Nitrogen critical loads and management alternatives for N-impacted ecosystems in California

M. E. Fenn, E. B. Allen, S. B. Weiss, S. Jovan, L. H. Geiser, G. S. Tonnesen, R. F. Johnson, L. E. Rao, B. S. Gimeno, F. Yuan, T. Meixner, A. Bytnerowicz

Research output: Contribution to journalReview articlepeer-review

182 Scopus citations


Empirical critical loads for N deposition effects and maps showing areas projected to be in exceedance of the critical load (CL) are given for seven major vegetation types in California. Thirty-five percent of the land area for these vegetation types (99,639km2) is estimated to be in excess of the N CL. Low CL values (3-8kgNha-1yr-1) were determined for mixed conifer forests, chaparral and oak woodlands due to highly N-sensitive biota (lichens) and N-poor or low biomass vegetation in the case of coastal sage scrub (CSS), annual grassland, and desert scrub vegetation. At these N deposition critical loads the latter three ecosystem types are at risk of major vegetation type change because N enrichment favors invasion by exotic annual grasses. Fifty-four and forty-four percent of the area for CSS and grasslands are in exceedance of the CL for invasive grasses, while 53 and 41% of the chaparral and oak woodland areas are in exceedance of the CL for impacts on epiphytic lichen communities. Approximately 30% of the desert (based on invasive grasses and increased fire risk) and mixed conifer forest (based on lichen community changes) areas are in exceedance of the CL. These ecosystems are generally located further from emissions sources than many grasslands or CSS areas. By comparison, only 3-15% of the forested and chaparral land areas are estimated to be in exceedance of the NO3- leaching CL. The CL for incipient N saturation in mixed conifer forest catchments was 17kgNha-1yr-1. In 10% of the CL exceedance areas for all seven vegetation types combined, the CL is exceeded by at least 10kgNha-1yr-1, and in 27% of the exceedance areas the CL is exceeded by at least 5kgNha-1yr-1. Management strategies for mitigating the effects of excess N are based on reducing N emissions and reducing site N capital through approaches such as biomass removal and prescribed fire or control of invasive grasses by mowing, selective herbicides, weeding or domestic animal grazing. Ultimately, decreases in N deposition are needed for long-term ecosystem protection and sustainability, and this is the only strategy that will protect epiphytic lichen communities.

Original languageEnglish (US)
Pages (from-to)2404-2423
Number of pages20
JournalJournal of Environmental Management
Issue number12
StatePublished - Dec 2010


  • Air pollution effects
  • California ecosystems
  • Chaparral
  • Coastal sage scrub
  • Critical loads
  • Desert
  • Epiphytic lichens
  • Eutrophication
  • Forest
  • Grassland
  • Management options
  • Nitrogen deposition
  • Nitrogen response thresholds
  • Oak woodland
  • Pinyon-juniper

ASJC Scopus subject areas

  • Environmental Engineering
  • Waste Management and Disposal
  • Management, Monitoring, Policy and Law


Dive into the research topics of 'Nitrogen critical loads and management alternatives for N-impacted ecosystems in California'. Together they form a unique fingerprint.

Cite this