Forest resilience to climate change and disturbance depends strongly on seedling establishment. Seedlings are especially vulnerable to climate change because they have narrower climatic tolerances than adults. Bioclimatic models that ignore the moderating effects of forest cover may misrepresent the impacts of climate change on tree regeneration. Here, we quantify the moderating effect of tree canopy cover on microclimate conditions experienced by seedlings and saplings, at six sites spanning climatic gradients across the northwestern US. At each site, we recorded hourly temperature and relative humidity for three growing seasons, at 10 cm and 2 m above the ground, in a clearing and at six surrounding paired plots with varying levels of canopy cover. We used the difference in temperature and vapor pressure deficit (VPD) between the clearing and the paired plots as a measure of the impact of canopy cover on microclimate. Additionally, we examined how microclimatic buffering varied with solar radiation and soil moisture using linear mixed effects models.
Results/Conclusions
Tree canopy cover strongly moderated microclimate; at 10cm above the ground the effect was more than twice that at 2 m above the ground on average. The moderating influence of forest canopy on daily temperature and VPD was best explained by the level of canopy cover, solar radiation, and their interaction. On average maximum daily temperature was 7 °C cooler, minimum daily temperature was 4 °C warmer, and maximum daily VPD was 2 mPa lower under high canopy cover than in openings near the ground surface. Microclimatic buffering at a site depended on the interaction between canopy cover and soil moisture, with limited to no buffering at drier sites and increased buffering at sites with greater soil moisture and evaporative demand. Our findings shed light on the importance of climatic context, and particularly the water balance, for understanding potential buffering of climate change impacts within forests. They also highlight forest disturbances that reduce canopy cover as mechanisms that can catalyze ecosystem transitions in western US forests.