Wildfire impacts on forest microclimate vary with biophysical context

AbstractIncreasing wildfire activity in western North America has the potential to remove forest canopy cover over large areas, increasing the vulnerability of understory plants and juvenile trees to microclimatic extremes. To understand the impacts of wildfire on forest microclimatic buffering, we monitored daily temperature and vapor pressure deficit (VPD) in 33 plots over the first two growing seasons following two wildfires from 2017. The Lolo Peak and Sunrise fires occurred during a regionally extensive fire season, burning mixed‐conifer and subalpine forests across complex mountainous topography in western Montana. Sensors were deployed from June to September in 2018 and 2019 in sites stratified by aspect, elevation, and fire severity (unburned, moderate, high) to capture a range of forest types, biophysical contexts, and fire effects. Loss of canopy and understory vegetation had marked effects on microclimate: On average, sites burned at high severity had 3.7°C higher daily maximum temperatures and 0.81 kPa higher daily maximum VPD relative to paired unburned sites. Differences between burned and unburned sites were most pronounced when ambient temperatures were high, across diurnal and seasonal time scales. Differences were also more pronounced at sites with less canopy cover, more bare ground postfire, and greater long‐term water availability (i.e., low climatic water deficit). Our results reveal fire‐caused changes in microclimate extremes that are biologically meaningful for the postfire establishment of tree seedlings and understory vegetation. These effects depend strongly on biophysical context, with cool‐wet forests more vulnerable to fire‐caused changes in microclimate compared with warm‐dry settings. Our results further highlight the functional importance of standing dead trees for moderating surface temperature in postfire environments. Anticipating forest ecosystem responses to increased warming and wildfire activity, and the potential for fire to catalyze vegetation changes, thus requires considering the substantial impacts of fire on microclimate.