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Different responses to water availability and evaporative demand of four co-occurring riparian tree species in NE Iberian Peninsula: Temporal and spatial sap flow patterns

Abstract:

Mediterranean riparian forests are natural refuges for boreal and temperate origin's tree species as black alder (Alnus glutinosa L. Gaertn.), black poplar (Populus nigra L.), and ash tree (Fraxinus excelsior L.). These riparian tree species are vulnerable when exposed to water scarcity. Under climate change scenarios drier and hotter environments are projected, worsening the growing conditions of these tree species. Furthermore, the expansion of human promoted invasive tree species, as black locust (Robinia pseudoacacia L.), are entering these ecosystems and competing with the autochthonous species. We analyze sap flow per unit sapwood area (Js) patterns of alder, poplar, black locust and ash tree using Granier's thermal dissipation method in a mixed riparian species forest. Potential maximum Js is greater in ash and black locust than in alder and poplar. We found significant differences in species phenology, with a larger growing period for alder and poplar than for ash tree and black locust. Spatial patterns were determined by a gradient of water availability, at increasing distance from the river stream, which in turn affects species distribution. We find alder, poplar and black locust by the riverside, poplar and black locust at intermediate conditions and ash tree and black locust at the driest ones. Black locust is the only tree species capable to grow all over the water availability gradient zones and its Js does not show any significant difference between zones. In dry seasons, we found no negative Js responses for alder, poplar and black locust trees. Conversely, ash tree appears sensitive to soil water availability reductions, showing a decrease in Js during the summer period. Black locust appears to be a very effective invasive tree species performing well all over the monitored zones of the water availability gradient. © ISHS 2013.