Drivers of spatial and temporal soil water isotope variability in a sub-arctic catchment

Research on ecohydrological separation and plant water use have been increasing in the last few years, and various studies indicate that trees can use winter precipitation as a dominant water source during the growing season. Such studies are of great importance to northern regions, where soil water recharge timing is predicted to be significantly altered due to climate change. In order to assess plant water use in sub-arctic environment, it is necessary to understand how soil water pools under different land covers evolve throughout the year and how cryogenic processes alter the isotope input signal. This field study was conducted from May 2019 to June 2020 in Pallas catchment, located in sub-arctic conditions in Finnish Lapland. Soil cores up to 1 meter depth with 5 cm increments and xylem water of dominant tree species were collected in 4 locations, ranging from forest to shrubland/forest transitional area, and to forested peatland. All locations are positioned on a snow survey, in the vicinity of previously installed groundwater wells and snow lysimeters, and within 2 kilometers of rain gauge. Additional spatial samples of topsoil and xylem water were collected throughout the catchment during 2019 growing season. Relative proportions of tree source water were calculated by Bayesian mixing model MixSIAR. We produce new data set that displays plot and catchment scale soil water heterogeneities in a snow dominated environment, and examine: i) How soil properties affect isotopic composition of soil water?; ii) What is the effect of rising groundwater level on soil water isotope composition?; and iii) How snowpack thickness and melt timing modify soil water isotope patterns? We analyze if these varying pools of water are reflected in tree xylem water. Soil water isotope dynamics under deep snowpack, during and after snowmelt reveal how snow accumulation and melt timing and magnitude influence plant available water for growing season.