Long-term declines in nitrogen and other nutrients in foliage of three northern hardwood species at the Hubbard Brook Experimental Forest

Forests of the northeastern US have experienced multiple environmental changes over the last three decades, including rising temperatures and atmospheric CO 2 concentrations and declines in acidic deposition, including nitrogen (N) deposition. We used measurements over a 32-year time span (1992–2023) from the Hubbard Brook Experimental Forest, New Hampshire, USA to assess the overall impact of these environmental changes on foliar chemistry (N, carbon, and phosphorus concentrations) and isotopic composition (δ 15 N and δ 13 C) of three dominant northern hardwood species: sugar maple, American beech, and yellow birch. Intrinsic water use efficiency estimated from foliar δ 13 C increased for all three species in response to rising atmospheric CO 2 concentrations. Foliar N concentrations showed large declines after peaks during 2001–2006. These responses are likely driven by the combination of rising atmospheric CO 2 and declining N deposition, but we cannot distinguish between these drivers. Foliar δ 15 N and N:P ratios declined over the study period for sugar maple but not for American beech and birch. Foliar C concentrations declined for all three species, but larger relative declines in foliar N than C led to increases in foliar C:N ratios, which could further slow N mineralization rates and amplify N limitation in forests where these impacts are present.