Human-altered fire regimes and the development of stand structure in Macrocarpae pines

This study is an investigation into the relationship of human-caused variation in the parameters of wildland fire regimes to the development of stand structure in two closely related species of Macrocarpae pines: Pinus torreyana Parry ex. Carr. and P. coulteri D. Don. These species define the ecocline between conifer woodland and chaparral in San Diego County, California. Thus their response to human alterations of local fire regimes reflects on the larger question of whether human fire management practices are affecting the areal and elevational distribution of conifer woodlands in this region.; The study is divided into three portions: a study of variation in the parameters of fire regimes in six generalized vegetation types, and studies of the responses of the two species of interest to variations in fire regimes and other disturbances. The first study compares the stand age, fire frequency and transitional stability of vegetation as classed by morphology. Differences between classes are quantified by using a "Manhattan" dissimilarity index. Brush dominated and woodland vegetation types display different characteristics and trends in the amount of area burned and in frequency of burning during the 20th century. Frequency of burning in brushlands is either increasing (sage scrub) or remaining stable (chaparral) while the occurrence of fire in both conifer and broadleaf woodlands is declining. The second and third studies use techniques of point pattern analysis to describe the spatial and temporal characteristics of stands that have experienced different frequencies and intensities of burning. A synthesis of the results of the three studies suggests that Macrocarpae pines are resilient to current trends, along the chaparral/conifer ecocline, in the frequency and intensity of burning. However, Macrocarpae pines may not be resilient to prescribed fires repeated at return intervals of less than 20 years.