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Biogeochemical monitoring for 45 years at the Hubbard Brook Experimental Forest in New Hampshire has revealed multiple surprises, seeming contradictions, and unresolved questions in the long-term record of ecosystem nitrogen dynamics. From 1965 to 1977, more N was accumulating in living biomass than was deposited from the atmosphere; the “missing” N source was attributed to biological fixation. Since 1992, biomass accumulation has been negligible or even negative, and streamwater export of dissolved inorganic N has decreased from ∼4 to ∼1 kg of N ha–1 year–1, despite chronically elevated atmospheric N deposition (∼7 kg of N ha–1 year–1) and predictions of N saturation. Here we show that the ecosystem has shifted to a net N sink, either storing or denitrifying ∼8 kg of N ha–1 year–1. Repeated sampling over 25 years shows that the forest floor is not detectably accumulating N, but the C:N ratio is increasing. Mineral soil N has decreased nonsignificantly in recent decades, but the variability of these measurements prevents detection of a change of <700 kg of N ha–1. Whether the excess N is accumulating in the ecosystem or lost through denitrification will be difficult to determine, but the distinction has important implications for the local ecosystem and global climate.

Winter is an understudied but key period for the socioecological systems of northeastern North American forests. A growing awareness of the importance of the winter season to forest ecosystems and surrounding communities has inspired several decades of research, both across the northern forest and at other mid‐ and high‐latitude ecosystems around the globe. Despite these efforts, we lack a synthetic understanding of how winter climate change may impact hydrological and biogeochemical processes and the social and economic activities they support. Here, we take advantage of 100 years of meteorological observations across the northern forest region of the northeastern United States and eastern Canada to develop a suite of indicators that enable a cross‐cutting understanding of (1) how winter temperatures and snow cover have been changing and (2) how these shifts may impact both ecosystems and surrounding human communities. We show that cold and snow covered conditions have generally decreased over the past 100 years. These trends suggest positive outcomes for tree health as related to reduced fine root mortality and nutrient loss associated with winter frost but negative outcomes as related to the northward advancement and proliferation of forest insect pests. In addition to effects on vegetation, reductions in cold temperatures and snow cover are likely to have negative impacts on the ecology of the northern forest through impacts on water, soils, and wildlife. The overall loss of coldness and snow cover may also have negative consequences for logging and forest products, vector‐borne diseases, and human health, recreation, and tourism, and cultural practices, which together represent important social and economic dimensions for the northern forest region. These findings advance our understanding of how our changing winters may transform the socioecological system of a region that has been defined by the contrasting rhythm of the seasons. Our research also identifies a trajectory of change that informs our expectations for the future as the climate continues to warm.

We propose an important revision to a previously published conceptual model of nutrient retention during terrestrial ecosystem succession, which predicted that ecosystem losses of limiting nutrients such as nitrogen (N) should increase as rates of biomass accumulation slow during late stages of succession. This revision explicitly recognizes that mineral soil horizons (the layers of subsoil beneath the organic‐rich surface layers) can behave as an “N bank”, serving as a source of N for growing forests early in secondary succession and as a sink for N later in succession as plant biomass accumulation slows. If this soil N sink were present, mature forests would continue to retain N for decades following the cessation of biomass accumulation. This conceptual model is consistent with long‐term data collected from the Hubbard Brook Experimental Forest in the US state of New Hampshire, where existing N budgets that exclude the mineral soil horizon indicate both a missing source early in succession and a missing sink as the forest has matured.

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