Controls on winter ecosystem respiration in temperate and boreal ecosystems

Abstract. Winter CO₂ fluxes represent an important component of the annual carbon budget in northern ecosystems. Understanding winter respiration processes and their responses to climate change is also central to our ability to assess terrestrial carbon cycle and climate feedbacks in the future. However, the factors influencing the spatial and temporal patterns of winter ecosystem respiration (R_eco) of northern ecosystems are poorly understood. For this reason, we analyzed eddy covariance flux data from 57 ecosystem sites ranging from ~35° N to ~70° N. Deciduous forests were characterized by the highest winter R_eco rates (0.90 ± 0.39 g C m⁻² d⁻¹), when winter is defined as the period during which daily air temperature remains below 0 °C. By contrast, arctic wetlands had the lowest winter R_eco rates (0.02 ± 0.02 g C m⁻² d⁻¹). Mixed forests, evergreen needle-leaved forests, grasslands, croplands and boreal wetlands were characterized by intermediate winter R_eco rates (g C m⁻² d⁻¹) of 0.70(±0.33), 0.60(±0.38), 0.62(±0.43), 0.49(±0.22) and 0.27(±0.08), respectively. Our cross site analysis showed that winter air (T_air) and soil (T_soil) temperature played a dominating role in determining the spatial patterns of winter R_eco in both forest and managed ecosystems (grasslands and croplands). Besides temperature, the seasonal amplitude of the leaf area index (LAI), inferred from satellite observation, or growing season gross primary productivity, which we use here as a proxy for the amount of recent carbon available for R_eco in the subsequent winter, played a marginal role in winter CO₂ emissions from forest ecosystems. We found that winter R_eco sensitivity to temperature variation across space (Q_S) was higher than the one over time (interannual, Q_T). This can be expected because Q_S not only accounts for climate gradients across sites but also for (positively correlated) the spatial variability of substrate quantity. Thus, if the models estimate future warming impacts on R_eco based on Q_S rather than Q_T, this could overestimate the impact of temperature changes.