The effects of climate change and nitrogen deposition on the Sierran mixed-conifer understory plant community

Advances in general circulation models have allowed for more localized climate change predictions. In the mixed-conifer forests of California's Sierra Nevada Mountains, these more localized predictions indicate that winter precipitation is likely to increase and warmer temperatures will result in a longer growing season. Coupled with changing climatic conditions is an increase in atmospheric nitrogen deposition resulting from fossil fuel consumption.; To determine species specific growth response and the influence of density on five Sierran mixed-conifer overstory species I sampled annual growth of 579 trees. Using the Kalman filter, an iterative state-space model, I examined species specific growth response to annual climatic fluctuations and the influence of stand density on growth response to climate. Under current high-density conditions, shade-tolerant white fir (Abies concolor) provided the best model for climate reconstruction. Shade-intolerant Jeffrey pine ( Pinus jeffreyi) had a lagged response to annual climatic fluctuations possibly because its roots may tap water reserves in granitic bedrock fissures. Open grown trees provided more accurate records of climate. Changes in forest density in this forest may have resulted in changes in species-specific response to annual climatic fluctuations.; To examine the impacts of alter altered precipitation, increasing nitrogen, and prescribed fire on understory species richness and biomass production, I utilized a full factorial treatment design in the central and southern Sierra Nevada. These data were used to parameterize and validate an understory growth model for use in fire management planning. Field results indicated that the understory community is most influenced by nitrogen and prescribed fire. At the central Sierra site, water increase coupled with increasing nitrogen and fire resulted in the highest herbaceous plant species richness. At the southern Sierra site, the nitrogen increase yielded the greatest post-treatment species richness.; Model runs under the expected increasing precipitation and nitrogen scenario indicated that understory biomass production would be significantly greater than under current precipitation and nitrogen deposition levels. Through a series of model experiments, I determined that decreasing the fire return interval from the historical 15-30 year return interval to 10 years maintained understory fuels at a level consistent with current precipitation and nitrogen deposition conditions.