Forest temperature buffering in pure and mixed stands: A high-resolution temporal analysis with generalized additive models

Forests foster buffered microclimates, but causal mechanisms have rarely been studied on longer timescales and in differently diverse stands. Here, we explore temperature regulation by a young experimental forest in Austria, focusing on four common colline broadleaf species (Acer platanoides L., Tilia cordata Mill., Quercus robur L., Carpinus betulus L.) in monocultures, two- and four-species mixed stands. Air temperature was monitored in 28 forest plots for two years and compared to open-field controls. Using generalized additive models (GAMs), we investigated direct temperature offsets and lags between open-field and sub-canopy temperatures, considering diurnal and seasonal changes, and causal factors such as global mean radiation, relative air humidity, wind, and leaf area index (LAI). Forests generally had a cooling effect during the summer and a warming effect in winter, where the cooling magnitude varied with species composition and environmental conditions. Specifically, Acer platanoides and Carpinus betulus demonstrated the highest cooling capacities, and Quercus robur the lowest. Mixed species stands exhibited higher temperature buffering effects relative to monospecific stands, suggesting that species diversity in forests can increase the ability to regulate microclimates. Solar radiation, relative air humidity, wind speed, and LAI all significantly influenced offsets. These findings are crucial for urban forestry and environmental planning, suggesting that careful selection of tree species can optimize temperature regulation, thereby improving human thermal comfort and ecosystem processes alike.