Jeffrey pine response to 8 years of nitrogen amendment: canopy health, bole growth, and litter production in mesic and xeric microsites

Air pollution produced in the San Francisco Bay area and the San Joaquin Valley is transported and deposited on the west slope of the Sierra Nevada. Although the southern Sierra Nevada currently experiences low nitrogen deposition rates (~6-7 kg ha^-1 yr^-1) in the upper montane, such deposition can alter plant function and resource cycling rates at the ecosystem level. Ammonium nitrate was added annually as a single dose in the autumn, at the rate of 50 kg ha^-1 to simulate higher N deposition rates expected with continued human population growth. This rate is typical of that in the Transverse Range, east of Los Angeles, CA. A single application was used to simulate N-deposition patterns observed in mediterranean-type forests: the first several autumn precipitation events redeposit the accumulated dry deposition from the canopy into the soil. Across 3 sites at a similar elevation, 96 N-amended and 128 control trees were assessed before treatment and every other year after treatment for the following attributes: branch and needle elongation growth, branch diameter, basal area increment, and needle retention, % N and ozone injury level. There were no statistically significant differences in litterfall under the canopy of N and control treatment, nor between xeric and mesic microsite trees. Trees in xeric microsites had lower growth, lower foliar retention, and slightly higher levels of ozone injury. N amendment stimulated growth more in mesic microsite trees, and improved attributes of canopy health, similar to that effected by prescribed burns.