Diurnal temperature range drives understory plant community composition in micro-climatically complex temperate forests

Adam Lee Mahood , David Barnard, Jacob Macdonald, Sarah Hall, David Pittenger, Paula Fornwalt

Diurnal temperature range (DTR) is hypothesized to be a key driver of plant community assembly, because areas with high DTR are exposed to both extreme high and extreme low temperatures on a daily basis. We established networks of temperature and relative humidity sensors along DTR gradients in two montane forest basins, and conducted plant surveys around each sensor (n=46). We examined which other microclimate measurements were associated with DTR, how community composition was correlated with our in situ microclimate measurements and compared that to how it was correlated with coarse-scale gridded climate products. DTR measured in situ was the only variable that explained more than 50% of the within-basin variation in species composition for both basins, but among basins coarser-scale climatic variables (actual evapotranspiration, topographic wetness index, and climatic water deficit) performed better. The Mean DTR was stable throughout the year, although it was more variable around the mean (i.e., the standard deviation was higher) in winter months. It achieved both time series stability and distinguishability in less than 100 days, and was most strongly associated with daily minimum vapor pressure deficit. Our simple model of DTR explained 64% of the variation, using only topographic wetness index and elevation as predictors. These findings illustrate how at broad scales, average temperature and moisture conditions drive the regional species pool, but the fine scale distribution of plant species within a basin is driven by microclimate. This study emphasizes the importance of accounting for fine-scale topoclimatic processes and highlights the need for better physically based models that capture topoclimate gradients, allowing for improved representation of complex ecological processes in hydrologic and earth systems models. Future studies should account for microclimate, especially DTR, when designing experiments as sampling across microclimates will introduce bias into community observations.