Landscape Influences on Microclimate and Tree Growth Cessation in a Semi-Arid Montane Forest

As climate change impacts the severity and frequency of drought, knowledge of hillslope-to-watershed scale ecohydrology is increasingly necessary to inform appropriate conservation, restoration, and management of forested ecosystems. In mountain environments, spatial patterns of water and energy organize forest productivity at plot, hillslope, and watershed scales. These microclimatic patterns are impacted by gradients in elevation, aspect, and local topographic convergence and divergence. In water-limited ecosystems, such patterns may be first-order drivers of tree growth. However, there is limited field-based research characterizing how seasonal drivers of forest growth may vary across complex terrain. Throughout the growing season, we measured soil moisture, vapor pressure deficit, soil and air temperatures, and radial growth at 27 Douglas-fir (Pseudotsuga menziesii) sites within Lubrecht Experimental Forest, Montana USA. Using these data, we assessed the influences of elevation, aspect, and local topographic position on hillslope-scale microclimates. These seasonal microclimates were then compared to the observed timing of tree growth cessation. Generally, trees located in high elevations, north-facing aspects, and convergent topographic positions were correlated with decreased temperatures, greater soil moisture, and reduced vapor pressure deficits, leading to later growth cessation. However, at the wettest positions, persistently saturated soil moisture conditions contributed to earlier cessation. Our findings highlight the landscape partitioning of forest microclimates and subsequently, their contribution to the spatial organization of growing seasons in semi-arid mountain watersheds. Lastly, we suggest that over longer timescales these seasonal microclimates influence tree growth year after year, contributing to the organization of cumulative forest growth previously observed within the watershed.