Drought legacy effects on post-fire vegetation dynamics in a Mediterranean shrubland of Central Spain

Post-fire regeneration often occurs under water scarcity conditions in Mediterranean ecosystems. This fact is likely to be more frequent in the future, as climate change projections for the Mediterranean show increased temperature and decreased rainfall, as well as higher fire weather danger. Therefore, studying how vegetation responds to fire and drought is critical for anticipating vegetation vulnerability to global warming. Here, we present the results of a manipulative field study in a Mediterranean shrubland of Central Spain in which 4 drought treatments were imposed before and after carrying out an experimental burning of the treated plots: natural rainfall, historical rainfall regime (2 months summer drought per year), moderate drought (ca. 25% rainfall reduction from historical records, 5 months drought), and severe drought (ca. 50% rainfall reduction, 7 months drought). We monitored the plant community during the first 4 years after fire under the drought treatments followed by additional 6 years once such treatments had ceased. We found that drought significantly reduced density, cover, and size of seeder species during the first post-fire years under treatment. In contrast, resprouter species were virtually unaffected. As a whole, the changes in woody species dynamics resulted in a community ‘herbalization’, which was richer and more diverse, but probably more prone to a new fire. Moreover, we found that the drought effects on the community, and especially on the seeders, were maintained various years after all plots started receiving natural rainfall. 10 years after the fire, the legacy effect of the drought had been diluted, although some effects on plant density or cover in certain seeder species were still present. Overall, this means that what happens during the first few years after fire is extremely important for vegetation recovery, and that the effects of a long drought can persist over various years, and likely over a whole fire cycle.