Abstract
Abstract. Severe droughts occurred in the western United States during recent decades, and continued human greenhouse gas emissions are expected to exacerbate warming and drying in this region. We investigated the role of water availability in shaping forest carbon cycling and morphological traits in the eastern Cascade Mountains, Oregon, focusing on the transition from low-elevation, dry western juniper (Juniperus occidentalis) woodlands to higher-elevation, wetter ponderosa pine (Pinus ponderosa) and grand fir (Abies grandis) forests. We examined 12 sites in mature forests that spanned a 1300 mm yr−1 gradient in mean growing-year climate moisture index (CMIgy ), computed annually (1964 to 2013) as monthly precipitation minus reference evapotranspiration and summed October to September. Maximum leaf area, annual aboveground productivity, and aboveground live tree biomass increased with CMIgy (r2 = 0.67–0.88, P < 0.05), approximately 50-, 30-, and 10-fold along this drier to wetter gradient. Interannual fluctuations in CMI affected the annual radial growth of 91 % of juniper, 51 % of pine, and 12 % of fir individuals from 1964 to 2013. The magnitude of the site-average growth–CMI correlations decreased with increased CMIgy (r2 = 0.53, P < 0.05). All three species, particularly fir, experienced pronounced declines in radial growth from c. 1985 to 1994, coinciding with a period of sustained below-average CMIgy and extensive insect outbreak. Traits of stress-tolerant juniper included short stature, high wood density for cavitation resistance, and high investment in water transport relative to leaf area. Species occupying wetter areas invested more resources in height growth in response to competition for light relative to investment in hydraulic architecture. Consequently, maximum tree height, leaf area : sapwood area ratio, and stem wood density were all correlated with CMIgy . The tight coupling of forest carbon cycling and species traits with water availability suggests that warmer and drier conditions projected for the 21st century could have significant biogeochemical, ecological, and social consequences in the Pacific Northwest.
Highlights
Consistent with predictions from our first hypothesis, forest ecosystem leaf area index (LAI), aboveground net primary productivity (ANPP), and aboveground live biomass (AGB) increased with CMIgy, yet residual variance in the regression models implies that the characteristics were influenced by additional bioclimatic or disturbance-related factors
Along with those of several prior field studies (Gholz, 1982; Grier and Running, 1977), indicate that water availability is the dominant constraint on forest leaf area of mature stands in the eastern Cascades
The constraint of water availability on forest leaf area in this environment is driven by the need to maximize carbon assimilation while simultaneously minimizing water loss
Summary
Droughts in the western United States had severe impacts on human and natural systems during the past century (Allen et al, 2010; Schwalm et al, 2012; Williams et al, 2012; Woodhouse and Overpeck, 1998), and sustained climatic warming due to human greenhouse gas emissions will likely exacerbate drought impacts over the coming century (Collins et al, 2013; Dai, 2013; Williams et al, 2012). Tree-ring-based reconstructions indicate that very persistent, severe, and extensive droughts (“mega droughts”) occurred over the past 1200 years in the western United States, especially during an abnormally warm period from around AD 900 to 1300 (Cook et al, 2004; Woodhouse et al, 2010), and suggest an increase in the area annually affected by drought over the 20th century (Cook et al, 2004). Our objective in this study was to investigate how forest carbon cycling and conifer morphological traits responded to variation in water availability across a network of field sites in the eastern Cascade Mountains, Oregon, which included forests dominated by western juniper, ponderosa pine, and grand fir. We anticipated that average stem wood density (WD) would decrease with increased CMIgy, whereas maximum tree height (Hmax) and LA : SA would increase with CMIgy
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
