Fire severity and prolonged drought do not interact to reduce plant regeneration capacity but alter community composition in a Mediterranean shrubland

Abstract

Disturbance-regime shifts are often a manifestation related to climate change. In Mediterranean ecosystems, summer-drought lengthening and high fire-severity may be among the most detrimental processes for plant recovery capacity. However, although isolated effects have been usually assessed, the effects when both disturbances interact are less known. This paper examined the possible interactive impacts of increased fire severity and summer-drought lengthening on Mediterranean vegetation recovery. Our initial hypothesis maintained that both disturbances would interact and cause greater recovery damage than in an isolated way. For this reason, we performed an experimental fire in summer by creating two fire severity levels: control fire severity (CSev) and increased fire severity by adding dry biomass (IncrSev). Following fire, and using rainfall exclusions roofs, we extended summer drought conditions toward the first autumn after fire (AutExcl treatment) and toward the first post-fire spring (SprExcl). All the treatment-levels combinations were replicated in five 0.5 × 0.5 m plots. Emerged seedlings were not affected by treatments, but plant establishment was significantly impaired by extended droughts at the end of the first post-fire year, particularly for Cistaceae and subshrubs. Nevertheless, we found no effects of increased fire severity on either isolation or combination with drought. Notwithstanding, the combination of some treatment levels brought about changes in plant composition. These changes were driven mainly by the detrimental effects of treatments on perennial forbs. This functional group was affected by treatments, which suggests that they may be more sensitive to changes in fire severity and severe droughts. Increased fire severity might not affect plant recovery either by itself or by interacting with drought because prolonged drought may mask increased fire severity impact on Mediterranean seeding species. However, fire-severity increases, together with sporadic drought events in the early stages of these communities, could imply long-lasting changes in community composition due to distinct functional-groups sensitivities. Nevertheless, these impacts depend on the considered species or functional group. These findings provide information about the impacts that Mediterranean-shrublands ecosystems might face if the trends of fire and drought regimes continue shifting.