SGER: Pandemic Conifer Forest Mortality and Dieback Under Extreme Drought in the Mountains of Southern California

University Of California-Riverside, Riverside CA

Investigators

Abstract

The winter of 2001-02 in southern California was the driest year since instrumental records began in 1850. Southern California mountain conifer forests exhibit symptoms of severe drought stress, including elevated tree mortality, active insect predation, premature leaf shed, absence of leader growth in early summer, and anomalous leaf-shed in both evergreen and winter deciduous broadleaf species. Drought is also coupled with wildland fire, which is integrated in the structure, species composition and distribution of southern California conifer forests. It follows that the rate of drought-induced tree mortality/dieback must be evaluated in relation to long-term fire regimes and vegetation dynamics. Because the region is now experiencing rare conditions of the climate, it is vital to document the state of the forests with an aerial photographic survey in September, 2002, using as a case study the San Bernardino Mountains and San Jacinto Mountains where water deficits are greatest. This research will use stereoscopic aerial photography at a scale of 1:24,000 to assess tree mortality patterns and compare them to 1983 photos in hand. The photo resolution of <1.0 m will permit the analysis of trees as small as saplings, 3.0 m tall. The objective of this study is to document and quantify the spatial pattern of the current forest dieback/mortality episode in SBM and SJM to test the following hypotheses: (1) drought-induced fire mortality reflects fire regime and life history traits of ecosystems, and (2) drought mortality reflects the post-fire successional status of stands in the patch mosaic, including changes arising from fire suppression. The study is relevant to other research including air pollution effects on forests, fire suppression and forest densification, and short-term fire risk to suburban mountain communities.