Restoring for the present or restoring for the future: enhanced performance of two sympatric oaks (Quercus ilex and Quercus pyrenaica) above the current forest limit

Reforestation is common to restore degraded ecosystems, but tree-species choice often neglects ongoing environmental changes. We evaluated the performance of planted seedlings of two oak species at two sites in a Mediterranean mountain (Sierra Nevada, SE Spain): one located within the current altitudinal forest range (1,600-1,760 m), and one above the upper forest limit (1,970-2,120 m). The forest service planted 1,350 seedlings of the deciduous Pyrenean oak and the evergreen Holm oak in a postfire successional shrubland. After 2 years, seedlings were monitored for survival, and a subset of 110 Pyrenean oaks and 185 Holm oaks were harvested for analyses of biomass and foliar nutrient status, C-13, and O-18. Both species showed the highest survival and leaf N status above the upper forest limit, and survival increased with altitude within each plot. The deciduous oak benefited most from planting at higher altitude, and it also had greater biomass at the higher site. Correlations between foliar N, O-18, and C-13 across elevations indicate tighter stomatal control of water loss and greater water-use efficiency with increasing plant N status at higher altitude, which may represent a so-far overlooked positive feedback mechanism that could foster uphill range shifts in water-limited mountain regions. Given ongoing trends and future projections of increasing temperature and aridity throughout the Mediterranean region, tree-species selection for forest restoration should target forecasted climatic conditions rather than those prevailing in the past. This study highlights that ecosystem restoration provides an opportunity to assist species range shifts under rapidly changing climate.