Understanding mixed conifer forests in Yosemite National Park: An historical analysis of fires regimes and vegetation dynamics.

摘要

This dissertation presents the results of an historical ecological analysis that quantifies the temporal and spatial variation in forest structure and fire regimes in the mixed conifer forests of Yosemite National Park. I used stand structural analysis and reconstructed fire regime characteristics to determine the relationship between fire regimes and forest structure (i.e. age, size, composition, and spatial patterning) at the plot scale. The structure and composition of the contemporary forest varied spatially with several environmental factors (i.e. slope aspect, TRMI, elevation), although there were no significant differences in fire regime parameters across environmental gradients and forest compositional groups. The contemporary forest was significantly different from the reconstructed forest in basal area, density and mean diameter. The contemporary forest was denser, contained more basal area, but the mean diameter decreased due to the infilling of younger trees since fire suppression. In addition, the majority of the increased density in the forest was due to increases in the number of fire intolerant species (e.g. white fir, incense cedar). The spatial pattern of trees in the contemporary forest also changed from the reconstructed forest with a smaller number of plots demonstrating a clustering of trees of similar ages. Several fire regime parameters varied at the study area scale (extent, severity). Fires of all sizes were present in the study area, although the majority of the fires were 250 ha in size. There was also a combination of low to moderate severity fires, with no evidence of high severity fires occurring. The relationship between fires and changing climate conditions was assessed by comparing the occurrence and extent of fires with several climate variables (e.g. Palmer Drought Severity Index, temperature). Strong relationships between climate variability and fire occurrence and extent were identified at both the interannual and interdecadal scale. At the interannual scale fire extent was linked to regional drought. At the interdecadal scale, fire occurrence and extent was significantly correlated with distinct phases of several interhemispheric climate patterns (e.g. El Nino-Southern Oscillation, Atlantic Multidecadal Oscillation), in addition to interactions between the climate patterns. The influence of fire regimes and forest structure on carbon sequestration was assessed by calculating the total biomass and carbon levels for both the reconstructed and contemporary forest. Total carbon storage increased significantly due to fire suppression. However, the location of the carbon in the forest changed due to the changing structure of the contemporary forest. More carbon was stored in smaller trees established since fire suppression, and fire intolerant species (white fir, incense cedar) stored a significantly greater amount of carbon than in the reconstructed forest. The results of this study indicate that there was a high degree of variability in forest structure and composition in the study areas, however, not all of the variability was related to fire regime characteristics. The influence of fire on the forest structure and carbon sequestration also varied over time, due to changing climate conditions. In addition, the significant role fire played in the historic forest structure is apparent from the high degree of change that occurred due to the removal of fire from the system for over 100 years. Contemporary mixed conifer forests in Yosemite National Park are much denser and homogeneous than historic forests.