Data from: Centennial-scale reductions in nitrogen availability in temperate forests of the United States

Forests cover 30% of the terrestrial Earth surface and are a major component of the global carbon
(C) cycle. Humans have doubled the amount of global reactive nitrogen (N), increasing deposition of
N onto forests worldwide. However, other global changes—especially climate change and elevated
atmospheric carbon dioxide concentrations—are increasing demand for N, the element limiting primary
productivity in temperate forests, which could be reducing N availability. To determine the long-term,
integrated effects of global changes on forest N cycling, we measured stable N isotopes in wood, a
proxy for N supply relative to demand, on large spatial and temporal scales across the continental U.S.A.
Here, we show that forest N availability has generally declined across much of the U.S. since at least
1850 C.E. with cool, wet forests demonstrating the greatest declines. Across sites, recent trajectories
of N availability were independent of recent atmospheric N deposition rates, implying a minor role for
modern N deposition on the trajectory of N status of North American forests. Our results demonstrate
that current trends of global changes are likely to be consistent with forest oligotrophication into the
foreseeable future, further constraining forest C fixation and potentially storage.