were quantified using field measurements in WAORCA and satellite remote
sensing data sets covering the western US. The analysis incorporated forests
in areas where CMIwy‾ was between -200 and 200 cm yr-1.
Summaries include (1) median forest
characteristics in the driest 5 % and wettest 95 % of pixels or plots, (2) the corresponding
change, and (3) Spearman's correlation (rs) between
CMIwy‾ and the median forest
characteristics computed at 10 cm yr-1 CMIwy‾ intervals. All correlations were statistically significant
at α < 0.001.
Domain
Variable
Units
Median of…
Change…
CMIwy‾ cor.
Driest 5 %
Wettest 95 %
Abs.
%
rs
WAORCA
NPPfield
Mg C ha-1 yr-1
2.2
5.6
3.4
155
0.93
BIOfield
Mg C ha-1
26
281
255
997
0.96
CRTfield
year
11
49
38
358
0.96
Western US
NPPsat
Mg C ha-1 yr-1
3.4
6.7
3.3
97
0.93
BIOsat
Mg C ha-1
32
165
133
410
0.97
CRTsat
year
10
26
16
160
0.99
Tree NPP, BIO, and CRT varied substantially across both WAORCA and the
broader western US in response to variation in CMIwy‾ (Figs. 1, 2; Table 2). We focused on forests in areas where
CMIwy‾ was between -200 and 200 cm yr-1, given the paucity of land and measurements in the 2 % of forest
area that was either drier or wetter. Median NPPfield, BIOfield,
and CRTfield all exhibited a strong, positive association with
CMIwy‾ (rs= 0.93–0.96, p < 0.001). Median NPPfield increased 155 % between the driest and
wettest 5% of sites in WAORCA (Fig. 2a), while median BIOfield and
CRTfield increased 997 and 358 %, respectively, between these
sites (Fig. 2b, c; Table 2). The relationship in each case was slightly
curvilinear. There were also strong, positive relationships among median
NPPfield, BIOfield, and CRTfield along the WAORCA bioclimatic
gradient (rs= 0.90–0.96, p < 0.001).
Broadly similar patterns were evident when tree NPPsat, BIOsat,
and CRTsat were examined across the western US using remote sensing
data sets (Figs. 1b–d, 2c, d; Table 2). Median NPPsat, BIOsat,
and CRTsat all showed a strong, positive relationship with
CMIwy‾ (rs= 0.93–0.99, p < 0.001).
Median NPPsat increased 97 % between the driest and wettest 5 % of forested areas along
the regional CMIwy‾ gradient (Fig. 2d, Table 2). Similarly, median BIOsat
and CRTsat increased 410 and 160 %, respectively, between the
driest and wettest areas (Fig. 2e, f; Table 2). The response of median
NPPsat, BIOsat, and CRTsat to increased
CMIwy‾ was more curvilinear than the
field measurements and plateaued in areas where CMIwy‾ exceeded ∼ 100 cm yr-1.
Furthermore, while magnitude of NPPsat and NPPfield response to
CMIwy‾ were similar, the magnitude of
BIOsat and CRTsat responses to increased
CMIwy‾ were much more muted than the
magnitude of response in BIOfield and CRTfield. Nevertheless,
field- and satellite-derived estimates of median NPP, BIO, and CRT were
strongly correlated (rs= 0.90–0.95, p < 0.001).
Furthermore, there were again strong relationships among median NPPsat,
BIOsat, and CRTsat along the western US bioclimatic gradient
(rs= 0.93–0.97, p < 0.001).
Discussion and conclusions
Climate moisture index
Water availability exerted a strong influence on tree NPP, BIO, and CRT
among mature forests in the western US. We chose to quantify water
availability using an index that accounted for both precipitation and
energy-mediated ET0, recognizing that both of these factors contribute
to the relative water stress experienced by plants within an ecosystem
(Webb et al., 1983). We acknowledge that this index has several
shortcomings. For instance, the index does not account for spatial
variation in soil water storage capacity, which can be crucial for
determining plant performance during drought (Peterman et al., 2013).
This might explain some of the variation in NPP and BIO among areas with
similar CMIwy‾; however, quantifying
soil water storage capacity even at individual sites is challenging given
uncertainty in soil structure and plant rooting capacity (Running, 1994).
The index also does not account for water added via fog drip, which has been
shown to supply 13–45 % of the water transpired by redwood forests
(Sequoia sempervirens)
(Dawson, 1998) and sustain other forest ecosystems along the California
coast (Johnstone and Dawson, 2010; Fischer et al., 2016). This
potentially explains why there were areas with low
CMIwy‾ along the central and northern
coast of California that supported forests with higher NPP and BIO than
other forests with similar CMIwy‾.
Furthermore, the index does not account for spatial variation in runoff and
thus likely overestimates water availability in the wettest areas since the
fraction of water lost as runoff increases with precipitation (Sanford
and Selnick, 2013). Despite its relative simplicity, prior studies showed
that CMI was a useful index for explaining interannual variability in fire
activity in the southwestern US (Williams et al., 2014), as well as forest
productivity in northern Siberia (Berner et al., 2013),
southern Canada (Hogg et al., 2002), and central Oregon (Berner and
Law, 2015). Several studies also found that the index, or its inverse (i.e.,
ET0-P), explained substantial spatial variability in mature forest
gross photosynthesis (Law et al., 2002), productivity, and
biomass across a range of ecosystems (Webb et al., 1983; Hogg et al.,
2008; Berner and Law, 2015). Our current study further demonstrates that CMI
is a useful empirical index for assessing climatic constraints on forest
ecosystems on large spatial scales.
Tree net primary productivity
Median tree NPP in mature stands approximately doubled between the driest
and wettest areas in both WAORCA and the western US, though in both
cases the rate at which NPP increased with CMIwy‾ slowed in the wettest areas. Prior field studies conducted
at a limited number of field sites in the western US over the past 4 decades have similarly documented increased tree NPP along spatial gradients
of increasing water availability (Whittaker and Niering, 1975; Gholz,
1982; Webb et al., 1983; Berner and Law, 2015). Building on these prior
efforts, our current study demonstrates a robust relationship between tree
NPP and water availability in mature forests using field measurements from
nearly 2000 inventory plots along with satellite remote sensing estimates
of NPP covering ∼ 18 Mha of forestland.
The NPP–CMIwy‾ relationship was similar
when NPP was assessed using field measurements from across WAORCA or using
MODIS covering the western US. The NPP estimates derived from MODIS did
level off in the wettest parts of WAORCA (CMIwy‾≈ 100–200 cm yr-1), whereas this was less
evident in the field measurements. The inventory sites and MODIS forestland
occurred at similar elevations along the CMIwy‾ gradient in WAORCA, suggesting that this discrepancy in NPP
was not due to MODIS systematically including cold, high-elevation areas not
sampled by the inventory sites. One possibility is that MODIS NPP did not
increase in the wettest areas because MODIS becomes less sensitive to
increases in the fraction of photosynthetically active radiation (FPAR)
absorbed by plant canopies in densely vegetated areas (Yan et al.,
2016). A recent MODIS analysis similarly found that the amount of
photosynthetically active radiation absorbed by plant canopies (APAR = FPAR × PAR) increased asymptotically with increasing mean annual
precipitation across plant communities in California (Jin and Goulden,
2014). Forests had higher APAR than other plant communities and,
furthermore, exhibited the smallest increase in APAR per unit increase in
precipitation of any plant community, suggesting that forest productivity
was less sensitive to changes in precipitation than productivity of other
plant communities (Jin and Goulden, 2014). In contrast with the field
measurements, the asymptotic response of MODIS NPP and APAR to increasing
water availability in wet areas suggests that climate impact assessments
based on MODIS could underestimate the sensitivity of plant productivity to
changes in water availability in wet areas with high biomass.
Mechanistically, the strong NPP–CMIwy‾
association reflects the coupling between carbon and water cycling at leaf
(Ball et al., 1987)-to-ecosystem scales (Law et al.,
2002). Tree NPP depends on regionally specific relations with leaf area
(Schroeder et al., 1982; Waring, 1983), which largely determine the
proportion of incoming solar radiation that is absorbed and thus potentially
available to fuel photosynthesis (Runyon et al., 1994). Leaf
photosynthesis inevitably leads to transpiration water loss (Ball et
al., 1987) that must be balanced against water uptake from the soil so as to
prevent the formation of excessive tension on the internal water column,
which could result in hydraulic failure (Williams et al., 1996; Ruehr et al.,
2014). As soil water availability increases, trees are able to support
greater leaf area while maintaining water column tensions within
physiologically operable ranges, which consequently leads to more
photosynthate available to fuel NPP, unless trees are limited by other
resources (e.g., nitrogen). The decreasing rate at which NPP increased with
CMIwy‾ in the wettest areas is likely
due to low temperatures constraining productivity at high elevations
(Runyon et al., 1994; Nakawatase and Peterson, 2006) and heavy
cloud cover limiting solar radiation and thus photosynthesis in coastal
areas (Zhao et al., 2010; Carroll et al., 2014). Tree NPP is affected by
many biotic (e.g., age) and abiotic factors (e.g., nutrients), yet water
availability emerges as a key environmental constraint in the western US.
Tree carbon stocks
Tree BIO increased notably with increasing CMIwy‾ in mature forests across both WAORCA and the broader
western US, reflecting underlying shifts in NPP and, likely, BIO mortality
rates due to natural disturbance. Tree biomass is determined by the rates at
which carbon is gained via NPP and lost due to tissue senescence and
mortality integrated over annual to centennial timescales (Olson, 1963).
Hence, the increase in NPP with increasing CMIwy‾ explains some of the concomitant increase in BIO. Our
analysis did not investigate how tissue senescence or mortality varied along
the regional bioclimatic gradient, though a recent study found that BIO
mortality rates due to bark beetles and fires were very low in the wettest
parts of the western US (e.g., Coast Range and Cascades), while considerably
higher in most drier areas (Hicke et al., 2013). Furthermore, the field
and satellite data sets also incidentally revealed that there was an increase in
the median age of stands over 100 years as conditions became wetter, with
median stand age at ∼ 140 years in the driest areas and 200–240 years
in the wettest areas. The general increase in BIO with increasing
water availability is thus likely due to higher rates of productivity and
potentially lower BIO mortality rates from natural disturbance.
The observed increase in BIO with increasing water availability was
generally consistent with prior field studies from this region, yet our
study demonstrates this response over a much broader bioclimatic gradient.
For instance, early work by Whittaker and Niering (1975) showed that
BIO in mature forests tended to increase with a moisture index inferred from
community composition along an elevational gradient in Arizona's Santa
Catalina Mountains. Subsequent studies, focused on five long-term ecological research (LTER) sites spread
across the conterminous US (Webb et al., 1983) and at 8–12 sites in
Oregon (Gholz, 1982; Berner and Law, 2015), similarly showed a general
increase in tree biomass with increasing water availability. Our study
included sites that ranged from dry woodlands with little BIO to temperate
rainforests with BIO exceeded in few other regions (e.g., max BIO ≈ 950 Mg C ha-1). Tree biomass in our study area has been reported to
reach over 2000 Mg C ha-1 in old-growth coastal redwood stands in
northern California (Waring and Franklin, 1979), which is thought to be
exceeded only by the > 3000 Mg C ha-1 attained by
old-growth Eucalyptus regnans stands in southern Australia (Keith et al., 2009). A global
synthesis suggested that average AGB among high-biomass stands in wet
temperate forests (∼ 377 Mg C ha-1) was over 2 times that
of high-biomass stands in wet tropical forests (∼ 179 Mg C ha-1) and nearly 6 times that of high-biomass stands in wet boreal
forests (∼ 64 Mg C ha-1) (Keith et al., 2009). The
range in BIO included in our analysis of WAORCA thus spanned much of the
observed global range in BIO.
Both field and satellite measurements revealed that median BIO increased
with CMIwy‾, yet the satellite data set
showed less of an increase than the field measurements. Median forest
BIOfield increased nearly 1000 % between the dry woodlands and
coastal temperate rainforests in WAORCA, yet the increase in BIOsat
with increasing CMIwy‾ was less
pronounced (∼ 410 % increase) when assessed across the
western US. Furthermore, median BIOsat plateaued around 175 Mg C ha-1
in areas where CMIwy‾ was
∼ 100–200 cm yr-1. The response of BIO to increasing
CMIwy‾ was likely more muted when
assessed using the satellite-derived maps than the field measurements for
several reasons. The maps are largely derived from optical, multispectral
satellite imagery that is not very sensitive to variation in BIO in
high-biomass forests. Additionally, areas with high BIO often occur as small
patches set in a matrix of stands with lower BIO (Spies et al.,
1994) and thus the moderate-resolution satellite imagery used in developing
these maps records the spectral signature of this larger area rather than
just the patch with high BIO. In other words, the satellite imagery has a
larger sampling footprint relative to that of a field plot, which thus
averages BIO over a larger area, reducing peak values. Advances in satellite
remote sensing, such as NASA's new Global Ecosystem Dynamics Investigation
Lidar (GEDI) instrument, are anticipated to help overcome some of these
challenges (Goetz and Dubayah, 2011). Nevertheless, current BIO
maps (e.g., Kellndorfer et al., 2012; Wilson
et al., 2013) proved to be valuable tools for ecologic and natural resource
assessments (Berner et al., 2012; Goetz et al., 2014; Krankina et al.,
2014).
Carbon residence time in tree biomass
We computed CRT as BIO / NPP and found that median CRTfield increased
persistently with CMIwy‾ from
∼ 11 years in the driest forests to over 49 years in the
wettest forests, highlighting a fundamental change in ecosystem function
along this broad bioclimatic gradient. One limitation of our study is that
computing CRT in this manner assumes that BIO is constant over time
(Friend et al., 2014). We focused on mature stands (> 100 years) to minimize the change in BIO over time, though we acknowledge that BIO
can gradually increase during subsequent centuries (Hudiburg et
al., 2009), which would lead to underestimated CRT. Conversely, drought
and insect-induced defoliation in the early 2000s could have suppressed NPP
(Schwalm et al., 2012; Berner and Law, 2015) without a
proportional reduction in BIO, which could have inflated our estimates of
CRT in some areas. Nevertheless, our results agree well with a prior study
focused on 11 long-term ecological research sites (LTERSs) spread across the conterminous US that found that CRT
increased from ∼ 2 years in a desert shrubland to
∼ 73 years in a 450-year-old Douglas-fir stand at the Andrews
LTER in the Cascade Mountains in Oregon (Webb et al., 1983). For
comparison, we looked at five old-growth Douglas-fir stands (336–555 years
old) near the Andrews LTER and found that CRTfield averaged
79 ± 23 years (±1 SD) among these stands. An increase in the CRT of
aboveground tissues was also observed among plant communities along an
elevational moisture gradient in the Santa Catalina Mountains of Arizona
(Whittaker and Niering, 1975) and across nine mature stands in a range
of forest communities in Oregon (Gholz, 1982). Although this pattern
has been previously documented on small scales, the underlying mechanisms
remain unclear.
We speculate that the increase in CRT with increased water availability was
associated with underlying changes in NPP allocation, BIO mortality rates,
and stand age. Trees invest a larger proportion of NPP into aboveground
tissue production as conditions become wetter and competition for light
intensifies (Runyon et al., 1994; Law et al., 2003). Our field
measurements suggested that the fraction of NPP allocated aboveground
increased from ∼ 0.45 in the driest areas to ∼ 0.64 in the wettest areas and, furthermore, that CRT in aboveground tissues
averaged twice as long as CRT in belowground tissues. Thus, a shift in NPP
allocation toward longer-lived aboveground tissues likely contributed to
longer CRT in wetter areas. Longer CRT in wetter areas could also be related
to forests in these areas (e.g., Coast Range) experiencing lower BIO
mortality rates from wildfire and bark beetles than forests in drier,
continental areas (Hicke et al., 2013). We also found that mature stands
tended to be older in wetter areas and that older stands tended to have
longer CRT, likely as a result of these stands having higher BIO and similar
NPP (Hudiburg et al., 2009). Consequently, the
CRT–CMIwy‾ relationships that we
observed incorporate an age-related effect; however, the effect was quite
small relative to the climate effect. This can be illustrated by comparing
median CRT between mature (100–200 years) and old (> 200 years)
stands occupying very dry (CMIwy‾ < -100 cm yr-1)
and very wet (CMIwy‾ > 100 cm yr-1) areas. Median CRT differed
by 6 % (16 vs. 17 years) between mature and old stands in very dry areas
and by 10 % (47 vs. 52 years) in very wet areas. Conversely, median CRT of
mature stands differed 98 % (16 vs. 47 years) between very dry and very
wet areas, while the median CRT of old stands differed 101 % (52 vs. 17 years)
between very dry and very wet areas. In other words, the difference
in CRT between stands in contrasting climates is much greater than the
difference in CRT between mature and old stands within the same climate zone. Our
study demonstrates that CRT in live-tree biomass was strongly influenced by
water availability. However, additional efforts are needed to determine the
underlying mechanism by which changes in water availability affect CRT,
particularly given that CRT is a primary source of uncertainty in global
vegetation model projections of future terrestrial carbon cycling
(Friend et al., 2014).
Predicting ecosystem response to environmental change
Water availability is projected to decline in much of the western US over
the coming century, in part due to higher temperatures increasing
atmospheric evaporative demand (Dai, 2013; Walsh et al., 2014; Cook et
al., 2015). Predicting the timing, magnitude, and extent of ecological
response to regional climate change remains a challenge. Our study showed
that water availability is a key determinant of forest structure and
function in the western US, broadly suggesting that chronic reductions in
regional water availability could reduce the NPP, BIO, and CRT in mature
stands. Nevertheless, it is problematic to predict the temporal response of
extant forest communities to near-term climatic change based on bioclimatic
relationships derived from spatial data. For instance, recent studies found
that the slope of the NPP–precipitation relationship was much steeper when
derived from spatial data than when derived from the temporal response of
NPP to interannual variation in precipitation (Jin and Goulden, 2014;
Wilcox et al., 2016). Near-term effects of climate variability depend on the
physiological characteristics of species in the extant plant community.
However, bioclimatic relationships derived from spatial data reflect gradual
adjustment of community composition and population size to climate over long
periods of time (Jin and Goulden, 2014; Wilcox et al., 2016).
Furthermore, bioclimatic models derived from spatial data cannot account for
other ecophysiological impacts of environmental change, such as (1) enhanced
plant water use efficiency from CO2 fertilization (Soulé and
Knapp, 2015), (2) increased likelihood of tree mortality due to hotter
drought (Adams et al., 2009), or (3) novel changes in disturbance regimes (Dale et al., 2001; Hicke et al.,
2006). Consequently, predicting ecological response to environmental change
over the coming century will require the use of mechanistic ecosystem models
that account for physiologic, demographic, and disturbance processes at fine
taxonomic and spatial scales (Hudiburg et al., 2013;
Law, 2014). Although spatial models may not be suitable for near-term
projection of ecosystem change, they do provide insight into long-term
ecosystem adaptation to local climate and, furthermore, can be used to
validate and refine mechanistic models if constructed from a representative
sample of forestland.
Summary and conclusions
Water availability varies widely across the western US, giving rise to
forests that range from dry, low-biomass woodlands to temperate rainforests
that are among the highest biomass forests found anywhere in the world. In this
study we quantified changes in tree productivity, live biomass, and carbon
residence time along spatial gradients in water availability using field
inventory measurements from WAORCA and satellite remote sensing data sets
spanning the western US. Our multi-method, multi-scale analysis revealed
that tree productivity, live biomass, and carbon residence time all
increased notably with water availability, which we computed using an index
that accounted for both precipitation and reference evapotranspiration. The
observed increase in productivity was likely due to the close coupling
between carbon and water cycling on leaf-to-ecosystem scales, while the
observed increase in live biomass was associated with higher productivity
and longer carbon residence. The increase in carbon residence time in wetter
areas was linked with greater carbon allocation to long-lived aboveground
tissues, older stand age, and, possibly, lower biomass mortality rates from
natural disturbances (e.g., bark beetles, fires). Tree productivity and
biomass derived from field and satellite measurements exhibited similar
responses to increasing water availability, though the satellite data sets
tended to plateau in the wettest areas, suggesting that additional efforts
are needed to better quantify productivity and biomass from satellites in
high-productivity, high-biomass forests. The pronounced increase in tree
productivity, biomass, and carbon residence time between the driest and
wettest areas illustrates the gradual adjustment of ecosystem structure and
function to long-term variation in water availability; however, the observed
bioclimatic relationships are not suitable for near-term projections of
future ecosystem response to regional drying. Predicting near-term ecosystem
response to drying and other environmental change (e.g., increased CO2)
will require mechanistic ecosystem models, which can be evaluated against
bioclimatic relationships developed using inventory sites from a
representative sample of forestlands (e.g., Forest Service inventory sites).
Overall, our results indicate that water availability is a key determinant
of tree productivity, live biomass, and carbon residence time in mature
stands ranging from dry woodlands to coastal temperate rainforests. Future
efforts are needed to anticipate and mitigate the impacts of projected
warming and drying on forest ecosystems in the western US and elsewhere
around the world.