Stable isotope insights into dryland ecohydrology

Isotopic tracing of water sources for plants is an increasingly common method that supports insight into climatic controls on water availability to plants and their use of this available water, especially in water-limited environments where isotopic endmembers are distinct. Recent advances in this field of research have enabled characterization of annual and seasonal water use by plants, whose water sources vary in contribution along a continuum between groundwater (isotopically light) to infiltrated precipitation (isotopically heavy). Xylem samples are commonly used to characterize real-time uptake of water from roots, and they can be contextualized with respect to endmember water sources via sampling of root zone water, providing these endmembers are isotopically distinct. The time integration of seasonally varying water source usage results in the annually recorded isotopic signal recorded in tree ring cellulose for temperate trees and shrubs, which reflects the dominant water source used in the season of growth. This has enabled dendro-isotopic methods that are commonly used to reconstruct past climates (isotopically light = colder/wetter; isotopically heavy = warmer/drier). However, questions have arisen about the utility of these annually integrated dendro-isotopic signatures, given the strong seasonal variations of water use that are particularly pronounced in dryland ecosystems, including notable water source switching by plants.      

In our recent work, we have been pushing isotopic methods in new directions to better understand what plants can tell us about how climate affected hydrology across dryland regions, and about the associated plant responses. Drylands pose interesting research challenges, since water is typically the key limiting factor on dryland plant growth, and it is fundamental to the health, functioning, composition, distribution, and evolution of vegetation communities. In drylands, water availability to plants may vary dramatically across space and time, creating challenges for simple analyses of annual water use signatures. To aid the understanding of climatically-controlled ecohydrology in drylands, we have developed a new tool (ISO-Tool) based on established biochemical fractionation theory, which allows for back-calculation of water sources used for growth from tree-ring isotopes. This tool generates critical knowledge for evaluating dendro-isotopic signatures within the same reference frame as sampled endmember water sources, and it can be used for both annual and seasonal analyses of plant water use. We have also been working on a set of interdisciplinary metrics we call water stress indicators (WSIs), which support corroboration of information on climatic forcing, water availability, plant water uptake, and ecological health of terrestrial vegetation.   

Using these new methods, we have been able to identify important hydroclimatic gradients in water usage for the same species that reflect the local expression of climate into plant-available water. We have also begun to understand the whole continuum from climate forcing to root-zone water availability to tree growth to canopy health. We believe this broader continuum perspective is critical for tackling key ecohydrological questions especially in drylands, where we expect large variability in water availability across space and time.