In the forests of New Hampshire, climate change is shifting the seasons in subtle yet profound ways. Winter is warming faster than any other season across northern latitudes, accelerating snowmelt and vegetation green-up. However, the rate of change is uneven, resulting in a longer transitional period between snowmelt and full forest canopy leaf-out. Scientists call this the “vernal window,” and its impact on forest ecosystems is becoming increasingly clear.

During the December 2024 American Geophysical Union Annual Meeting data from Hubbard Brook and other locations will be presented as part of “Earlier snowmelt drives higher rates of growing season soil respiration in mesic, temperate forests“. For the past decade, researchers have monitored six forest sites in New Hampshire, collecting year-round data on snow depth, soil temperature, soil moisture, and soil respiration using a statewide soil sensor network. According to the authors, “the vernal window was longer following warmer winters with reduced snow cover, largely due to the influence of winter season severity on snowmelt timing.” This extended period of warming before the trees leaf out has significant implications for carbon cycling.

The authors found that “monthly average soil moisture was lower following a longer vernal window due to later forest canopy green-up, while monthly average soil temperature was higher because of earlier snowmelt.” These changes boost soil microbial activity, increasing the release of carbon dioxide into the atmosphere. The study demonstrates how shifting seasonality affects the delicate balance of carbon and water cycling in mesic temperate forests.

“Growing season soil respiration was also higher if the preceding vernal window was longer, in part because warmer soil temperatures drove higher respiration rates,” the authors explain. These findings challenge assumptions about forests as stable carbon sinks and highlight the intricate ways climate change reshapes ecosystem dynamics.

The authors’ work underscores the importance of understanding these transitions, as forests play a critical role in global carbon and water cycles.

Authors: Alexandra Contosta, Elizabeth Burakowski, Serita D. Frey, Mark Green, Danielle S. Grogan, Julia Olson, Apryl Perry, Ruth K. Varner