Water balance in boreal forests: the role of season-specific responses in a warming climate

Abstract

The water balance in boreal forests is dominated by snow in winter and rain in summer, but is expected to change in unpredictable ways by forecasted temperature rise and irregular precipitation patterns. Here we use 30 years of hydrological and environmental data representative of a mixed boreal forest including precipitation, tree transpiration (T), evapotranspiration (ET), vapor pressure deficit, and discharge, combined with a reconstructed 150-year historic climatic record. We show that contrasting seasonal conditions and their lag effects, common in high latitude ecosystems, reduce, buffer, or amplify the boreal forest’s response to extreme weather. Additionally, the season when extreme weather occurs and its preexisting conditions, define how the water balance is partitioned. We show that similarly hot-summer years, (e.g., 2018, 2021), can result in contrasting water balances, primarily because when precipitation is reduced, summer ET returns it rapidly to the atmosphere. Early snowmelt in spring due to high temperatures, accompanied with increased T during spring, reduces summer watershed discharge, storage, and can lower groundwater depths, several months after snowmelt. Using long-term tree T and environmental data, we show the individual importance of seasons in the partitioning of water balance, and highlight the major role boreal forests will play under future climate.

Peatland hydrology in boreal Sweden: Modelling, long-term data analysis, and experimental rewetting

“…precipitation between 1991 and 2021 was 636 mm, and for mean air temperature was 2.4 0C (Lopez and Laudon, 2023). Snow accumulates from early November to late April, and constitutes approximately 40% of annual precipitation.…”

“…The Trollberget Experimental Area (TEA) is a headwater stream situated in the same region as KCS (64° 10′ N, 19° 51′ E). The TEA includes six treated experimental catchments which make the BACI design for this study possible (Laudon et al 2023). Two catchments (R1 and R2 with drainage areas of 47 and 60 hectares, respectively) were used for peatland rewetting.…”