Skip to main content
2023 ESA Annual Meeting Main banner
Icon Legend
This session is not in your schedule.
This session is in your schedule. Click again to remove it.
Back

Contributed Talk

Session: : Fire/Fire Management 2

COS 106-2 - Herbaceous vegetation responses to experimental fire in savannas and forests

Tuesday, August 8, 2023
1:45 PM – 2:00 PM PDT
Location: E141
Abstract: Fire is an important process in savanna as well as forest ecosystems, but its effects on herbaceous vegetation across biomes remain unelucidated. In theory, fire-vegetation feedbacks maintain the vegetation structure of both savannas and forests. In savannas, a continuous layer of highly flammable herbaceous vegetation promotes frequent fires, which inhibit the maturation of juvenile trees. Meanwhile, tree cover in forests inhibits herbaceous vegetation accumulation, thereby limiting fire spread and maintaining the closed canopy. Accordingly, vegetation responses to prescribed fires should depend on the initial condition of the system as either a savanna or forest. Excluding fire in savannas should result in a decrease in herbaceous vegetation due to increased shading from woody vegetation that has been released from fire-induced topkill. In contrast, introducing more frequent fires into forests should induce weaker responses than in savannas because mild surface fires are insufficient to affect mature trees. Work at individual sites provides patchy support for these hypotheses, but the relative magnitude of responses is unclear and insights across studies are further obscured by differences in climatic factors. We synthesized results across studies to ask whether savanna herbaceous vegetation has greater responses to changes in fire frequency, what components of the herbaceous vegetation layer (e.g. grass vs. forbs, C3 vs. C4 plants, etc.) drive responses to fire, and if responses depend on climatic factors.

We examined herbaceous vegetation responses to experimental fire at 30 sites spanning four continents. Across a variety of metrics and climates, herbaceous vegetation generally increased in abundance where fire was applied. Biome had the strongest effect on the response of herbaceous vegetation to fire, with an estimated 4.8 (±0.4) times larger herbaceous vegetation response to fire in savannas than in forests. In particular, grass cover decreased with fire exclusion in savannas, largely via decreases in C4 grass cover, whereas changes in fire frequency had a relatively weak effect on grass cover in forests. Climate also mattered: for every additional 100mm in mean annual precipitation, the estimated response of herbaceous abundance to fire was 20 ± 3.9% larger, indicating that herbaceous vegetation in wetter sites was more responsive to fire. Responses were also larger in less seasonal and/or in cooler sites. These differential responses underscore the importance of fire for maintaining the vegetation structure of savannas and forests. Managed fire is important for conserving open savanna vegetation structure and preventing woody encroachment without necessarily leading to savannization in forests.