COS 20-10
From the coast to the desert in southern California: Atmospheric deposition and ecological effects
The highest levels of nitrogen (N) deposition in North America occur in southern California, a region where ecological effects of N deposition are found from western coastal habitats to shrublands, forests and deserts located inland. The primary drivers of this deposition are NO2, HNO3, NH3 and particulate N. Steep dry deposition gradients lead to highly variable deposition across the landscape. Deposition and exposures have recently been more fully characterized by the use of passive samplers for gaseous pollutants and ion exchange resin throughfall samplers. Broader scale deposition across the region has also been estimated with the CMAQ simulation model and a GIS-based inferential method. N deposition effects and critical loads have been described for coastal sage scrub (CSS), chaparral, forest and desert scrub ecosystems along a western transect from the coast to the eastern desert. Ecosystem processes and components reportedly affected by chronic N deposition in the region include altered epiphytic lichen communities, increases in invasive annual grasses, changes in mycorrhizal communities, increased nitrate leaching, soil acidification, and decreased forest sustainability.
Results/Conclusions
In some instances N deposition contributes to major changes to plant communities and to other biotic communities. Elevated N levels in soils of CSS and desert habitats favors growth of exotic invasive annual grasses causing loss of native shrub and forb cover in CSS ecosystems. These exotic grasses can also lead to fuel buildup and fire in desert scrub leading to vegetation type change. In chaparral and mixed conifer forests nitrate levels in streamwater and groundwater become elevated with chronic N deposition. In chaparral communities, lichen tissue metrics (%N, C:N) and abundance indices for eutrophic and moderately N-sensitive species, mesotrophs, show clear correlations to N deposition. Only eutrophs were found at sites with deposition ≥ 5 kg N ha-1 yr-1. In forests, eutroph abundance indices were closely correlated with N deposition under Pinus (r2 = 0.77 – 0.94), evidence that eutrophs respond to multiple forms of N and not just ammonia as past studies suggest. The use of passive atmospheric sampling techniques have been key in these deposition gradient studies. Empirical deposition measurements are also important in ground-truthing simulation models. Ecosystem restoration and protection in southern California will require further reductions of NOx and greater regulation of NH3 emissions.