Mechanisms of fire-maintained plant species diversity in species-rich wet pine savannas

Temperate savannas and grasslands maintained by frequent, low-intensity disturbances such as fire contain among the most species-rich plant communities in the world. Precisely how these disturbances maintain such high fine-scale diversity is poorly understood. This study examined the effects of the frequency of simulated fire (clipping combined with litter removal) and the relative importance of recruitment and survival on species diversity and trait and species composition at each of two pine savannas in southeastern Mississippi (USA) that had not been recently burned. Ten 2 x 2 m plots at each site were clipped/cleared annually from 2014 to 2019 and again in spring 2021 (annual frequency). The other 10 clipping plots were not clipped from 2018 to 2020 (reduced frequency). Vegetation in small subplots in annual frequency and reduced frequency plots was compared in August 2021 to test the effects of a short period without clipping on diversity and composition. To test the relative importance of recruitment and survival on diversity and composition, four 0.25 x 0.25 m quarter plots were established within each of 10 annual frequency plots per site following a clipping treatment in fall 2019 and assigned a 2 x 2 factorial arrangement of transplantation of sods from long-unburned areas and herbicide application. Reducing the frequency of clipping reduced plant diversity and altered composition at both sites. A comparison of diversity and trait composition responses to transplant and herbicide treatments revealed how recruitment and survival combined to affect species diversity. Partial or complete recovery of diversity following clipping and litter removal at both sites was driven by rapid increases in short-lived, resilient species that show fire-stimulated emergence from a seed bank and the persistence of long-lived species capable of surviving the prolonged period without fire or clipping. Species with reduced resilience and persistence were more likely to be lost in the reduced frequency treatment. Results are consistent with a model of short-term coexistence of maximum species diversity maintained by the most frequent fire regime fuels will permit.