Applying mycorrhizal critical loads of nitrogen to US federal lands containing forested and Mediterranean shrubland ecosystems
Authors: Clemens Cody , Bell Michael , Felker-Quinn Emmi , Bell Michael , Clemens Cody , Felker-Quinn Emmi 
Published: August 2024

Mycorrhizal fungi (MF) play a crucial role within ecosystems by forming mutually symbiotic relationships where they share water, carbon, nutrients, and other benefits with plants they have inoculated. Since these plant communities depend on MF for survival, the status of MF communities can be used as a proxy for the health of the plants they support. Anthropogenic nitrogen (N) deposition can disrupt the symbiotic relationship between the plants and MF by causing a shift in mycorrhizal community composition, and in turn a shift in the plant structure within an ecosystem. Lilleskov et al. (2019) summarized recent research on critical loads (CLs) of N for a shift from N-sensitive to N-tolerant mycorrhizal and tree communities, setting CLs to protect conifer-associated MF (5?6 kg-N ha-1 yr-1), and broadleaf-associated MF (10?20 kg-N ha-1 yr-1). Allen et al. (2016) developed CLs of N for shifts in arbuscular MF communities in California coastal sage scrub which contribute to the conversion of native shrubland to exotic annual grassland (10?11 kg-N ha-1 yr-1).
Here we apply the CL-N for forest mycorrhizae to forested areas in the contiguous United States (CONUS), and the CL-N for southern California shrubland mycorrhizae to shrublands and grasslands within the Mediterranean ecoregion of California. The National Atmospheric Deposition Program (NADP) Total Deposition model (TDep) for total N deposition from 2018?2020 (v2018.02) is used to calculate exceedances of the CLs and evaluate potential risk to forest health. The areas of exceedance are calculated for all units within CONUS managed by the Fish and Wildlife Service (FWS), National Park Service (NPS), and Forest Service (USFS).
Within CONUS, 68% of federal land managed by the three agencies is forest: 76% is conifer, 17% is broadleaf, and 7% is a mixture of both tree communities. The USFS manages 93% of this forested land, and 30% of USFS forests are in exceedance of CLs-N for MF. The NPS manages a smaller portion of federal forested land at 6%, with exceedance occurring in 29% of that area. The FWS controls only 1% of forested land but has exceedance of CLs in 43% of that area.
Within Mediterranean California (CA), 29% of federal land managed by the three agencies is shrubland or grassland. Federal shrublands are in exceedance of the CL-N for MF in 25% of the area. The USFS manages 91% of federal shrublands in Mediterranean CA, with 27% of USFS CA shrublands in exceedance of the CL. The NPS manages 8% of federal shrublands in Mediterranean CA, of which only 3% is experiencing exceedance. The FWS is only responsible for 1% federal shrublands in Mediterranean CA, with exceedance occurring in 22% of that area.
This analysis shows the effects that N deposition has on forest and shrubland ecosystems occurring within federal lands across CONUS. Knowing the extent of exceedance observed within federal lands informs natural resource management. On smaller scales, this analysis enables federal land managers to better understand what is happening on their lands, effectively guiding local management decisions. On larger scales, the analysis outlines a portion of the overall effects that N deposition has on ecosystems within CONUS, providing policymakers with the scientific evidence required to support future policy actions aimed at protecting natural resources.