Sub-annual Fluctuations in Water Sources Utilised by Mediterranean RiparianTrees Determined Through Highly Resolved Oxygen Isotope Analysis of Tree-ring Cellulose

Abstract:

The sensitivity of trees to water availability within their rooting zones is a major determinant of tree and forest health. Yet, we have a poor understanding of subterranean water availability and its fluctuations due to climate. Such shortcomings limit our ability to predict how climatic variability will impact water availability to trees, and corresponding forest health. Understanding of water partitioning within the ‘critical zone’ of riparian areas are particularly lacking, especially in the vulnerable Mediterranean climate regimes. A substantial body of research uses isotope dendrochronology to assess riparian forest-water relations at annual (tree-ring) timescales, which integrate variability in seasonal hydrology. However, the sub-annual variations in water availability have been largely overlooked, which may have important ramifications for riparian ecohydrology.

We present a new method for determining the sub-annual hydrologic variability within a floodplain forest using two co-occurring Mediterranean tree species along the Rhône River, southern France. We conducted oxygen isotope (δ¹⁸O) analysis of cellulose for 11 microslices within each tree ring to detect sub-annual patterns in δ¹⁸O that reflect the variability in hydrological partitioning. We back-calculated the seasonal time series of source waters used by the trees via a mechanistic model. Differences in rooting between the species allow us to constrain fluctuations in water availability and use between the vadose and phreatic zones. The two different species of streamside trees use distinct water sources and their seasonal patterns of water use are also fundamentally different. We develop strong links between these sub-annual patterns of δ¹⁸O signatures and the climatic characteristics of the hydrological year. We also present isotopic analyses of source waters from the vadose and phreatic zones, precipitation, and the Rhône to bolster our interpretations of water partitioning.

This research provides new insight for disentangling the impacts of seasonal hydrologic variability on ecohydrological processes both in the past and under future climate regimes. These sub-annual fluctuations within the critical zone are likely to have a disproportionate influence on the response of the biosphere to such climatic trends.