Landscape Ecology of Large Fires in Southwestern Forests, USA
This dissertation has been moved to the following series:
The recent increase in large fires in southwestern forests has prompted concern regarding their ecological consequences. Recognizing the importance of spatial patterns in influencing successional processes, I asked: (1) How do large fires change plant communities?; (2) What are the implications of these changes for ponderosa pine forests?; and (3) What is the relationship of fire severity to gradients of climate, fuels, and topography? To address the first two questions, I studied succession in the woody plant community at two sites that burned in high-severity fire: La Mesa fire in northern New Mexico (1977) and Saddle Mountain in northern Arizona (1960). After large fires, abiotic conditions, associated prefire plant distributions, and spatial patterns of burning interacted to result in particular successional outcomes. Variation in abundance and diversity of species that spread from a refuge of seed sources remaining after the fire followed the model of wave-form succession. I investigated the implications of large fires for ponderosa pine by examining the influence of spatial patterns of burning on regeneration. Tree density corresponded most closely with particular scales of seed dispersal kernel and neighborhood severity metrics. Spatial patterns of burning remained influential even after consideration of variables describing subsequent burning and the physical and biotic environment. Age structure of young forests indicated that populations spread in a moving front and by long-distance dispersal. To explore the relationship between fire severity and climate, I investigated how the spatial heterogeneity of high-severity patches varied among 20 fires across gradients in fire size and climate. The largest fires generally occurred during cool dry La Niña climates, however, several fires deviated from this trend. Some spatial properties of severity did not correspond to fire size or to changes in climate. Characteristics of fuels and topography altered spatial patterns of severity, but interactions with extreme burning conditions may have disrupted these local influences in both La Niña and El Niño fires. Spatial patterns of fire severity are central to understanding ecological dynamics following large fires in southwestern forests. Moreover, simplistic assumptions regarding the relation of fire severity to fire size and climate should be viewed with caution.