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

Questions Global change projections predict biodiversity changes and increased drought intensity in the Mediterranean Basin in the decades to come. This will presumably affect plant performances, 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 of France (43°22′ N; 5°25′ E). Methods A rainfall manipulation experiment was conducted in a Mediterranean shrubland in southern France to examine the performances of four dominant shrub species in response to increased drought. The study assemblages spanned a gradient of 4 shrub richness levels (mono-, bi-, tri-, and tetraspecific plant mixtures) and 15 different shrub combinations. Throughout the two years of non-destructive experiment (2012-2014), we measured plant litterfall, stand necromass and plant cover of the 4 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 4 species and a decrease of 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 monospecific plots, aggravated drought conditions led to a decrease of litterfall of Cistus, Rosmarinus and Ulex and an increase of necromass of all 4 species. In general, these negative drought effects disappeared as number of shrub species in the plant community increased. Under rain exclusion treatment, Rosmarinus cover decreased in monospecific plots whereas Quercus cover increased in bispecific plots and Rosmarinus cover increased in tetraspecific 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 could modulate the negative impacts of aggravated drought conditions. Our results pointed to a probable shift of interspecific relationships in response to water resource 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 favoring the most drought-resistant species, i.e. Quercus and Rosmarinus here. Maintaining a high diversity level appears critical to mediate drought effect for less resistant species (Cistus and Ulex in our study). This article is protected by copyright. All rights reserved.