Drought-induced mortality of an endemic leaf succulent, Argyroderma delaetii, in a South African biodiversity hotspot

The biodiversity of the Knersvlakte, located in the Succulent Karoo biodiversity hotspot of South Africa, is predicted to experience an increase in severe drought events during this century. This region harbours a high diversity of endemic leaf succulents and it is unclear whether these will be resistant or vulnerable to drought as experimental and observational studies for this are few and contradictory. This study quantifies the effect of a recent drought in the Knersvlakte on the survival of juvenile and reproductively mature individuals of a quartz plain-adapted leaf succulent, Argyroderma delaetii. The progression of the drought and an experimental watering treatment was quantified using a Standard Precipitation Index from May 2017 to May 2018. The effect of the drought and the watering treatment on the survival of A. delaetii was determined by monitoring individuals using repeat photography for one year. The mortality of individuals in each treatment and life stage was derived from the photographs and a comparative survival analysis was used to quantify the threat posed by the drought conditions. The results indicate that the Knersvlakte experienced severe drying during the 2017 drought in relation to historical records and a median rainfall year. This decreased A. delaetii survival probability three-fold when compared to mortality in a median rainfall year. Mortality was likely driven by an interplay between carbon starvation and hydraulic failure in the leaf succulent due to leaf water reserves not being replenished by the reliable winter rains of the Succulent Karoo, to which this species is adapted. The larger, reproductively mature individuals were more resistant to drought-induced mortality than the smaller juveniles, likely because the mature individuals are significantly larger than juveniles with higher water storage capacity and reduced risk of passing critical osmotic thresholds, allowing for maintained photosynthesis through the drought.