DOI: 10.18739/a2ft8dj8h
title: Arctic and boreal plant evapotranspiration and meteorology, near Toolik Field Station and Caribou Poker Creeks Watershed, Alaska, 2015-2018

description: In the tundra and boreal forest, snowmelt is the principal hydrologic event of the year, when large amounts of water are released during a 2-3 week period. Most snowmelt water flows into rivers to constitute the peak annual flows, because the ground is mostly frozen during snowmelt. The remainder of the snowmelt water is attributed to sublimation or evaporation during the snowmelt period, or to groundwater recharge in areas with sufficient pathways. Most permafrost hydrology models assume plant water storage to be negligible. Alaskan tundra and boreal forest biomes are experiencing a widespread expansion of deciduous shrubs and trees. Many studies highlight the consequences of deciduous vegetation expansion for energy balance, snow accumulation, permafrost, or trace gas fluxes. Data from a small watershed in Interior Alaska shows that deciduous trees take up 25-40% of the snowmelt water prior to leaf out and draw from these stores throughout the summer, including during lengthy dry periods. Preliminary data also show that arctic deciduous shrub stems rise and fall diurnally as the snow melts, which we suspect is linked with water uptake, but the link between shrub water content and snowmelt is unknown. Water uptake by deciduous trees and shrubs, and the observed range expansion of deciduous vegetation, has important ecosystem implications, from water balance to climate feedbacks to coping strategies of deciduous vegetation during dry periods. Intellectual Merit: Ecohydrology is understudied in northern regions, despite strong interactions between permafrost, hydrology, and vegetation. This proposal addresses emerging avenues of ecohydrology by quantifying the role of deciduous plants on the pathways and reservoirs of snowmelt water, and by assessing the potential for stored snowmelt water to buffer vegetation during dry periods. The transformative nature of the proposed work lies in reconsidering the dogma that vegetation plays a minor role in snowmelt hydrological processes, given our evidence to the contrary. This reconsideration is salient given the observed widespread expansion of deciduous trees and shrubs and observed advancing snowmelt dates. Finally, a lack of severe water stress observed in deciduous trees during dry periods suggests that they are drawing on reserves in their trunks. This work will produce a fundamental understanding of ecohydrological interactions across the tundra and boreal forest biomes. The overarching question of this study was: what are the consequences of woody deciduous vegetation and its expansion on the ecohydrology of northern ecosystems? Specific questions we addressed were: To what extent do deciduous vegetation (trees and shrubs) store and flux snowmelt water? What are the ecological consequences (such as buffering from weather variability) of the storage and use of snowmelt water? We propose to explore these critical unknowns: (1) the amount of snowmelt water taken up by deciduous shrubs and trees, (2) the consequences of deciduous tree and shrub expansion on the fate of snowmelt water, and therefore, on the most significant event in the arctic and subarctic hydrologic cycle, (3) the consequences of the snowmelt water uptake for “buffering” deciduous plants from seasonal weather variability and drought, and (4) the spatial variability of shrub and tree water uptake (beyond the small watershed and the single species from which preliminary data were collected). Exploring the above unknowns will contribute toward (1) understanding the fate of snowmelt water, (2) predicting the effect of increased deciduous vegetation cover on the water balance of northern ecosystems, (3) identifying the hydrologic nature of vegetation-snow interactions and feedbacks, and (4) quantifying water uptake by different tree and shrub species to inform spatial extrapolation. We implemented a field study across varied climate zones in Alaska to measure snowmelt water uptake and water flux by deciduous vegetation in the arctic (shrubs) and subarctic (trees and shrubs). We measured sap flux (transpiration), TDR (time domain reflectometry) and environmental variables.