The long-term viability of a forest depends in large part on forest regeneration. However, the forest understory often exists in a microclimate that can be quite different from the regional environment, thus decoupling regeneration dynamics from more regional environmental processes. For example, seedlings may be partially buffered from climate warming because canopies reduce the load of solar radiation, reduce wind speed near the ground, and decrease temperatures through evapotranspiration. Understanding how microclimate variability is influenced by canopy structure can therefore help us understand how forest understories will be influenced by climate change. Here, we report on a study at Mt Rainier National park (MORA, 46.8529° N, 121.7604° W), an old growth forest, conducted by instrumenting custom micro-metstations in two summers (2018, 2019) to collect fine scale microclimate data (temperature, wind and solar radiation) along an elevational gradient (650 – 1450 m). We hypothesized that canopy structure and height from the forest floor would influence the extent to which understory microclimates are decoupled from regional climates (i.e. buffering).
Results/Conclusions
Our findings suggest that understory microclimates are influenced by elevation and canopy structure as expected; with mean temperatures that are cooler at higher elevations and under dense canopies. However, we also found more complex drivers of microclimate variability. For example, diurnal variation (daily max – min temperature) was higher in gaps than under dense canopies. Additionally, diurnal variation in temperature varied according to height above the forest floor, with temperatures at or just above the ground surface experiencing lower diurnal variability in temperature than at 1 meter height above the forest floor. In all, our research suggests that the microclimate seedlings experience will depend not only on macroscopic topographic features (e.g. elevation), but also on the finer-scale effects of canopy structure and seedling size. More broadly, we believe that considering the role of canopy in influencing microclimate can help elucidate the complex effects of climate change on regeneration in old growth forests.