Drought response of fire-adapted Mediterranean shrubs under elevated CO2.

The first summer after fire poses a high risk of drought mortality for species that rely on postfire recruitment for their persistence in fire-prone ecosystems (postfire seeders). However, little is known about how future conditions of elevated atmospheric CO2 (eCO2) and its interaction with severe drought will affect this key life history strategy in Mediterranean shrublands. We investigated the extent to which eCO2 modifies plant traits and resource allocation, as well as how eCO2 and drought intensity interact to modify plant water status, water fluxes and hydraulic behavior in 8-month-old seedlings of five seeder shrub species. Among the studied species, three regenerate postfire exclusively by seed (non-resprouting seeders), while two can also resprout after fire (resprouting seeders). We found that eCO₂ significantly reduced specific leaf area for all species and increased above- and belowground biomass for three species but had no species-specific effects on stomatal density or root-to-shoot ratios. When species were grouped by resprouting ability, eCO2 reduced root-to-shoot ratios in resprouting species but not in non-resprouting species. Importantly, eCO2 did not affect plant water relations under drought either at the species level or when grouped by resprouting ability. However, resprouting and non-resprouting seeders exhibited distinct water-use strategies under drought, independent of CO₂ treatment. Overall, eCO₂ is unlikely to mitigate drought stress for postfire seeders in future climates. Considering functional differences across life history strategies, as well as the variability of traits within them, is key to predicting future patterns of biodiversity in a drier and more fire-prone world.