UC Riverside

In the 20th century, a policy of fire suppression provided the opportunity for regeneration of vegetation in excess of historical levels of density, with disproportionately high amounts of shade tolerant, more flammable trees across mixed conifer forests of the western US. Historical data on levels of density and cover of vegetation is fragmentary. Gaps in historical data make the task of ecological restoration challenging. One alternative is to use a proxy condition to guide restoration. The Sierra San Pedro Mártir (SSPM), Baja California, Mexico, is a Jeffrey pine mixed conifer forest similar to forests of southern California (SoCal) and elsewhere that has not experienced programmatic levels of fire suppression The hypothesis tested is that if Jeffrey pine mixed conifer forests in southern California and the SSPM, Baja California, Mexico, are a continuation of the California Floristic Province, then locations with similar fire histories should have similar species composition and stand structure. Chapter 1 results indicate that historical species associations are still intact after a century of fire suppression, but levels of shade-tolerant more flammable species are higher, not only because of the time since the last fire of record, but also because of long fire intervals that have elapsed between the most recent fire and the previous fire of record. Chapter 2 results indicate that time-since-fire is a strong predictor of stand structure and composition. Sites with similar fire histories and species composition had similar stand structure except where long intervals between fires left more dense residual stands after a fire event, or where levels of precipitation in the driest sites were found to result in lower levels of recruitment in the understory, but statistically similar levels of canopy trees. Chapter 3 results for downed woody debris indicate that accumulation of rotten woody debris is strongly related to time-since-fire, while sound woody debris is deposited as a continuous rain of material from the canopy. As sound wood decays, it adds to the accumulation of rotten wood that is largely consumed by the next fire event. Results were found to be statistically similar in both SoCal and the SSPM.