COS 218-2 - Effect of invasive species on the Woolsey Fire: Monitoring monthly post-wildfire non-native and native vegetation succession

Wednesday, August 9, 2023

3:45 PM – 4:00 PM PDT

Location: E141

Presenting Author(s)

RR

Richard M. Rachman

Boise State University, United States

Coauthor(s)

JC

Joey Curti

University of California, Los Angeles, United States
Ct

Casey terHorst, PhD.

Associate professor
California State University, Northridge, United States
HS

H. Bradley Shaffer

UCLA, United States

Abstract: Climate change and anthropogenic disturbance are driving the spread of invasive plant species. Disturbed shrublands in southern California with high abundances of invasive species have been shown to have an increased frequency of wildfires but decreased burn severity. Shrublands are a heterogenous patchwork of species composition and fire history, and measuring the burn severity using low-resolution remotely sensed imagery could be inaccurate. The Santa Monica Mountain National Recreation Area (which abuts Los Angeles and is the world's largest urban national park unit) burned in the largest wildfire in its history in November 2018. This wildfire presented an opportunity to study the succession of native and non-native plants and their relationship to various aspects of wildfire, such as burn severity. In February 2019, we found average remaining twig thickness in the burned area indicated lower burn severity in vegetation communities with more non-native and species ranked as invasive by the California-Invasive Plant Council (Cal IPC). Conversely, we showed higher burn severity in vegetation communities with fewer non-native and invasive individuals. The most recently burned areas had the highest abundance of non-native species. Species richness was highest in areas with the most frequent burns and shortest average fire return interval. Transects with the highest native plant species richness and most frequent burns had a higher abundance of non-native species and fire-adapted fire followers. We examined plant phenology from February to May of 2019 and found that native plants only weakly changed in abundance over time. Non-native species, annual grass species, and invasive species abundance significantly changed over the four months, increasing from February to March, then decreasing into May. Transects differed significantly from one another spatially and temporally in terms of species abundances, but no specific species were driving differentiation in transects. Our categories for species such as non-native or invasive, also differed spatially and temporally, dependent on the transect. Type conversion, such as chaparral converting to annual invasive grassland due to frequent disturbance, was only potentially observed in our very small time frame of sampling, demonstrating the value of long-term vegetation monitoring before and after wildfires to understand these successional dynamics better. Management of invasive plants in already degraded vegetation communities during the first and second growing season months following a wildfire could be an essential way to reduce the seedbank of invasive plants and minimize the negative effects of invasive species on future wildfires.