Seasonal patterns of water uptake in Populus tremuloides and Picea glauca on a boreal reclamation site is species specific and modulated by capping soil depth and slope position

Abstract

Soil water availability is important for tree growth and varies with topographic position and soil depth. We aim to understand how two co-occurring tree species with distinct rooting and physiological characteristics respond to those two variables during two climatically distinct growing seasons. Growing season (May to September) sap and transpiration fluxes were monitored using heat ratio method sap flow sensors on Populus tremuloides and Picea glauca in 2014 and 2015 growing along a hillslope with two different soil cover depths providing different rooting spaces. Across the two growing seasons, a shallow rooting space was the main factor limiting aspen’s leaf area and cumulative sap flux, whereas responses of white spruce were more limited by topographical position. Generally, sap and transpiration fluxes decreased with the season; however, a large precipitation event during the 2015 summer triggered a significant recovery in sap and transpiration fluxes in white spruce, while in aspen this response was more muted. The two species distinct rooting and physiological characteristics produced contrasting water uptake and water use dynamics in response to rooting space, soil water availability and climate, inviting a more detailed exploration of sap flux and its interactions with climatic and edaphic variables.