Linking Topography, Hydrology, Climate, and Ecology in Semi-Arid Forests: Within Catchment Annual Tree Growth and Water Use Efficiency

Topography and climate play an integral role in the spatial variability and annual dynamics of aboveground carbon sequestration. Topographic, climatic, and hydrologic dynamics interact to drive vegetation spatial distribution, growth patterns, and physiological processes. Despite knowledge of vegetation - climate - topography relationships on the landscape and hillslope scales, little is known about the influence of complex topography coupled with hydrologic and topoclimatic variation on tree growth and physiology at the catchment scale, especially in semi-arid forests. Climate change predictions for the semi-arid west include increased temperatures, more frequent and extreme drought events, and decreases in snowpack, all of which put forests at risk of altered species ranges and physiological processes and enhanced susceptibility to disturbance events. In this study, we determine how species-specific tree growth patterns and water use efficiency respond to interannual climate variability and how this response varies with topographic position. We found that tree growth and water use efficiency respond directly to climatic and topographic parameters and species vary in their response to these parameters. Pinus contorta and Pinus ponderosa both show significant decreases in growth with water-limiting climate conditions. Topographic position mediates this response. Carbon isotope analyses show increased water use efficiency during drought for Pinus contorta, but indicate no significant difference in water use efficiency of Pinus ponderosa between a drought year and a non-drought year. Varying responses from different species and climate conditions indicate that semi-arid forests are especially susceptible to changes and risks posed by climate change and that topographic variability will likely play a significant role in determining the future vegetation patterns of semi-arid systems