The application of highly resolved tree-ring isotopes for reconstructing the ecohydrological interactions within riparian forests

Abstract

The research conducted within this thesis aims to advance our understanding of plant-water interactions, specifically the identification of the source(s) of water available for tree growth within riparian zones. Through highly resolved tree ring isotopic analyses, it is possible to deconvolve the δ¹⁸O signature of water utilised by a tree during the growing season and relate this to dynamic hydroclimatic processes. In doing so, this methodology allows for the reconstruction of the seasonal evolution of tree source water availability for the historical period. The utility of this method is initially demonstrated for the determination of the isotopic signature of water utilised by individuals of co-occuring riparian tree species over ten consecutive years. It is found that species-specific seasonal source water variability is indicative of tree rooting depth and the hydroclimatic partitioning of water to each of the floodplain hydrological reservoirs. The technique is then developed in to a methodological ‘toolkit’, comprised of tool groups representing differing levels of data availability. The aim is to enable continued progress in this research area, even under conditions of sub-optimal data availability, whilst also making the suite of methods accessible to other researchers and resource managers. Finally, the hydroclimatic expression of seasonal tree source water avaialbility is characterised across a climate gradient utilising over 800 δ¹⁸O cellulose measurements. The results show that the seasonal progression of tree water availability responds to near- and far-field hydroclimatic processes, but this is mediated by species’ functional type and site lithology. Where near-field processes dominate water availability, there is a potential for increased vulnerability to localised drought conditions and species-specific competition for water resources. Retrodictions of seasonal water source use may prove useful in forest restoration and rehabilitation efforts, assessment of forest vulnerability to future hydroclimatic regimes and for improving agroforestry and sustainable water management practices.