The response of forests to climate change depends in part on whether the photosynthetic benefit from increased atmospheric CO2(∆Ca= future minus historic CO2) compensates for increased physiological stresses from higher temperature (∆T). We predicted the outcome of these competing responses by using optimization theory and a mechanistic model of tree water transport and photosynthesis. We simulated current and future productivity, stress, and mortality in mature monospecific stands with soil, species, and climate sampled from 20 continental US locations. We modeled stands with and without acclimation to ∆Caand ∆T, where acclimated forests adjusted leaf area, photosynthetic capacity, and stand density to maximize productivity while avoiding stress. Without acclimation, the ∆Ca-driven boost in net primary productivity (NPP) was compromised by ∆T-driven stress and mortality associated with vascular failure. With acclimation, the ∆Ca-driven boost in NPP and stand biomass (C storage) was accentuated for cooler futures but negated for warmer futures by a ∆T-driven reduction in NPP and biomass. Thus, hotter futures reduced forest biomass through either mortality or acclimation. Forest outcomes depended on whether projected climatic ∆Ca/∆T ratios were above or below physiological thresholds that neutralized the negative impacts of warming. Critically, if forests do not acclimate, the ∆Ca/∆T must be aboveca. 89 ppm⋅°C−1to avoid chronic stress, a threshold met by 55% of climate projections. If forests do acclimate, the ∆Ca/∆T must rise aboveca. 67 ppm⋅°C−1for NPP and biomass to increase, a lower threshold met by 71% of projections.

SummaryElevated forest mortality has been attributed to climate change-induced droughts, but prediction of spatial mortality patterns remains challenging. We evaluated whether introducing plant hydraulics and topographic convergence-induced soil moisture variation to land surface models (LSM) can help explain spatial patterns of mortality.A scheme predicting plant hydraulic safety loss from soil moisture was developed using field measurements and a plant physiology-hydraulics model, TREES. The scheme was upscaled to Populus tremuloides forests across Colorado, USA, using LSM-modeled and topographymediated soil moisture, respectively. The spatial patterns of hydraulic safety loss were compared against aerial surveyed mortality.Incorporating hydraulic safety loss raised the explanatory power of mortality by 40% compared to LSM-modeled soil moisture. Topographic convergence was mostly influential in suppressing mortality in low and concave areas, explaining an additional 10% of the variations in mortality for those regions.Plant hydraulics integrated water stress along the soil-plant continuum and was more closely tied to plant physiological response to drought. In addition to the well-recognized topo-climate influence due to elevation and aspect, we found evidence that topographic convergence mediates tree mortality in certain parts of the landscape that are low and convergent, likely through influences on plant-available water.

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The mechanistic understanding of drought‐induced forest mortality hinges on improved models that incorporate the interactions between plant physiological responses and the spatiotemporal dynamics of water availability. We present a new framework integrating a three‐dimensional groundwater model, Parallel Flow, with a physiologically sophisticated plant model, Terrestrial Regional Ecosystem Exchange Simulator. The integrated model, Parallel Flow‐Terrestrial Regional Ecosystem Exchange Simulator, was demonstrated to quantify the susceptibility of riparian cottonwoods (Populus angustifolia, Populus deltoides, and native hybrids) in southwestern Canada to sustained atmospheric drought and variability in stream flow. The model reasonably captured the dynamics of soil moisture and evapotranspiration in both wet and dry years, including the resilience of cottonwoods despite their high vulnerability to xylem cavitation. Unrealistic predictions of mortality could be generated when ignoring lateral groundwater flow. Our results also illustrated a mechanistic linkage between streamflow and cottonwood health. In the absence of precipitation, normal streamflow could sustain 94% of cottonwoods, and higher streamflows would be required to sustain all of the floodplain cottonwoods. Further, the risk of mortality was mediated by plant hydraulic properties. These results underpin the importance of integrating groundwater processes and plant hydraulics in order to analyze the forest response to sustained severe drought, which could increase in the future due to climate change combined with increasing river water withdrawals.

The reliance of 10 Utah (USA) aspen forests on direct infiltration of growing season rain versus an additional subsurface water subsidy was determined from a trait‐ and process‐based model of stomatal control. The model simulated the relationship between water supply to the root zone versus canopy transpiration and assimilation over a growing season. Canopy flux thresholds were identified that distinguished nonstressed, stressed, and dying stands. We found growing season rain and local soil moisture were insufficient for the survival of 5 of 10 stands. Six stands required a substantial subsidy (31–80% of potential seasonal transpiration) to avoid water stress and maximize photosynthetic potential. Subsidy dependence increased with stand hydraulic conductance. Four of the six “subsidized” stands were predicted to be stressed during the survey year owing to a subsidy shortfall. Since winter snowpack is closely related to groundwater recharge in the region, we compared winter precipitation with tree‐ring chronologies. Consistent with model predictions, chronologies were more sensitive to snowpack in subsidized stands than in nonsubsidized ones. The results imply that aspen stand health in the region is more coupled to winter snowpack than to growing season water supply. Winters are predicted to have less precipitation as snow, indicating a stressful future for the region's aspen forests.

Snowmelt-driven streamflow in the mountain watersheds of western North America is the primary water source for urban, agricultural, natural ecosystems, and hydropower generation throughout the region. The

Prediction of ecosystem responses to a changing climate is challenging at the landscape to regional scale, in part because topography creates various habitats and influences ecosystem productivity in complex ways. However, the effects of topography on ecosystem function remain poorly characterized and quantified. To address this knowledge gap, we developed a framework to systematically quantify and evaluate the effects of topographic convergence, elevation, aspect, and forest type on the long-term (1986-2011) average and interannual variability of remotely sensed ecosystem productivity. In a forested watershed in the Rocky Mountains, spanning elevations from 1,800 to 4,000 m, we found a prevalent and positive influence of topographic convergence on long-term productivity. Interannual growing season productivity was positively related to precipitation, with higher sensitivity in low elevation and highly productive areas and lower sensitivity in convergent areas. Our findings highlight the influence of topographic complexity on both long-term and interannual variations of ecosystem productivity and have implications for understanding and prediction of ecosystem dynamics at hillslope to regional scales.

Drought predisposes conifer forests to bark beetle attacks and mortality. Although plant hydraulic stress mechanistically links to tree mortality, its capacity to predict trees' susceptibility to beetle attacks has not been evaluated. Further, both tree size and water supply could influence plant hydraulic stress, but their relative importance remained unknown. In this study, we modeled plant hydraulic stress of individual trees in a mixed forest of Lodgepole pine (Pinus contorta), Engelmann spruce (Picea engelmannii), and Subalpine fir (Abies lasiocarpa) in southern Wyoming, using an integrated model of plant hydraulics and hydrology, ground surveys of tree size as well as physiological and geophysical measurements. Based on the established link between plant hydraulic stress and tree mortality, we found interspecific differences in the relative importance of water availability and tree size. Pine mortality was best explained by the combination of tree size and water supply, and fir mortality was best explained by variations in water supply. We next compared the prediction of beetle attack by modeled plant hydraulic stress versus tree size and found tree size best explained beetle attack consistently for all three species. Taken together, our results suggested beetle attack was primarily influenced by beetle preference for large trees, potentially as food sources, rather than more hydraulically stressed trees. These findings highlighted the importance of integrated understanding of biotic/abiotic factors and their mechanistic pathways in order to accurately predict the sustainability of forests susceptible to drought and beetle outbreaks.