Zomer_et_al_CO2

Post-fire plant recruitment is occurring in conditions of increased drought and elevated atmospheric CO₂ (eCO₂), and thus post-fire seeder species, those that recruit from a persistent seedbank after fire, are becoming especially vulnerable. eCO₂generally enhances photosynthesis and thereby plant growth and water demand, but may also ameliorate plant stress during drought via reduced stomatal conductance and consequently, reduced water loss. Our current understanding of how eCO₂ modifies the drought response of post-fire seeders in Mediterranean shrublands is scarce. Some of these species also have the ability to resprout after fire, a trait that is related with increased belowground allocation of resources. We hypothesized that the ability to resprout would thus be a key trait modifying the effect of eCO₂ on plant resource allocation and water-use during drought. We grew seedlings of five post-fire seeders (two resprouters and three non-resprouters) in ambient and eCO₂, and subjected them to a three phase, progressively severe, drought. Our results show that eCO₂ had direct effects on plant characteristics and resource allocation, reducing SLA and increasing above and belowground biomass for all species, as well as reducing root-to-shoot ratios for resprouting plants. During drought, eCO₂did not ameliorate plant drought stress or modify plant water-use. Rather, the ability to resprout was a significant determinant of plant drought response, even in eCO₂, in that non-resprouting species tolerated better mild droughts, but resprouting species maintained higher water potentials during severe droughts. In conclusion, it is unlikely that eCO₂ will buffer drought stress of post-fire seeders in future climate conditions, and could increase drought vulnerability by enhancing plant size and water demand, particularly for resprouting seeders. Our results highlight the importance of considering differences across growth forms, but also the variability of traits within them when predicting future patterns of biodiversity in a drier and fierier world.