Growth and water use performance of four co-occurring riparian tree species in a Mediterranean riparian forest

Mediterranean riparian zones act as vegetation shelters for several deciduous tree species at the edge of their bioclimatic distribution, e.g. alder (Alnus glutinosa), black poplar (Populus nigra) or ash (Fraxinus excelsior). Current global warming and human induced disturbances may worsen their growing conditions. Under such circumstances, black locust (Robinia pseudoacacia) is outcompeting autochthonous tree species. Here, we provide evidences of black locust better growth and water use performance than alder and ash. We compare the temporal and spatial patterns of transpiration and the stem basal area increments of alder, black poplar, common ash and black locust, all of them co-occurring in a mixed riparian Mediterranean forest. Black locust presented the lowest transpiration values per basal area unit (4.0 mm·m−2·growing season−1). Although tree transpiration was mainly driven by energy availability instead of water, ash transpiration was constrained by water availability at soil water contents below 0.08 cm3·cm−3. Black locust was the only tree species growing all over the water availability gradient present in the study site, and it did not present any significant difference in sap flow values across this gradient. Furthermore, black locust and black poplar were the species with higher growth-based water use efficiency (5.4 g·cm−1·m−3 and 3.6 g·cm−1·m−3, respectively); ash and alder were the less efficient ones (2.8 g·cm−1·m−3 and 1.9 g·cm−1·m−3respectively). The good performance of black locust is relevant to understand its great successful invasion of Mediterranean riparian forests, particularly after human-induced disturbances, as forest management.