The terrestrial water cycle contains large uncertainties that impact our understanding of water b... more The terrestrial water cycle contains large uncertainties that impact our understanding of water budgets and climate dynamics. Water storage is a key uncertainty in the boreal water budget, with tree water storage often ignored. The goal of this study is to quantify tree water content during the snowmelt and growing season periods for Alaskan and western Canadian boreal forests. Deciduous trees reached saturation between snowmelt and leaf-out, taking up 21–25% of the available snowmelt water, while coniferous trees removed <1%. We found that deciduous trees removed 17.8–20.9 billion m 3 of snowmelt water, which is equivalent to 8.7–10.2% of the Yukon River's annual discharge. Deciduous trees transpired 2–12% (0.4–2.2 billion m 3) of the absorbed snowmelt water immediately after leaf-out, increasing favorable conditions for atmospheric convection, and an additional 10–30% (2.0–5.2 billion m 3) between leaf-out and midsummer. By 2100, boreal deciduous tree area is expected to increase by 1–15%, potentially resulting in an additional 0.3–3 billion m 3 of snowmelt water removed from the soil per year. This study is the first to show that deciduous tree water uptake of snowmelt water represents a large but overlooked aspect of the water balance in boreal watersheds. Like the carbon cycle, the terrestrial water cycle contains large uncertainties that impact our understanding of local to global water budgets and climate dynamics. For example, increased global terrestrial water storage (~3200 billion m 2 or gigatons, as reported by Reager et al. 1) due to climate variability may have slowed a decade of sea level rise 1. Yet, a limited understanding of the type, location, and climate-responsiveness of the different terrestrial water storage pools, including vegetation, contributes to large uncertainty in the " climate-driven land water storage " estimates (+ /− 900 billion m 3 of water) 1. As described by Reager et al. 1 , understanding this uncertainty will require focusing on regions with significant freshwater export to the oceans and surface water hydrology that is rapidly and substantially impacted by climate change. Most of freshwater inflow into the Arctic Ocean is generated from the boreal forest 2. Changes in storage – due to thawing permafrost, shifts in tree species and distributions, snow accumulation and ablation, and deepening of the seasonally thawed soil – has implications for streamflow and ultimately freshwater export from the boreal forest 2. Of this list, the most under-examined aspect of the boreal water budget is tree water storage 2,3. Further, a key uncertainty in the climate of boreal regions is the current and future state of net precipitation (precipitation minus evapotranspiration or ET) 4. Transpiration composes a large fraction of ET 5 and is not well constrained in boreal regions. Tree water use dynamics impact processes across multiple scales, from soil moisture and ecosystem water vapor efflux 6 to regional climate 7 and hydrology 6,8. Many studies focus on transpiration but few on tree water content. However, water content is important for quantifying tree water storage, which impacts hydrology 8,9 , tree response to drought 10–12 , and the coupling of tree water use, soil moisture, and climate 7,8,12. The primary source for tree water storage, whether it is rainfall or snowmelt, has consequences for watershed water balance and the connections between tree water use, storage, and drought stress. Tree water storage can buffer the impact of drought on physiological activity. Existing studies found that trunk and stem water content sustains transpiration rates during dry (up to a week, in some cases) and wet periods alike 10–12. A decoupling between transpiration and growing season rainfall may occur if snowmelt water is the primary water source for
The terrestrial water cycle contains large uncertainties that impact our understanding of water b... more The terrestrial water cycle contains large uncertainties that impact our understanding of water budgets and climate dynamics. Water storage is a key uncertainty in the boreal water budget, with tree water storage often ignored. The goal of this study is to quantify tree water content during the snowmelt and growing season periods for Alaskan and western Canadian boreal forests. Deciduous trees reached saturation between snowmelt and leaf-out, taking up 21–25% of the available snowmelt water, while coniferous trees removed <1%. We found that deciduous trees removed 17.8–20.9 billion m 3 of snowmelt water, which is equivalent to 8.7–10.2% of the Yukon River's annual discharge. Deciduous trees transpired 2–12% (0.4–2.2 billion m 3) of the absorbed snowmelt water immediately after leaf-out, increasing favorable conditions for atmospheric convection, and an additional 10–30% (2.0–5.2 billion m 3) between leaf-out and midsummer. By 2100, boreal deciduous tree area is expected to increase by 1–15%, potentially resulting in an additional 0.3–3 billion m 3 of snowmelt water removed from the soil per year. This study is the first to show that deciduous tree water uptake of snowmelt water represents a large but overlooked aspect of the water balance in boreal watersheds. Like the carbon cycle, the terrestrial water cycle contains large uncertainties that impact our understanding of local to global water budgets and climate dynamics. For example, increased global terrestrial water storage (~3200 billion m 2 or gigatons, as reported by Reager et al. 1) due to climate variability may have slowed a decade of sea level rise 1. Yet, a limited understanding of the type, location, and climate-responsiveness of the different terrestrial water storage pools, including vegetation, contributes to large uncertainty in the " climate-driven land water storage " estimates (+ /− 900 billion m 3 of water) 1. As described by Reager et al. 1 , understanding this uncertainty will require focusing on regions with significant freshwater export to the oceans and surface water hydrology that is rapidly and substantially impacted by climate change. Most of freshwater inflow into the Arctic Ocean is generated from the boreal forest 2. Changes in storage – due to thawing permafrost, shifts in tree species and distributions, snow accumulation and ablation, and deepening of the seasonally thawed soil – has implications for streamflow and ultimately freshwater export from the boreal forest 2. Of this list, the most under-examined aspect of the boreal water budget is tree water storage 2,3. Further, a key uncertainty in the climate of boreal regions is the current and future state of net precipitation (precipitation minus evapotranspiration or ET) 4. Transpiration composes a large fraction of ET 5 and is not well constrained in boreal regions. Tree water use dynamics impact processes across multiple scales, from soil moisture and ecosystem water vapor efflux 6 to regional climate 7 and hydrology 6,8. Many studies focus on transpiration but few on tree water content. However, water content is important for quantifying tree water storage, which impacts hydrology 8,9 , tree response to drought 10–12 , and the coupling of tree water use, soil moisture, and climate 7,8,12. The primary source for tree water storage, whether it is rainfall or snowmelt, has consequences for watershed water balance and the connections between tree water use, storage, and drought stress. Tree water storage can buffer the impact of drought on physiological activity. Existing studies found that trunk and stem water content sustains transpiration rates during dry (up to a week, in some cases) and wet periods alike 10–12. A decoupling between transpiration and growing season rainfall may occur if snowmelt water is the primary water source for
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Papers by Jordi Rosselló