Plant mortality on ultramafic soils after an extreme heat and drought event in the Mediterranean area

Plant mortality associated with the growing frequency and intensity of heatwaves and dry spells is increasingly occurring in various ecosystems worldwide. However, the impacts of extreme events on plant communities of ultramafic soils are still unknown. Here, we describe a first case of plant mortality on serpentine outcrops in Italy following a heat and drought event that occurred between autumn 2016 and summer 2017. Mortality of perennial plants was assessed in thirty plots representing ten major serpentine areasalong a SW-NE gradient, with the sparse vegetation of ultramafic soils. Statistical modelling was based on temperature and precipitation data and local site conditions. Responses of single species were largely different and not related to their taxonomic osition, growth form or level of edaphic specialization for serpentine soils. Notably, obligate serpentine endemics were not less impacted than non-endemics. As expected, species mortality decreased with increasing chromosome number, suggesting higher tolerance and adaptive capacity in taxa of likely polyploid origin. Mortality increased with decreasing number of rainy days and duration of the heatwave and decreased with increasing distance from the Tyrrhenian coastline, reaching 60% in the southern and western areas with a Mediterranean climate. Local site conditions such as increasing soil depth and north-facing aspect significantly increased plant survivorship. Our findings show that extreme drought and heat can have a previously unrealized impact on ultramafic plant communities in the short term. Further studies should examine the recovery capacity and resilience of serpentine plants, together with the long term effects.