Effects of ozone, nitrogen deposition, and other stressors on montane ecosystems in the Sierra Nevada

Abstract

The greatest air pollution impacts in forests of California are the physiological disturbances imposed on trees as a result of the combined effects of excess N and phytotoxic ozone exposure (Takemoto et al., 2001). In highly-polluted stands in the San Bernardino Mountains in southern California, fine root biomass is greatly reduced and C cycling within the tree and within the ecosystem is also significantly altered. Air pollution effects appear to be more subtle over most of the Sierra Nevada. Individual trees with significant amounts of ozone injury in the southern and western edge of the Sierra have been identified in previous surveys. Additional significant environmental impacts of N deposition in southern California forest and chaparral ecosystems include high NO 3 − concentrations in streamwater and groundwater and increased greenhouse gas emissions from soil. Nitrogen deposition in the Sierra Nevada does not appear to be sufficiently high to cause major physiological impacts or widespread deterioration of water quality, although it is possible that chronic N deposition may be at least partially offsetting the depressive growth effects of ozone in the southern Sierra. However, unusually high nitrate concentrations frequently occur in a chaparral catchment with high N deposition inputs in Sequoia National Park. Preliminary results from N deposition measurements, streamwater analyses for NO 3 − , and soil and plant indicators of N enrichment suggest that N cycling in the mixed conifer forests in the Mountain Home State Park region in the southwestern Sierra Nevada is being altered by N deposition to a greater extent than similar forests in Sequoia National Park. Ozone and N deposition levels are relatively low in high-elevation ecosystems of the Sierra Nevada and do not appear to have severe impacts, although N deposition in the southern Sierra may contribute to the natural peak in nitrate in runoff during early snowmelt. In forests throughout California, periodic droughts and stand densification from long-term fire suppression are major risk factors responsible for reduced tree vigor, greater mortality and predisposition to disease and insect attack; the latter a common ultimate cause of tree mortality. Current land management plans for the Sierra Nevada focus on decreasing overstocking of stands and reducing fuel loads and wildfire risk.