Thesis

Shifts in species dominance in a Mediterranean ecosystem under a changing fire regime

Over a span of 70 years, portions of the Santa Monica Mountains in southern California have seen a change from an Adenostoma fasciculatum dominated chaparral community to a co-dominant A. fasciculatum – Ceanothus megacarpus community. Each of these species has a different adaptation to fire. A. fasciculatum is a facultative seeder, while C. megacarpus is an obligate seeder. In conjunction with 70 years of vegetation modification by humans, environmental conditions have also changed: fire frequency has increased, and drier conditions are becoming more common. Recognizing that species compositional change in the Santa Monica Mountains chaparral community may be somewhat unexpected, this study attempts to identify the drivers of this apparent transformation. It hypothesizes that the observed trends can be explained by increased fire frequency and drought. Consequently, it is expected that C. megacarpus will perform better than A. fasciculatum under increasing fire frequency and drought. Using GIS, an overlay analysis was conducted to identify the areas where shift of dominant species occurred, together with related fire history. Two datasets were examined for species composition and fire history. The first dataset referenced vegetation from1934 to 2004, and was analyzed using a Geographic Information System (GIS) program while the second consisted of field data acquired in 2004 and in 2014. In contradiction to the stated hypothesis, the analysis suggested that A. fasciculatum was better adapted for higher fire frequency than C. megacarpus. This was observed in the 1934/2004 dataset. The 2004/2014 dataset did not yield enough areas of change to analyze the relationship between vegetation dynamics and fire frequency. The complexity of this system could not be sufficiently explained by this study, but the research did suggest that A. fasciculatum is better adapted to increased fire frequency than C. megacarpus.

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