Predicting the microclimate buffering capacity of twelve future tree species using Plant Area Index and stand structural characteristics

Forest microclimates mitigate heat extremes and support biodiversity, but the strength and drivers of microclimate buffering remain insufficiently understood in young forests and experimental afforestations. Using a one-year dataset (September 2024 to August 2025) from a 17 year-old afforestation experiment in the Upper Rhine valley in southwestern Germany, we (Q1) quantified seasonal dynamics of temperature buffering, (Q2) identified structural predictors of buffering strength and (Q3) evaluated three non-destructive indirect optical methods for estimating plant area index (PAI). Understory air temperature was recorded continuously at 48 plots representing 12 tree species and buffering capacity was assessed as temperature offsets between the forest understory and open-land reference, as well as a stability index indicating the thermal variability per month. Cooling and warming effects in forest understory and canopy temperatures occurred throughout the year but varied seasonally in their magnitude. Cooling of daily maximum temperatures peaked in early summer (up to -6 °C), whereas warming of daily minimum temperatures was strongest in early spring (up to +3 °C). The variables basal area and tree height were the most robust predictors of buffering: dense and tall stands enhanced cooling during warm months and increased thermal stability in colder months. Plant area index (PAI) explained less seasonal variation in buffering and showed inconsistent relationships depending on the method used. Among the three methods for estimating PAI, the LAI-2000 aligned best with the structural variables basal area and tree height and microclimate metrics but still had only limited predictive power. Our findings highlight that forest microclimate buffering is a seasonally dynamic process, primarily explained by the forest structural variables basal area and tree height. Maintaining structurally dense and tall canopies may therefore help sustain cooler conditions in recreational forests and provide microrefugia for temperature-sensitive species under increasing heat extremes. Improving PAI estimation and integrating longer-term microclimate monitoring will be critical for predicting the capacity of future forests to buffer temperature extremes.