Fire increases the reproduction of the dominant grass Brachypodium retusum and Mediterranean steppe diversity in a combined burning and grazing experiment

Questions: What are the effects of fire and grazing on vegetative growth and sexual reproduction of Brachypodium retusum, the dominant species of Mediterranean Thero-Brachypodietea grasslands (“steppes”) and on species composition of the associated plant community? Location: “La Crau”, South-Eastern France. Methods: We set up a split-plot experiment testing the effects of fire season (winter, summer) and grazing on plant cover, inflorescence number, seed set and germination of B. retusum in traditionally grazed grasslands. We further analyzed plant species composition and diversity in vegetation releves. The same fire treatments were tested in a second experiment in long-term grazing exclosures. Results: B retusum showed a rapid post fire recovery, but other species of the associated plant community, in particular annuals, recovered as fast or even faster. Fire increased B. retusum inflorescence production and seed set per inflorescence and this effect was stronger in the summer fire treatment. At community level, fire significantly increased species richness, evenness and Bray-Curtis dissimilarity in the second post-fire season and again, the summer fire effect was stronger. Grazing exclusion for two seasons had only a small effect on B. retusum and the associated plant community. The effect of both fire treatments on B. retusum was similar in long-term exclosures. In these exclosures, fire resulted in a community shift towards the species composition of traditionally grazed steppes. Conclusions: The concomitant positive effects of experimental burning on B. retusum reproduction and on plant diversity of Mediterranean steppe vegetation suggest that the system is adapted to fire as an important driver of community composition. Annual species surviving as seeds are as successful in post-fire recovery as perennial resprouters. Prescribed burning may be an alternative strategy to restore community structure in abandoned (ungrazed) steppes showing a decline in typical grassland species after several years of grazing abandonment.