Water availability limits tree productivity, carbon stocks, and carbon residence time in mature forests across the western US

Abstract

Water availability constrains the structure and function of terrestrial ecosystems and is projected to change in many parts of the world over the coming century. We quantified the response of tree net primary productivity (NPP), live biomass (BIO), and mean carbon residence time (CRT= BIO/NPP) to spatial variation in water availability in the western US. We used forest inventory measurements from 1953 mature stands (>100 years) in Washington, Oregon, and California (WAORCA) along with satellite and climate data sets covering the western US. We summarized forest structure and function in both domains along a 400cmyr^-1 hydrologic gradient, quantified with a climate moisture index (CMI) based on the difference between precipitation and reference evapotranspiration summed over the water year (October-September) and then averaged annually from 1985 to 2014 (CMI_{<span style="text-decoration: overline">wy</span>}). Median NPP, BIO, and CRT computed at 10cmyr^-1 intervals along the CMI_{<span style="text-decoration: overline">wy</span>} gradient increased monotonically with increasing CMI_{<span style="text-decoration: overline">wy</span>} across both WAORCA (r_s = 0.93-0.96, p<0.001) and the western US (r_s = 0.93-0.99, p<0.001). Field measurements from WAORCA showed that median NPP increased from 2.2 to 5.6MgCha^-1yr^-1 between the driest and wettest 5% of sites, while BIO increased from 26 to 281MgCha^-1 and CRT increased from 11 to 49 years. The satellite data sets revealed similar changes over the western US, though these data sets tended to plateau in the wettest areas, suggesting that additional efforts are needed to better quantify NPP and BIO from satellites in high-productivity, high-biomass forests. Our results illustrate that long-term average water availability is a key environmental constraint on tree productivity, carbon storage, and carbon residence time in mature forests across the western US, underscoring the need to assess potential ecosystem response to projected warming and drying over the coming century.

Biogeosciences, 14, 365378, 2017 www.biogeosciences.net/14/365/2017/ doi:10.5194/bg-14-365-2017 Author(s) 2017. CC Attribution 3.0 License.

Logan T. Berner1, Beverly E. Law1, and Tara W. Hudiburg2

1Department of Forest Ecosystems and Society, Oregon State University, 321 Richardson Hall, Corvallis, Oregon, OR 97331-2212, USA

2Department of Forest, Rangeland, and Fire Sciences, University of Idaho, 875 Perimeter Drive, Moscow, Idaho, ID 83844-1133, USA

Correspondence to: Logan T. Berner ([email protected])

Received: 1 October 2016 Published in Biogeosciences Discuss.: 5 October 2016 Revised: 26 December 2016 Accepted: 9 January 2017 Published: 24 January 2017

Abstract. Water availability constrains the structure and function of terrestrial ecosystems and is projected to change in many parts of the world over the coming century. We quantied the response of tree net primary productivity (NPP), live biomass (BIO), and mean carbon residence time (CRT = BIO / NPP) to spatial variation in water avail

ability in the western US. We used forest inventory measurements from 1953 mature stands (> 100 years) in Washington, Oregon, and California (WAORCA) along with satellite and climate data sets covering the western US. We summarized forest structure and function in both domains along a 400 cm yr1 hydrologic gradient, quantied with a climate moisture index (CMI) based on the difference between precipitation and reference evapotranspiration summed over the water year (OctoberSeptember) and then averaged annually from 1985 to 2014 (CMIwy). Median NPP, BIO, and CRT computed at 10 cm yr1 intervals along the CMIwy gradient increased monotonically with increasing CMIwy across both WAORCA (rs = 0.930.96, p < 0.001) and the western

US (rs = 0.930.99, p < 0.001). Field measurements from

WAORCA showed that median NPP increased from 2.2 to5.6 Mg C ha1 yr1 between the driest and wettest 5 % of sites, while BIO increased from 26 to 281 Mg C ha1 and

CRT increased from 11 to 49 years. The satellite data sets revealed similar changes over the western US, though these data sets tended to plateau in the wettest areas, suggesting that additional efforts are needed to better quantify NPP and BIO from satellites in high-productivity, high-biomass forests. Our results illustrate that long-term average water availability is a key environmental constraint on tree produc-

Water availability limits tree productivity, carbon stocks, and carbon residence time in mature forests across the western US

tivity, carbon storage, and carbon residence time in mature forests across the western US, underscoring the need to assess potential ecosystem response to projected warming and drying over the coming century.

1 Introduction

Water availability strongly constrains the distribution of plants on Earths 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 inuence over forest

Published by Copernicus Publications on behalf of the European Geosciences Union.

ditions 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; 19852014)

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 (OctoberSeptember). Furthermore, we focused on forest stands that were at least 100 years old because eld 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 eld 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 rst 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, eld measurements of forest structure and function, while satellite remote sensing provides spatially continuous, albeit modeled, estimates of forest structure and function across large domains.

2 Materials and methods

2.1 Data sets and preprocessing

2.1.1 Field estimates of tree biomass, productivity, and carbon residence time

We used eld measurements to estimate tree BIO (BIOeld, Mg C ha1), NPP (NPPeld, Mg C ha1 yr1), and

CRT (CRTeld, year) at 1953 forest inventory sites located in mature stands spread across WAORCA (Table 1). These 1 ha sites were surveyed by the US Forest Service Forest Inventory and Analysis (FIA) program between 2001 and 2006 and comprise a representative sample of forest lands (tree cover > 10 %) in the region (Bechtold and Patterson, 2005). The inventory sites occurred at elevations ranging from 5 to 3504 m, with an average (1 SD) elevation of

1429 677 m. We included sites in our analysis when stand

age was at least 100 years. Stand age was dened as the av-

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366 L. T. Berner et al.: Water limitations on forests in the western US

structure and function. For instance, average annual precipitation varies over 500 cm yr1 across this region, with particularly steep hydrologic gradients in the Pacic 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 eld 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 reect 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 eld 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 eld 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 eld 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 specic 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-r for

est (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 ne roots and foliage vs. long-lived stem wood) and disturbance regimes (Girardin et al., 2010). Together, these eld 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 con-

L. T. Berner et al.: Water limitations on forests in the western US 367

erage age of the oldest 10 % of trees, where individual tree age was determined at survey plots using increment cores (Van Tuyl et al., 2005). Tree BIOeld and NPPeld were computed for each site as part of a prior study (Hudiburg et al., 2011). Tree BIOeld was estimated using regional allometric equations for tree components (e.g., stem, branch, bark, foliage, and coarse roots) based on tree diameter and/or height (Means et al., 1994; Law et al., 2001), along with estimates of ne-root mass derived from a relationship with leaf area index (LAI, m2 leaf m2 ground; Van Tuyl et al. 2005). Tree

NPPeld was estimated based on changes in above and below-ground woody biomass over a 10-year interval plus annual foliage and ne-root turnover. See Hudiburg et al. (2011) for additional details. We then computed CRTeld in live-tree biomass as the ratio of BIOeld to NPPeld.

2.1.2 Remote sensing estimates of tree biomass, productivity, and carbon residence time

We used satellite remote sensing and ancillary data sets to estimate BIO (BIOsat), NPP (NPPsat), and CRT (CRTsat) across mature forests in the western US (Table 1). Tree BIOsat included the same component carbon pools as BIOeld (i.e, stem, branch, bark, foliage, coarse roots, and ne roots). We quantied the amount of carbon in stems, branches, and bark using an ensemble of three satellite-derived data sets that depicted live-tree aboveground biomass (AGB, excluded foliage) circa 2000 to 2008 (Blackard et al., 2008; Kellndorfer et al., 2012; Wilson et al., 2013). Each map was generated using satellite and geophysical (e.g., climate, topography) data sets to spatially extrapolate forest inventory measurements over the conterminous US. We acquired these maps at 250 m spatial resolution and then converted two of the maps (Blackard et al., 2008; Kellndorfer et al., 2012) from dry biomass to carbon assuming a 50 % conversion factor (Smith et al., 2006). We then reprojected these maps onto a uniform grid in an equal area projection, masked them to the common forest extent, and then averaged the AGB for each pixel across the three biomass maps. We used the biomass map ensemble average in the subsequent analysis, recognizing that pixel-wise estimates of AGB can vary notably among individual maps (Neeti and Kennedy, 2016).

After deriving spatial estimates of carbon storage in AGB, we then estimated carbon storage in coarse roots, ne roots, and foliage for each 250 m forested pixel. As with AGB, we assumed that roots and foliage were 50 % carbon (Smith et al., 2006; Berner and Law, 2016). We computed coarse-root biomass based on an empirical relationship with AGB (Cairns et al., 1997) and ne-root biomass based on an empirical relationship with peak summer LAI (Van Tuyl et al., 2005). Spatial estimates of LAI were available globally at 1 km resolution from NASAs MODIS as part of the MOD15A2 (Collection 5) data set (Myneni et al., 2002). We obtained these LAI estimates at 8-day intervals during July and August (late summer) from 2000 to 2014 for the west-

ern US. We then (1) excluded poor-quality pixels using the quality control ags, (2) computed average late-summer LAI over the 15-year period, and (3) reprojected and resampled the data set to the common 250 m resolution equal area grid.We used average late-summer MODIS LAI to compute both ne-root biomass (as described above) and foliage biomass.Foliage biomass was estimated for each pixel by dividing LAI by the average specic leaf area (SLA, g C m2 leaf) of the forest type found in that pixel. We aggregated an existing map of forest type (Ruefenacht et al., 2008) into nine classes (e.g., Pinus ponderosa, true r) and then varied SLA among classes using species-, genus-, or division-specic estimates of average SLA from a recent leaf trait synthesis (Berner and Law, 2016). We then estimated BIOsat for each 250 m resolution pixel by summing the above and belowground carbon pools.

We quantied regional NPP using the satellite-derived MODIS primary productivity data set (NPPsat, MOD17A3v. 55). The MODIS light-use efciency model predicts annual NPP at 1 km resolution across global terrestrial ecosystems by incorporating estimates of absorbed photosynthetically active radiation (APAR), LAI, and land cover derived from MODIS, together with plant physiological characteristics and climate data (Running et al., 2004; Zhao et al., 2010). The model rst predicts daily gross primary productivity (GPP) based on APAR and the efciency with which APAR is converted to biomass ("), which is affected by low temperatures (frost) and high vapor pressure decit (VPD), inducing stomatal closure. The model then estimates plant respiration (R) at daily-to-annual increments and subsequently computes annual NPP as the cumulative difference between GPP and R. These estimates thus reect NPP allocated both above and belowground. We acknowledge a degree of circularity in relating NPPsat to CMI given that both computations incorporate temperature data, specically, temperature effects on VPD. We obtained annual NPP estimates from 2000 to 2014 for the western US, reprojected the data onto an equal area grid, and then averaged the data over years.

Several additional preprocessing steps were required after deriving forest BIOsat and NPPsat. These included masking both BIOsat and NPPsat to areas mapped as forest by the MODIS land cover map (Friedl et al., 2010) and then further masking these data sets to include only areas where stand age was at least 100 years. The map of stand age reected conditions around 2006 and was produced by Pan et al. (2011) by combining forest inventory measurements, information on historical res, and optical satellite imagery.We applied these 1 km resolution masks to the 250 m resolution BIOsat assuming homogenous land cover and stand age within each 1 km pixel. We then aggregated BIOsat from 250 m to 1 km resolution and computed CRTsat as the ratio of BIOsat to NPPsat.

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368 L. T. Berner et al.: Water limitations on forests in the western US

2.1.3 Climate data sets and derivation of the climate moisture index

We quantied water availability using a climate moisture index (CMI) that was computed at monthly time steps as precipitation minus ET0 (Webb et al., 1983; Hogg, 1994).

We summed monthly CMI over each water year (October in year t-1 to September in year t) from 1985 to 2014 and then averaged over the years to produce a 30-year climatology (CMIwy, cm yr1). The water year represents the approximate annual cycle of soil water recharge and withdrawal (Thomas et al., 2009). We obtained estimates of monthly precipitation from the Parameter-elevation Relationships on Independent Slopes Model (PRISM; Daly et al., 2008), which interpolated weather station measurements onto a 4 km resolution grid. We then estimated monthly ET0 using the Food and Agricultural Organizations (FAO)

PenmanMonteith equation (FAO-56; Allen et al., 1998), where

ET0 =

were summarized separately for the eld and remote sensing data sets. There was a minimum of 10 and a maximum of 114 eld sites in each bin. We then assessed the association between the median forest characteristic (i.e., NPP, BIO, and CRT) in each bin and CMIwy across the bioclimatic gradient using nonparametric Spearmans rank correlation. This test yields a coefcient (rs) between 1 and +1, where a value

of +1 indicates a perfect monotonically increasing relation

ship, a value of zero indicates no covariation between the

two variables, and a value of 1 indicates a perfect mono

tonically decreasing relationship. The test is analogous to Pearsons correlation where the data were rst ranked. We assessed the association between forest characteristics and CMIwy using Spearmans correlation rather than nonlinear regression because our intent was to describe the general relationship rather than develop a predictive model. We performed data preprocessing, analysis, and visualization using ArcGIS 10 (ESRI, Redlands, CA) and Rstatistical software (R Core Team, 2015), relying extensively on the R packages raster (Hijmans and van Etten, 2013) and dplyr (Wickham and Francois, 2015).

3 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 yr1 in southern California and Arizona

to a maximum of 490 cm yr1 in the Olympic Mountains in northwestern Washington. Forests mapped by MODIS occurred in areas where CMIwy was between 340 and

490 cm yr1, though 98 % of forest area occurred between

200 and 200 cm yr1, and 72 % occurred between 100

and 100 cm yr1. Average (1 SD) CMIwy in forested ar

eas was 40 80 cm yr1. The Coast Range and Cascade

Mountains in Washington and Oregon were the wettest areas, with CMIwy generally > 100 cm yr1. 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 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 yr1,

given the paucity of land and measurements in the 2 % of forest area that was either drier or wetter. Median NPPeld,

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0.408[Delta1](Rn G) +

900 U(es ea) [Delta1] + (1 + 0.34U)

[parenleftBig]

. (1)

Variables included net incoming radiation (Rn), soil heat ux (G), mean daily temperature (T ), wind speed (U), and both saturation (es) and actual vapor pressure (ea), as well as the psychrometric constant ( ) and the slope of the vapor pressure curve ([Delta1]). We quantied Rn and U using monthly climatologies from the North American Land Data Assimilation System-2 (NALDAS-2, 12 km resolution) that were

based on measurements from 1980 to 2009 (Mitchell et al., 2004). We derived G, T , es, and ea from PRISM temperature data following Zotarelli et al. (2010). We also computed CMIwy based on ET0 derived using the modied Hargreaves approach (Hargreaves and Samani, 1985; Droogers and Allen, 2002) and found that our analysis was robust to differences in methods used to compute ET0 (results not shown). After computing CMIwy, we then resampled these data using the nearest neighbor approach to match the footprints of both the 1 km NPP and 250 m BIO remote sensing data sets.

2.2 Analysis

We quantied the response of tree NPP, BIO, and CRT to changes in CMIwy across both WAORCA and the broader western US. We specically focused on areas where CMIwy was between 200 and 200 cm yr1, conditions which oc

curred both in WAORCA and in the broader region. This range encompassed 98 % of forest area in the western US; the paucity of data in the remaining 2 % of forest area that was either drier or wetter precluded rigorous analysis. We divided the landscape along this gradient into 10 cm yr1 nonoverlapping bins and then summarized forest characteristics in each bin by computing the median, along with the 10th, 25th, 75th, and 90th percentiles. Forest characteristics

L. T. Berner et al.: Water limitations on forests in the western US 369

Figure 1. Mean climatic moisture index (CMIwy, cm yr1), tree net primary productivity (NPP, Mg C ha1 yr1), and live-tree biomass (BIO, Mg C ha1) in the western US. (a) Tree BIO derived from eld measurements (BIOeld) at mature sites (> 100 years) in WAORCA.

For visual clarity only 20 % of the 1953 sites are depicted. (b) The CMIwy was computed as monthly precipitation minus reference evapotranspiration summed over the annual water year (OctoberSeptember) and then averaged from 1985 to 2014. (c) Mean annual NPP was quantied using MODIS satellite data from 2000 to 2014 (NPPsat). (d) Tree BIO was quantied using satellite-derived estimates of carbon stocks (BIOsat).

Table 1. Summary of tree net primary productivity (NPP, Mg C ha1 yr1), live biomass (BIO, Mg C ha1), and carbon residence time (CRT, year) for stands over 100 years old across both WAORCA and the broader western US. These forest characteristics were quantied for WAORCA using eld measurements from 1953 sites and for the western US using satellite-derived data sets covering 18 Mha of mature forest. Satellite data sets included MODIS NPP and an estimate of BIO derived by combining existing maps of aboveground biomass with additional estimates of carbon storage in coarse roots, ne roots, and foliage. These carbon stocks and uxes combine above and belowground components.

Domain Variable Units Time span Mean (SD) Range

WAORCA NPPeld Mg C ha1 yr1 20012006 4.3 (2.5) 0.620.9 BIOeld Mg C ha1 20012006 158 (135) 2947

CRTeld year 20012006 33 (19) 2137Western US NPPsat Mg C ha1 yr1 20002014 5.3 (2.0) 0.122.7

BIOsat Mg C ha1 2000-2008 83 (54) 2669

CRTsat year 20002008 15 (9) 21390

BIOeld, and CRTeld all exhibited a strong, positive association with CMIwy (rs = 0.930.96, p < 0.001). Median

NPPeld increased 155 % between the driest and wettest 5% of sites in WAORCA (Fig. 2a), while median BIOeld

and CRTeld 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 NPPeld, BIOeld, and CRTeld

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370 L. T. Berner et al.: Water limitations on forests in the western US

Tree NPP fi el d(Mg C ha

500

80

9

1 yr1 )

1 )

400

Tree BIO fi el d(Mg C ha

60

6

300

40

200

3

20

100

0

0

200 100 0 100 200

200 100 0 100 200

0

200 100 0 100 200

200 100 0 100 200

200 100 0 100 200

Average climate moisture index (cm yr1)

Average climate moisture index (cm yr1)

Tree CRT fi el d(yr)

Average climate moisture index (cm yr1)

(d) Western US

300

(e) Western US

(f) Western US

9

40

1 yr1 )

1 )

Tree NPP sat(Mg C ha

Tree BIO sat(Mg C ha

Tree CRT sat(yr)

6

200

20

3

100

0

200 100 0 100 200

200 100 0 100 200

200 100 0 100 200

Average climate moisture index (cm yr1)

Figure 2. Tree net primary productivity (NPP, Mg C ha1 yr1), live biomass (BIO, Mg C ha1), and carbon residence time (CRT, years) increased with increasing water availability across both WAORCA (ac) and the broader western US (df). Forest characteristics were derived from eld measurements at 1953 inventory plots in WAORCA (ac) and from satellite remote sensing data sets across 18 Mha of mature forest in the western US (df). Tree NPPsat was characterized using MODIS data averaged annually from 2000 to 2014. Tree BIOsat was quantied based on an ensemble of aboveground biomass maps plus estimates of coarse-root, ne-root, and foliage biomass. Tree CRT was computed for each eld plot and pixel as BIO / NPP. Water availability was quantied using a climate moisture index (CMI = P ET0) summed

over the water year (OctoberSeptember) and then averaged from 1985 to 2014 (CMIwy). The region was partitioned into 10 cm yr1 (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 2575th percentiles, and the light gray band the 1090th percentiles.

Note the different y axis scales between (b) and (e), as well as (c) and (f).

Table 2. Changes in tree net primary productivity (NPP, Mg C ha1 yr1), live biomass (BIO, Mg C ha1), and carbon residence time (CRT, year) for stands over 100 years old along gradients in a climate moisture index (CMIwy, cm yr1) in both WAORCA and the broader western US. Forest characteristics were quantied using eld 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

Average climate moisture index (cm yr1)

0

Average climate moisture index (cm yr1)

0

1. Summaries include (1) median forest characteristics in the driest 5 % and wettest 95 % of pixels or plots, (2) the corresponding change, and (3) Spearmans correlation (rs)

between CMIwy and the median forest characteristics computed at 10 cm yr1 CMIwy intervals. All correlations were statistically signicant at < 0.001.

Domain Variable Units Median of. . . Change. . . CMIwy cor.

Driest 5 % Wettest 95 % Abs. % rs

WAORCA NPPeld Mg C ha1 yr1 2.2 5.6 3.4 155 0.93

BIOeld Mg C ha1 26 281 255 997 0.96 CRTeld year 11 49 38 358 0.96 Western US NPPsat Mg C ha1 yr1 3.4 6.7 3.3 97 0.93

BIOsat Mg C ha1 32 165 133 410 0.97

CRTsat year 10 26 16 160 0.99

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L. T. Berner et al.: Water limitations on forests in the western US 371

along the WAORCA bioclimatic gradient (rs = 0.900.96,

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. 1bd, 2c, d; Table 2).Median NPPsat, BIOsat, and CRTsat all showed a strong, positive relationship with CMIwy (rs = 0.930.99, 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 eld measurements and plateaued in areas where CMIwy exceeded 100 cm yr1. Furthermore,

while magnitude of NPPsat and NPPeld 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 BIOeld and CRTeld. Nevertheless, eldand satellite-derived estimates of median NPP, BIO, and CRT were strongly correlated (rs = 0.900.95, p < 0.001).

Furthermore, there were again strong relationships among median NPPsat, BIOsat, and CRTsat along the western US bio-climatic gradient (rs = 0.930.97, p < 0.001).

4 Discussion and conclusions

4.1 Climate moisture index

Water availability exerted a strong inuence 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 1345 % 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 re 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 ), ex

plained 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.

4.2 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 eld studies conducted at a limited number of eld 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 eld measurements from nearly 2000 inventory plots along with satellite remote sensing estimates of NPP covering 18 Mha of forestland.

The NPPCMIwy relationship was similar when NPP was assessed using eld 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 100200 cm yr1), whereas this was

less evident in the eld 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 sen-

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372 L. T. Berner et al.: Water limitations on forests in the western US

sitive to changes in precipitation than productivity of other plant communities (Jin and Goulden, 2014). In contrast with the eld 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 NPPCMIwy association reects 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 specic 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 photo-synthesis 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.

4.3 Tree carbon stocks

Tree BIO increased notably with increasing CMIwy in mature forests across both WAORCA and the broader western US, reecting 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 bio-climatic gradient, though a recent study found that BIO mortality rates due to bark beetles and res 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 eld 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

200240 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 eld 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 Arizonas Santa Catalina Mountains. Subsequent studies, focused on ve long-term ecological research (LTER) sites spread across the conterminous US (Webb et al., 1983) and at 812 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 rain-forests with BIO exceeded in few other regions (e.g., max BIO 950 Mg C ha1). Tree biomass in our study area has

been reported to reach over 2000 Mg C ha1 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 ha1 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 ha1)

was over 2 times that of high-biomass stands in wet tropical forests ( 179 Mg C ha1) and nearly 6 times that of

high-biomass stands in wet boreal forests ( 64 Mg C ha1)

(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 eld and satellite measurements revealed that median BIO increased with CMIwy, yet the satellite data set showed less of an increase than the eld measurements. Median forest BIOeld 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 ha1 in areas where CMIwy was 100

200 cm yr1. The response of BIO to increasing CMIwy was likely more muted when assessed using the satellite-derived maps than the eld 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 eld plot, which thus averages BIO over a larger area, reducing peak values.Advances in satellite remote sensing, such as NASAs new

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L. T. Berner et al.: Water limitations on forests in the western US 373

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).

4.4 Carbon residence time in tree biomass

We computed CRT as BIO / NPP and found that median CRTeld 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 inated 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-r stand at the Andrews LTER in the Cascade Mountains in Oregon (Webb et al., 1983).For comparison, we looked at ve old-growth Douglas-r stands (336555 years old) near the Andrews LTER and found that CRTeld 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 intensies (Runyon et al., 1994; Law et al., 2003). Our eld 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 below-ground 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 re-

lated to forests in these areas (e.g., Coast Range) experiencing lower BIO mortality rates from wildre 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 CRTCMIwy 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 (100200 years) and old (> 200 years) stands occupying very dry (CMIwy < 100 cm yr1) and very wet

(CMIwy > 100 cm yr1) 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 inuenced 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).

4.5 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 NPPprecipitation 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 reect gradual adjustment of community composi-

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374 L. T. Berner et al.: Water limitations on forests in the western US

tion 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 efciency from CO2 fertilization (Soul 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 ne 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 rene mechanistic models if constructed from a representative sample of forestland.

4.6 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 quantied changes in tree productivity, live biomass, and carbon residence time along spatial gradients in water availability using eld 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, res).Tree productivity and biomass derived from eld 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 ob-served 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.

5 Data availability

The data sets that we used in this analysis are nearly all publicly available online. In particular, MODIS NPP are available from the Numerical Terradynamic Simulation Group at the University of Montana (NTSG, 2017; http://www.ntsg.umt.edu/project/mod17#data-product

Web End =http://www.ntsg. http://www.ntsg.umt.edu/project/mod17#data-product

Web End =umt.edu/project/mod17#data-product http://www.ntsg.umt.edu/project/mod17#data-product

Web End = ) and MODIS LAI can be accessed using the NASA Reverb data portal (NASA, 2017; https://reverb.echo.nasa.gov/reverb/

Web End =https://reverb.echo.nasa.gov/reverb/ ). The satellite-derived biomass data sets are available from the Oak Ridge National Laboratory Distributed Active Archive Center for Biogeochemical Dynamics (ORNL DAAC, 2017; https://daac.ornl.gov/

Web End =https:// https://daac.ornl.gov/

Web End =daac.ornl.gov/ ) and the US Forest Service (USFS, 2017; https://www.fs.usda.gov/rds/archive/

Web End =https://www.fs.usda.gov/rds/archive/ ). The forest inventory data can be obtained from the USFS Forest Inventory and Analysis program (USFS FIA, 2017; https://www.fia.fs.fed.us/

Web End =https://www.a.fs.fed. https://www.fia.fs.fed.us/

Web End =us/ ), while the derived estimates of forest biomass and productivity are available on request.

Author contributions. L. T. Berner designed the study, analyzed the data, and prepared the manuscript with contributions fromB. E. Law and T. W. Hudiburg, who both also contributed data sets to this effort.

Competing interests. The authors declare that they have no conict of interest.

Acknowledgements. This work was supported by NASA Headquarters under the NASA Earth and Space Science Fellowship Program (Grant NNX14AN65H), the USDA National Institute of Food and Agriculture (Grant 2013-67003-20652), and the ARCS Foundation Scholar program. T. W. Hudiburg was supported by the National Science Foundation Idaho EPSCoR Program (Grant IIA-1W301792). We thank Charles Bourque for serving as the handling associate editor for our paper and also thank the four anonymous reviewers for their constructive feedback.

Edited by: C. BourqueReviewed by: four anonymous referees

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L. T. Berner et al.: Water limitations on forests in the western US 375

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