Native and non-native plant regrowth in the Santa Monica Mountains National Recreation Area after the 2018 Woolsey Fire

Mediterranean-type ecosystems are under chronic ecological stress. By assessing changes in plant species and functional groups across the landscape, identifying ecosystem degradation is possible. The Santa Monica Mountains are in close proximity to a densely populated urban area where non-native species invasions are being driven by changing fire regimes, climate change, and anthropogenic disturbances. Non-native growth impacts biodiversity levels and vegetation distributions of native plant communities that are critical for ecosystem health. This study uses analyses of line-point data from the National Park Service Inventory and Monitoring Program to assess ecosystem health in the Santa Monica Mountains from 2014 to 2020 and, in particular, to assess the effects of the 2018 Woolsey Fire on the balance of native and non-native species in the region. Results of this analysis show an increase in non-native cover since 2014 and rapid regrowth of non-native annual grasses and herbaceous cover after the 2018 Woolsey Fire while native communities regrew more slowly. A hotspot of non-native cover was identified in the Northern Simi Hills region, and rapid regrowth was seen after the Woolsey Fire. The hotspot is dominated by non-native annual grasses and annual herbaceous species, some of which returned to prefire populations within a year after the Woolsey Fire. These results raise concerns for the future of native vegetation composition and function throughout the park and highlight the damage densely populated non-native plant communities accumulate in the wake of disturbance events.

Automated summary

Mediterranean-type ecosystems are facing ecological stress, with non-native species invasions driven by fire regimes, climate change, and human disturbances. This study assesses ecosystem health in the Santa Monica Mountains from 2014 to 2020, focusing on the effects of the 2018 Woolsey Fire. The analysis shows an increase in non-native cover and rapid regrowth of non-native species after the fire, raising concerns for the future of native vegetation and highlighting the damage caused by densely populated non-native plant communities.