Latitudinal variation in the functional response of Quercus suber seedlings to extreme drought

Many plant species are being threatened by increasingly drought conditions due to current climate change at planetary scale. This global trend is leading to the scientific community to investigate the potential role of local adaptations through intraspecific differences in functional traits that may boost conservation strategies by modulating the plant responses to reduced water availability. We assessed under controlled conditions the effect of four different drought intensities on the survival time and morphological traits of Quercus suber seedlings collected from nine populations covering the complete latitudinal distribution of the species. Functional morphological traits related to biomass allocation and leaf and root display were analyzed. We then related these traits with the survival time after a terminal desiccation, used as a drought-resistance proxy and expressed as survival time without watering. Abundant watering availability allowed seedlings to survive for a longer period compared to drier conditions. Further, all morphological traits differed across watering levels, showing a very plastic response. Acorns from southern latitudes produced very large seedlings compared to those gathered from northern latitudes. However, the larger biomass implied higher evaporative water loss, inducing lower survival of southern populations under extreme drought conditions. We further found a clear trend toward maximizing those traits related with belowground growth (i.e., root surface area, root average diameter and root volume) in southern populations aimed to increase water uptake, overcoming the most limiting factor for plant growth in that area. Our results support that increased root development allow cork oak to maintain its functioning after being subjected to damage caused by reduced water availability, whereas high aerial biomass allocation is a handicap for survival under drought stress conditions. This study identifies drought-resistant populations and morphological traits related to drought resistance, which can be applied to improve restoration actions under a warmer climate.