Spatio-temporal dynamics and controls of forest-floor evapotranspiration across a managed boreal forest landscape

Forest-floor evapotranspiration (ET_ff) is a major pathway for water loss in terrestrial ecosystems, often accounting for more than half of ecosystem evapotranspiration. However, our understanding of the environmental and stand structural controls on the spatio-temporal dynamics of ET_ff across the managed boreal forest landscape remains limited. In this study, we conducted chamber-based flux measurements of ET_ff and its components, i.e., soil evaporation (E_s) and forest-floor understory transpiration (T_u), on natural and vegetation removal plots across 50 diverse forest stands (ranging 5–211 years old) in Northern Sweden over two contrasting growing seasons. We found manifold variations in the growing season means of ET_ff, E_s, and T_u, ranging from 0.008 to 0.048 mm h⁻¹, 0.004 to 0.034 mm h⁻¹, and 0.002 to 0.030 mm h⁻¹, respectively, across the 50 forest stands. The contribution of E_s and T_u to ET_ff ranged from 19 to 83 % and 38 to 85 %, respectively, with the average E_s:T_u ratio shifting from 0.84 in 2017 to 0.63 during 2018, the latter experiencing an exceptional summer drought. Seasonal variations in ET_ff and its component fluxes were mainly controlled by below-canopy air temperature, while radiation was the main driver of their spatial variations across the forest stands. At the landscape-level, stand age was the dominant control of ET_ff by modifying overstory tree characteristics such as biomass and leaf area index. In contrast, neither tree species nor soil type had any effect on ET_ff or T_u. However, E_s was higher in sediment compared to till soils. Thus, our results suggest that environmental and stand structural factors jointly control the spatio-temporal dynamics of ET_ff across the managed boreal forest landscape. Our study furthermore highlights the need for an in-depth understanding of ET_ff and its components when assessing the water cycle feedbacks of the boreal forest to changes in forest management and climate.