Stable isotopes in the cold Critical Zone: Snowmelt water contributes significantly to tree water storage refilling

Abstract:

Our understanding of the source of transpiration and how phenological and hydrological processes affect tree water use are based mainly on temperate and tropical zones. In cold regions, as trees emerge from winter dormancy, tree water storage refilling sinks large volume of water. Understanding the timing of snowmelt, tree water storage refilling, and the source of water uptake during this period is important to comprehend the impact of forests on water availability in cold regions. Here, we leverage high-resolution stem radius change measurements and stable isotope of δ¹⁸O and δ²H sampling of xylem, soil, precipitation and snowmelt water in two distinct boreal forest sites in Saskatchewan Canada, dominated by Pinus banksiana (OJP) and by Picea mariana (OBS) to answer: 1) Does the timing of tree water storage refilling overlap with snowmelt? 2) What is the water source for storage refilling and transpiration during rehydration? Results show that both tree species rely on snowmelt water to refill storage and commence transpiration in the spring while in tree water deficit. Stem radius analysis showed that tree dehydration occurred during the first week of November in the fall. We found more ²H and ¹⁸O‐enriched xylem water during this period that persisted throughout the winter. In the spring, soil water isotopic composition became significantly more depleted in both ²H and ¹⁸O at both sites in the end of snowmelt during the last week of April. Our weekly xylem water isotope data showed a clear shift from enriched to depleted isotopic signature in both ²H and ¹⁸O on May 2^nd at OBS and 9^th at OJP. Stem radius analysis showed that OBS and OJP trees started to transpire and refill storage during snowmelt (April 25-26^th). Our stem radius analysis results show synchronous tree storage refilling and snowmelt. Delays in changes to the stem water isotopic signature can be related to slow sap flow velocities as xylem samples were collected at the canopy, and/or because of persistent enriched signatures from the fall and winter. Our results highlight that shifts in snowmelt timing or tree rehydration in the spring caused by climate change can potentially affect forest production and water availability in cold regions. Cold regions with well-defined seasons and tree phenological phases can provide new insights into mechanisms of tree water use.