The Role of Winter in Forest Ecosystem Carbon Metabolism Across the Northeastern United States

Northeastern U.S. ecosystems are experiencing warmer winters and declining seasonal snowpack. Site-level evidence from this region indicates that altered winter conditions with continued climate change will lead to a complex suite of changes to forest nutrient and carbon cycling. However, existing studies are limited in temporal and spatial extent. The northeast region experiences high variability in winter conditions across a relatively small latitudinal gradient, and it is likely that responses to winter climate change depend on the historic conditions to which plants and soil organisms are adapted. Therefore, the importance of winter for forest productivity may vary considerably across a latitudinal and winter climate gradient. Here, we used long-term meteorological and eddy-covariance carbon exchange measurements from forest ecosystems spanning a latitudinal gradient across the northeastern U.S., along with modeled snow depth, to explore the role of winter in ecosystem carbon metabolism. Our findings suggest that the influence of winter climate variability depends on a combination of local mean winter climate conditions and forest functional type (e.g. deciduous versus evergreen). We found that both the proportion of annual ecosystem respiration occurring during winter months and the length of the transition between snowmelt and the start of the carbon uptake period increase with decreasing latitude and warmer mean winter temperatures in deciduous forest ecosystems. However, carbon uptake precedes snowmelt in evergreen ecosystems highlighting the differences in seasonal transitions across forest types. We conclude that site-level effects of winter climate change are observable at the flux footprint scale and that the importance of winter climate in annual ecosystem carbon cycling at individual sites across the region will likely shift as climate continues to warm.