Translating physiological drought into tree stress and forest response

Background/Question/Methods

Warmer temperatures, reduced snow accumulation and earlier melt, and changes in precipitation patterns all influence forest productivity and mortality rates. In the complex topography of mountain environments, how forests respond to drought stress is likely to vary along gradients between water and energy limited environments, and with soil drainage characteristics and historic disturbance regimes. Process-based models are key tools used to link spatial datasets with ecologic and hydrologic theory in order to examine the complex spatial-temporal patterns that give rise to forest vulnerability to drought . RHESSys (Regional hydrologic ecosystem simulation system) is an open source coupled model of spatially –distributed hydrology and ecosystem biogeochemical cycling that has been used to examine the geography of forest drought stress vulnerability and linkages among forest water use, productivity, carbon cycling, drought-related mortality, soil moisture and streamflow. One of the strengths of this tool is the use of multiple data sources for parameterization and evaluation. 

Results/Conclusions

Refinements to and applications of RHESSys provide an integrated systems-oriented perspective on forest drought stress and mortality and allow us to disentangle the relative importance of multiple controls on forest structure and function and the implications for downstream flows.  Results demonstrate that the model can accurately capture observed within-watershed spatial patterns of pine mortality during a drought in the early 2000s as well as carbon and moisture flux estimates. Model estimates indicate that micro-climates and spatial heterogeneity in soil properties can be first-order controls on forest mortality patterns. Interestingly results also highlight the key role played by long-term (decadal) growth and disturbances patterns in setting the stage for vulnerability to particular drought events. Ultimately our case-study model applications highlight the utility of a system-oriented, place-based perspective for estimating how forests are likely to respond to drought and how successful forest management practices may be in mediating these effects.