Trembling aspen is a foundational tree species of the boreal mixedwood forests of Canada, and apart from its ecological importance, a valuable resource for the forestry industry. Large uncertainties in climate change predictions hinder our ability to predict aspen’s future performance and distribution, as does the lack of detailed knowledge of its role in tree water uptake dynamics. Recent reports of dieback of trembling aspen across western Canada, invites more research on the ecophysiology of aspen and its relationship to climatic variation. The objectives of the study were to: (i) investigate the dynamics of water uptake in individual trembling aspen trees, and (ii) explore the coupling of water use with environmental factors controlling water uptake and tree phenology and their variation during the growing season. In 2014 and 2015, heat ratio method (HRM) sap flow sensors were mounted on 27 individual trembling aspen trees on a hill slope in the boreal forest region of Alberta, Canada to monitor water uptake. Hourly, daily and cumulative sap flow and transpiration measurements were obtained. Tree characteristics, climate and soil moisture data were also recorded during the two years.
Results/Conclusions
The dynamics of seasonal sap flow closely followed the surface soil layer moisture content for both years. Shifts in the timing and duration of initial spring warming during the growing season lead to significant differences in sap flow and cumulative water uptake over the growing season between the two years. Sap flow velocity and transpiration rates were strongly correlated to climatic variables such as vapor pressure deficit, precipitation events, air temperature and solar radiation; however, the strength of the correlation differed within and between growing seasons as rainfall and drying events occurred. A large rainfall event late in the growing season during a dry period provided an opportunity for increased water uptake over a short period of time, increasing cumulative water uptake for the season and potentially carbon gain. As averaged climatic conditions may become more restrictive to aspen growth during the growing season, these large but sporadic events may become crucial in determining its future performance as drier growing seasons are forecasted, highlighting the importance of addressing the consequences of increased climatic variability on tree performance in the boreal forest region.