Water source partitioning among plant functional types in a semi-arid dune ecosystem

Abstract

Question(s): The aim of this work was to characterize the main water sources used by the woody plant community of a semi-arid coastal dune ecosystem. We ask: Do the seasonal water sources used by plants differ between functional types? Does the seasonal drought promote shifts towards deeper soil layers in all plants? Does greater use of deep soil water enables the maintenance of a more favorable plant water status? Do water-sources-use strategies mirror overall drought strategies?. Location: Semi-arid coastal dune system in Doñana Biological Reserve, southwest Spain. Methods: We analyzed the oxygen isotope composition of xylem water of fourteen coexisting woody species and compared it to water sources (soil water at different depths, rain and groundwater), both in spring and dry summer. Bayesian isotope mixing models were used to estimate the proportion of each water source used by plants. We tested the influence of different (categorical) traits on plants’ water-source-use. We evaluated the relationships between the use of deep soil water and ecophysiological parameters related to water economy (leaf δ13C and reflectance water-index). Results: A greater similarity between species was found in periods of higher water availability (spring). Contrastingly, during the dry period the traits considered showed a significant effect on the water sources used and a high inter-specific differentiation was observed. Accordingly, species clustered in five water-use functional groups, exploring water from shallower to deeper soil layers. Greater use of deep soil layers in the dry season was linked to a better seasonal maintenance of plant water status. Conclusions: Coexisting plant functional types segregated along a spectrum of water-source-use under extremely dry conditions, evidencing great soil water partitioning. Relevant seasonal water-use shifts towards deeper soil layers were observed, but not in all species. Furthermore, we confirmed that the water-source-use behavior accompanied broader strategies of drought resistance.