Water use partitioning of native and non‐native tree species in riparian ecosystems under contrasting climatic conditions.

One of the suggested mechanisms behind the success of non‐native plants in recipient ecosystems is competition avoidance with natives by means of different resource‐use strategies, such as deeper water uptake under dry conditions.We aimed at evaluating water source partitioning between native and non‐native tree species coexisting in central Spain floodplains; determining the dependency on drought stress of such water sources use; and assessing if the reliance on deeper water sources relates with physiological and growth performance.We assessed water uptake depth, leaf functional traits related to physiological performance and growth of native (Populus alba) and non‐native trees (Ailanthus altissima, Robinia pseudoacacia) coexisting in riparian forests under different drought conditions (drier, intermediate and wetter). We analysed δ2H and δ18O isotopes in xylem water and in soil water from top, mid and deep soil depths and determined the contribution of each water source to overall plant xylem water. Leaf traits related with resource use and secondary growth were assessed for each species.We found stronger differences between sites than between species, with all species taking more deep water in the driest site (~45% of the xylem water) than in the wettest (~15%). However, under drier conditions, species differences were significant for top‐soil water use, with R. pseudocacia withdrawing more superficial water (~22%) than A. altissima (~8%). These results indicate stronger water partitioning under drier conditions. Non‐native species showed a physiological strategy characterized by greater leaf N, water content, and enriched δ13C and δ15N values independently of the deep‐water uptake. However, a positive relationship between deep soil water use and such strategy was found for P. alba.We highlight that those native and non‐native species differences were more evident regarding physiological performance at leaf level than for deep‐water uptake or growth. Furthermore, our results suggest that differences in water sources used by coexisting species may increase under drier conditions. Read the free Plain Language Summary for this article on the Journal blog. [ABSTRACT FROM AUTHOR]