Helen Mills, M.Sc. (2002)
Structural and compositional change in vegetation of Chiricahua National monument, Cochise county, Arizona.
Vegetation structure and composition within U.S. National Parks has changed in response to fire suppression. Understanding how plant populations respond to alteration of fire regimes is an essential step in evaluating the feasibility of returning vegetation to a pre-fire suppression state. Spatial vegetation patterns and trends during the 20th century in the Chiricahua National Monument (CHIR) have been identified using aerial and repeat ground photography. While photographic interpretation characterizes coarse-scale shifts in vegetation types over time and space, it does not describe changes in species abundances, or the mechanisms responsible for change. This study quantifies structural and compositional changes of vegetation in CHIR at the species level, using gradient analysis and stand structure analysis, and relates these to plant regeneration strategies. Sampling points were selected based on georeferenced maps of vegetation change between 1935 and 1993. Vegetation was sampled using belt transects in areas of change and no change. Recently burned sites were also sampled to assess the post-fire regeneration response of species. Spatial patterns of plant population distributions and abundances were significantly correlated with potential soil moisture and elevation. Age and size data of tree species indicate an increase in recruitment of fire sensitive tree species under fire suppression and a decrease in fire tolerant species. Post-fire responses of sprouting and seeding species indicate that fire can be used as a tool to guide efforts to restore vegetation to a pre-fire suppression state, but that a complete return is not realistic. Information from this study should be used as a framework for shaping future plant populations through fire use.
