Distribution, trade-offs and drought vulnerability of a high-mountain Pyrenean endemic plant species, Saxifraga longifolia

High-mountain ecosystems are biodiversity hotspots, providing habitats that shelter a great variety of unique species. Climate change is increasing the severity and recurrence of drought events, potentially endangering many species. We studied the distribution of a monocarpic perennial plant, Saxifraga longifolia and evaluated altitudinal changes in leaf physiological performance, with a particular emphasis on trade-offs and drought vulnerability in nine natural populations. Drought vulnerability of the studied populations covering almost the entire distribution range of the species was considered taking into account both ecological and physiological markers. The distribution of S. longifolia outside the Pyrenees was limited to the high eastern and North-west mountains of the Iberian Peninsula, constituting geographically highly isolated populations. We found that trade-offs between the activation of defense responses against abiotic and biotic stresses occur in this species, with populations occurring at high elevation showing less sensitivity to drought stress but higher endogenous contents of biotic defensive compounds. Furthermore, drought stress appeared to be a key factor in the distribution of this species, with populations growing at lowest elevations being the most exposed to the potential negative effects of climate change. It is concluded that, despite S. longifolia has developed complex mechanisms to adapt to the harsh environmental conditions of high-mountain ecosystems, global change is threatening its survival, most particularly in the regions most exposed to drought events.