Topoclimatic influences on climatic water balance in complex terrain: Implications for modeling tree species distributions

Background/Question/Methods

Mountains create steep biophysical and climatic gradients that drive fine-scale hydrologic and ecological variation, challenging our ability to model ecological processes in mountains at relevant scales. Using the Penman-Monteith model for evapotranspiration, we examine the influence of fine-scale variation in temperature, humidity wind speed and radiation on climatic water balance in the Bitterroot Valley, western Montana. Using distributed networks of inexpensive sensors, we develop very high resolution air temperature and humidity models. We model solar radiation, accounting for topographic shading and address the influence of wind speed on potential evapotranspiration using daily, 30-year Windninja simulations. We compare our water balance model with a similar model derived from PRISM data. We then examine the influences of topoclimate on conifer tree species distributions using Forest Inventory and Analysis data.   

Results/Conclusions

Interactions between incident radiation, wind speed and spatial variation in surface air temperature and humidity result in complex spatial patterns of deficit and evapotranspiration. Cold air drainage and high relative humidity at night as well as lower wind speeds reduce evaporative demand in valley bottoms. A comparison of modeled species occurrences using topoclimatically informed and naïve models illustrates that multiple physical factors may support climatic refugia in mountains. Accounting for topographic influences on biophysical and hydrologic processes may be essential for understanding how climate change will influence vegetation dynamics in complex mountainous terrain.