Shrub species richness decreases negative impacts of drought in a Mediterranean ecosystem

AIMS: Global change projections predict biodiversity changes and increased drought intensity in the Mediterranean Basin in the decades to come. This will presumably affect plant performance, plant community composition and ecosystem functioning. Shrublands are widespread in the Mediterranean area, yet we have no clear understanding of shrubland responses to the combined effects of increased drought and changes in plant diversity. LOCATION: Natural Mediterranean shrubland, Marseille, south France. METHODS: A rainfall manipulation experiment was conducted to examine the performance of four dominant shrub species in response to increased drought. The study assemblages spanned a gradient of four shrub richness levels (mono‐, bi‐, tri‐ and tetra‐specific plant mixtures) and 15 different shrub combinations. Throughout the 2 yr of non‐destructive experiments (2012–2014), we measured plant litter fall, stand necromass and plant cover of the four dominant plant species: Cistus albidus, Quercus coccifera, Rosmarinus officinalis and Ulex parviflorus. RESULTS: Increasing shrub richness levels in the plant community led to higher litter production by all four species and a decrease in dry shoots for Quercus and Ulex. Shrub species responses were affected by the rain exclusion treatment, but the effects were mainly dependent on shrub richness level. In mono‐specific plots, aggravated drought conditions led to a decrease in litter fall of Cistus, Rosmarinus and Ulex and an increase in necromass of all four species. In general, these negative drought effects disappeared as number of shrub species in the community increased. Under rain exclusion treatment, Rosmarinus cover decreased in mono‐specific plots whereas Quercus cover increased in bi‐specific plots and Rosmarinus cover increased in tetra‐specific plots. We observed two drought tolerance patterns, with Quercus and Rosmarinus being more tolerant species and Ulex and Cistus less tolerant. CONCLUSIONS: The magnitude of drought effects on shrub performance is species‐specific, and plant species combination‐dependent. High shrub richness levels modulate the negative impacts of aggravated drought conditions. Our results point to a probable shift of interspecific relationships in response to water shortage. As drought impacts are not mediated in low‐diversity communities, species‐specific responses to drier conditions could lead to shifts in plant community composition favouring the most drought‐resistant species, i.e. Quercus and Rosmarinus. Maintaining high diversity appears critical to mediate drought effects for less resistant species (Cistus and Ulex in our study).