Long-term drought promotes invasive species by reducing wildfire severity

Area of Science:

• Ecology
• Climate Change Science
• Fire Ecology

Background:

• Climate change increases drought, wildfire frequency, and invasive species.
• Drought impacts on post-wildfire native vegetation recovery are not fully understood, especially in non-forested ecosystems.
• Coastal sage scrub ecosystems are vulnerable to altered fire regimes.

Purpose of the Study:

• To test if reduced precipitation, fuel load, and fire severity convert native shrubs to invasive grasses in Southern California coastal sage scrub.
• To understand the role of drought in post-wildfire plant community recovery.

Main Methods:

• Utilized an existing rainfall manipulation experiment with three treatments: added water, ambient water, and reduced water.
• Measured plant community composition before and after the 2020 Silverado wildfire.
• Assessed fuel load, vegetation cover, and fire severity across rainfall treatments.

Main Results:

• Drought treatments reduced fuel load and vegetation cover, leading to decreased fire severity.
• Native shrubs had higher pre-fire cover in added water plots compared to reduced water plots.
• Post-fire, reduced water plots showed lower native plant cover and higher invasive grass cover compared to added and ambient water plots.

Conclusions:

• Fuel load significantly influences fire severity and ecosystem-scale plant community composition.
• Drought exacerbates the shift from native shrubs to invasive grasses following wildfire.
• Management strategies should prioritize reducing fire frequency and controlling invasive species to enhance coastal sage scrub resilience.
• Controlled burns are not recommended in these systems as they can promote invasive plant proliferation.