Periodic Reporting for period 1 - BoTWU (Tree water use under changing snow/rain seasonality: The forgotten tree-induced upward preferential flow in boreal soils)
Période du rapport: 2023-10-01 au 2025-09-30
Trees start growing in spring. In boreal forests, it is known that trees do not rely only on rainfall during the growing season, but also use snowmelt water from winter. Under climate change, the seasonal pattern of winter snowfall and growing-season rainfall is changing in northern environments. Winter snow has increased in some regions and decreased in others, and growing-season rainfall has become more variable in terms of amount, duration, and intensity. Do these changes alter boreal tree water use during the growing season, which strongly influences tree growth in regions where timber productivity is critical for the local economy? Meanwhile, as a dominant component of the forest water budget, how does altered tree water use further modify soil moisture conditions in boreal forests, thereby affecting local water resource dynamics? Answering these questions is critical for providing a scientific basis and new insights for developing sustainable and climate-smart forest and water resource management in boreal regions, in order to cope with the rapid changes in seasonal precipitation occurring in high-latitude areas. Therefore, by applying an array of interdisciplinary methods in an experimental setup with innovative high-resolution, in-situ, and non-destructive water isotope and water flux measurements, BoTWU explores boreal tree water uptake strategies under changing rain-snow seasonality. At the same time, BoTWU research and knowledge exchange between the researcher and the host boosted the researcher’s career development as a forest ecohydrologist and expanded the research scope of boreal forest (eco)hydrology at the host institution.
BoTWU established and maintained an elaborate and comprehensive forest ecohydrology research infrastructure through collaboration at the host institution. The key component of this infrastructure is an in-situ and high-resolution water flux and water isotope measurement system, which was applied to roots and trunks, as well as to soils, under three long-term snowpack treatments: natural, reduced, and increased snowpack. Meanwhile, seasonal forest water inputs were collected as snowmelt water, throughfall, and stemflow, with their isotopic compositions measured in the laboratory. These measurements allowed BoTWU to analyse how much water is taken up by roots and trees, when uptake occurs, and where the water originates, for example, from which soil layers and from which seasonal water inputs (e.g. late-winter snowmelt, growing-season rainfall, or even rainfall from the previous autumn). Building on the improved understanding of water isotopic processes along the tree water uptake pathway and the high spatio-temporal characterization of soil moisture, the effects of long-term snowpack changes and the combined effects of heatwaves and droughts on root water absorption and tree water uptake were tested. The results and outcomes of BoTWU have been disseminated widely and timely to the scientific communities of forest ecohydrology, forest hydrology, isotope hydrology, tree hydraulics, forestry, and climate change ecology through international publications, invited talks, research visits, conferences, meetings, and summer schools held in Europe, Asia, and Oceania, as well as via social media.
By testing and applying the innovative in-situ water isotope sampling technique in northern environments for the first time, BoTWU examined the long-term effects of changing snowpack on root water absorption and tree water uptake in boreal forests. Systematic process-based mechanisms of tree water uptake strategies under changing seasonal precipitation were explored by considering water uptake processes along both roots and trunks, and by accounting for the seasonal influences of both late-winter snowmelt and growing-season rainfall. These findings can support the modelling development of tree hydraulics and tree growth models in boreal regions, which may help forest industry stakeholders reduce economic losses associated with changing seasonal precipitation in high-latitude areas. In addition, the results can improve hydrological modelling in northern environments, contributing to more effective local water resource management.