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Wildfire impacts on forest microclimate vary with biophysical context

Citation

Wolf, Kyra (2021), Wildfire impacts on forest microclimate vary with biophysical context, Dryad, Dataset, https://doi.org/10.5061/dryad.47d7wm3c6

Abstract

Increasing 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 plots 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, plots burned at high severity had 3.7 °C higher daily maximum temperatures and 0.81 kPa higher daily maximum VPD relative to paired unburned plots. Differences between burned and unburned plots 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 post-fire, 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 post-fire establishment of tree seedlings and understory vegetation. These effects depend strongly on biophysical context, with cool-wet forest more vulnerable to fire-caused changes in microclimate compared to warm-dry setting. Our results further highlight the functional importance of standing dead trees for moderating surface temperature in post-fire 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.

Methods

Near-ground microclimate conditions (temperature and relative humidity) were monitored in two wildfires which burned in 2017 in northern Rocky Mountain conifer forests. Microclimate was monitored over June-Sept. 2018 and 2019 at 33 sites spanning gradients in local water balance (via elevation and aspect) and fire severity, including 11 unburned sites. Field data describing site characteristics and fire severity were also collected, including ground cover of bare ground, litter, and understory vegetation; scorch height; distance to seed source; tree mortality; live and dead canopy cover; and live and dead basal area. Site climate information derived from gridded datasets are also reported.

Microclimate data were quality-checked by visually examining values with >5 degrees Celsius change within a half-hour timestep, with less than 0.02% of data excluded from analysis. Microclimate data were collected using two sensor models, and a laboratory calibration was conducted to ensure comparability among measurements. Microclimate data were used to calculate vapor pressure deficit (VPD) and aggregated to daily maximum temperature and VPD. See Wolf et al. (2020) Ecosphere for detailed methods. 

Usage Notes

Please reference the following files:

README_for_Wolf_et_al_2020_Ecosphere_Data

README_for_Wolf_et_al_2020_Ecosphere_Code

Funding

Joint Fire Science Program, Award: 18-1-01-53