Divergent apparent temperature sensitivity of forest-floor respiration across a managed boreal forest landscape

Understanding the apparent temperature sensitivity (Q10) of forest floor respiration (Q10R_ff) and its plant autotrophic (Q10R_a) and soil heterotrophic (Q10R_h) components is critical for determining the strength and direction of forest carbon cycle-climate feedbacks. However, the spatial variability of Q10R_ff and its controlling factors across the spatially heterogeneous boreal forest landscape remains poorly understood. In this study, we used chamber-based respiration measurements conducted on paired natural and trenching plots over three growing seasons (2016-2018) to determine Q10R_ff, Q10R_h and Q10R_a to soil temperature across 50 diverse forest stands (ranging 5-211 years old) in a managed landscape in northern Sweden. Additionally, we estimated ecosystem- and landscape-scale Q10 based on conventional and tall tower eddy covariance measurements, respectively, conducted over the same landscape. Our results suggest manifold variations (i.e., 10th to 90th percentiles) of Q10R_ff (1.7 to 6.6), with Q10R_a (0.9 to 7.4) showing a greater range than Q10R_h (1.6 to 5.3). Forest-floor understory biomass was the main control of variations in Q10R_a, whereas soil properties explained best those of Q10R_h. Furthermore, a seasonal hysteresis of Q10R_ff was observed, with higher values mostly occurring during the late growing season. The magnitude of this hysteresis scaled with stand age in response to concurrent changes in net primary production. The Q10R_ff to air temperature declined with increasing stand age, which was attributed to the increased decoupling of air and soil temperatures with canopy development. Despite the considerable stand-level variations, when averaged over the 50 forest stands, Q10R_ff to air temperature converged with that of both ecosystem- and landscape-scale respiration. In conclusion, our study highlights the complexity of Q10R_ff, which needs to be considered in predictions of the interactions of climate change and management with the carbon cycle of managed boreal forests.