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

Abstract

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.

Figure 1

Locations of 49 sites sampled for wood δ¹⁵N in the continental U.S.A. Woody cover (green) is from the Commission for Environmental Cooperation. Full site information is available in Supplementary Table 1. Map was generated in ArcGIS 10.1 (https://www.arcgis.com/features/index.html).

Figure 2

Changes in wood δ¹⁵N values since 1850 C.E. The change in wood δ¹⁵N anomaly averaged at the decadal scale. All wood δ¹⁵N data for each tree core was arithmetically adjusted to have a mean wood δ¹⁵N of 0‰ from 1970–2015 and then wood δ¹⁵N anomalies were averaged for each decade (black symbols). Gray symbols represent unsummarized individual adjusted wood δ¹⁵N values (N = 8934). Thin black lines and blue area represent 95% confidence interval for the loess-smoothed curve of wood δ¹⁵N values.

Figure 3

The effect of three site-specific variables on trajectories of wood δ¹⁵N since 1970 C.E. (a) log of mean annual precipitation (estimate −0.069 ± 0.018, P < 0.001) (b) mean annual temperature (estimate 0.0028 ± 0.0008, P < 0.001) (c) atmospheric N deposition (estimate −0.0014 ± 0.0009, P = 0.12). (d) map of modeled wood δ¹⁵N trajectories since 1970 C.E. using these regression relationships for forested areas in the U.S. Units are ‰ y⁻¹. Data source for forested area: Commission for Environmental Cooperation. For panel d, the projection of δ¹⁵N trends was generated by using free software R (R Core Team (2013). R: A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna, Austria. URL: https://www.R-project.org/) and the map was generated in ArcGIS 10.1.