LB 18-183 - Vegetation recovery one and three years post-fire in Southern California chaparral

Background/Question/Methods:

Wildfires are increasing in severity, size, and frequency across the Western US, particularly in California. With these increases, plant communities are experiencing new levels of disturbance relative to historic fire regimes, creating variable post-fire responses and recovery patterns. Chaparral shrublands are important harbors of biodiversity that provide many vital ecosystem services, including stabilizing soils and preventing runoff. As fire frequency in chaparral increases, land managers need better frameworks to study post-fire recovery and more information on how changes in fire regimes will impact chaparral community compositions. In order to better understand post-fire recovery on short time intervals, we examined how environmental characteristics and fire history influence the regrowth of chaparral vegetation after the 2017 Thomas Fire in Los Padres National Forest, CA. We surveyed vegetation species composition, canopy cover, and shrub density at 25 sites along 30 meter transects in 2018 and 2021 across burned areas of the Los Padres National Forest. We compared the impact that fire history (time since last fire, fire return interval, and condition class) and environmental predictors (elevation, distance from coast, slope, and aspect) have on these recovery measurements one and three growing seasons after the Thomas Fire.

Results/Conclusions:

We anticipated that vegetation plots with shorter fire return intervals, higher departures from historic fire intervals, and lower elevations will host lower proportions of shrub species and lower shrub density. From 2018 to 2021, we found an increase in average live canopy cover (36.6% to 62.04%), an increase in average vegetation height (13.07cm to 60.01cm), and a decrease in live shrub density (11.28 per m2 to 2.93 per m2) across both inland and coastal sites. These findings are consistent with patterns of early post-fire recovery in chaparral. We also found that time since last fire prior to Thomas fire may be associated with decreasing shrub density in 2021 across inland and coastal sites. Our next steps involve examining recovery based on more site characteristics and species functional type. This project will provide guidance for chaparral restoration and fire suppression projects in the future by suggesting target areas for conservation based on risk of low shrub recovery and lack of recovery’s impact on debris flow potential and other ecological concerns. The project will also help determine how soon after a fire sampling should be undertaken to assess areas that may need restoration to promote chaparral regrowth.