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

Authors: Mahood, Adam; Barnard, David; MACDONALD, JACOB - Colorado State University; FORNWAULT, PAULA - Us Forest Service (FS); PITTENGER, DAVID - National Park Service; HALL, SARAH - National Park Service

Submitted to: Environmental Research: Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/6/2025
Publication Date: N/A
Citation: N/A

Interpretive Summary: Why plants grow where they do is a classic problem in ecology that is hard to predict. We found that diurnal temperature range and the average daily minimum of vapor pressure deficit were consistently tied to plant community composition much more strongly than variables like mean temperature or precipitation that are typically used to model species communities. We created a model that predicts fine-scale diurnal temperature range that researchers can use in ecological analysis. This provides researchers with basic guidelines on what spatial scale is important for these variables and how to derive them from publicly available data.

Technical Abstract: Understanding how plant species will respond to climate change is an ongoing challenge, with observations of a substantial number of species not tracking gradients in elevation or latitude as expected. One potential explanation is that microclimatic variation is much greater than coarse-scale climate variation, so species do not have to travel as far as would be expected from coarse-scale climate estimations. Another explanation is that variation in microclimate is driven by cold air drainage (CAD), which is not captured by coarse-scale products. This process results in conditions that are contrary to the lapse rate, and divides the landscape into areas with high diurnal temperature range (DTR) and lower daily minimum temperatures, and other areas with low DTR and high Tmin. Organisms in high DTR areas will be exposed to extremes in both high and low temperatures, and we hypothesized that high-DTR areas would have lower species richness. We established a network of 48 RH/T sensors along topographic gradients at two basins in the Southern Rocky Mountains Ecoregion, Manitou Experimental Forest and Valles Caldera National Preserve, and documented plant species occurrence at each sensor. Areas with high DTR were strongly associated with lower minimum vapor pressure deficit, and weakly associated with minimum temperatures. When each basin was analyzed separately, in situ measurements of diurnal temperature range (DTR) and daily minimum VPD were strongly correlated with species composition at both sites, while more commonly used variables like mean temperature were less predictive (also Tmin was weak). When analyzing both basins together, DTR was not associated with community composition, and coarse-scale water balance metrics were. DTR was stable throughout the year, and so may be more generalizable than seasonally fluctuating variables, and able to improve fine-scale species distribution estimations. We created a model that explained 64% of the variation in DTR using only elevation and topographic wetness index as predictors. At broad scales, variables representing 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 illustrates the importance of accounting for 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 need to account for microclimate when designing experiments as sampling across microclimates will introduce bias/error into community observations.