THE ROLE OF PLANT HYDRODYNAMICS AND PHENOLOGY IN PLANT WATER SOURCE APPORTIONMENT

Understanding plant water sources, their apportionment in time, and the age distribution of transpired water remains a major challenge in ecohydrology. While we know that transpiration is a dominant flux in the terrestrial water cycle, and that precipitation has been increasingly partitioned into evapotranspiration rather than runoff, the understanding of where trees get their water remains a major research gap. Basic questions of what sources of water are being taken up by roots, how that water travels through the xylem, how long takes for water to reach the stomata and then diffuse back to the atmosphere are key to identify mechanisms that control the partitioning of soil water storage into streamflow and transpiration. Trees use and store significant amounts of water. Their roots can create preferential flow paths in the soil, redistribute water within the soil profile, and reach far-deep water storages. Yet, the role of tree hydrodynamics and phenological processes have not been explored in the context of source apportionment age distribution of transpiration. This challenges our ability to fully understand how forests use, store, and cycle water. I undertook a high-resolution weighing lysimeter experiment located in Switzerland, and a large field-based investigation at two long-term studies sites in Canada. In these studies, tree water transport, stem radius change, tree water status, and phenological transition phases were monitored along with high-temporal resolution measurements of stable isotopes in trees, soil and precipitation. The goal was to determine the mechanisms that control tree water use in space and in time and factors that may influence observations of stable isotopes within trees. The major findings of this research were firstly that xylem water analysis using direct vapor equilibration on laser spectroscopy, results in spectral contamination introduced by organic compounds. But 17O-excess can be used as a tool to flag and quantify the degree of spectral contamination in direct vapor analysis and overcome the lack of flagging software or tools to detect contamination in vapor mode. Second, tree water status drives source water apportionment. Soil drying triggers changes in water status and results in shift in water uptake. Thus, measurements of tree water status in high-temporal resolution can improve ours understanding of short-term shifts in tree water sources. High-temporal resolution measurements of tree water deficit offer new opportunities to understand patterns in tree water use when combined with stable isotopes. Third, phloem water is more depleted in heavy isotopes than xylem water. The difference between phloem and xylem water is larger during phloem water refilling and in periods of tree water deficit. These observations led to proposing a phloem refilling hypothesis and illustrated the need to better understand water transport within trees and potential isotope fractionation associations, as well as how this can affect tree water use observations. Fourth, this dissertation proposes a simple method to identify transpiration phenological phases in the boreal forest. This approach shows good alignment and agreement with timing of phenological changes also observed with ecosystem evaporation fluxes, and canopy phenological processes. Lastly, the onset of stem rehydration and transpiration overlap with snowmelt, the largest hydrological event in northern ecosystems. Trees seem to rely on snowmelt water to start transpiring and snowmelt isotopic signatures dominate the transpiration stream in subsequent weeks. This investigation also showed that source water signatures in the xylem are controlled by tree water transit times. The high-temporal resolution observations of tree water use along with the understanding of tree hydrodynamics suggests that ecohydrological separation illustrates the different velocities of flow paths, influenced by dynamic tree water use in space. Overall, through the coupled tree hydrodynamic measurements of tree water transport and water status, transpiration phenology, and stable isotope dynamics of trees, this research has advanced the understanding of tree water use in space and time.

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