Patterns and predictors of crown fire induced type conversion in dry conifer forests

Since 1996, high-severity crown fires in Southwestern ponderosa pine forests have produced large treeless areas, which are unprecedented in the regional historic record. Other dry conifer forests, similar to ponderosa pine, are also experiencing extensive high-severity crown fires that are creating conditions unique in the evolutionary history of these forests. Recent crown fires have reset large portions of southwestern forest landscapes, creating opportunities for successional vegetation communities to alter ecological trajectories away from a return to the pre-fire forest type, which can lead to type conversion. I conducted a synthesis of the peer-reviewed scientific literature and carried out an empirical and modeling study to examine patterns of landscapes change and predictors of ecosystem type conversion.;For the literature review and synthesis, I examined general post-fire response patterns from studies completed following crown-fires in dry conifer systems. I determined that severely burned areas are often set on successional trajectories that lead to conversion from conifer forest ecosystem types to a non-forested types by following one of three pathways: 1) climate change or prolonged drought limits moisture availability, which prevents regeneration of the pre-fire conifer species, favoring more drought-adapted shrublands or grasslands; 2) changes to the fire regime of the area as a result of early successional ecosystem establishment excludes conifer regeneration; or 3) post-fire interspecific competition favors the persistence of early successional plant communities, to the exclusion of conifer regeneration. The complex linkages among particular land use histories, landscape changes in vegetation and 'fuels', climate variability and change, and differing fire patterns along with variation through time and across geographic space in different landscapes, leads to interactions between the 3 'pathways,' which will likely increase the probability of type conversion in any given landscape.;For my empirical and modeling study, I hypothesized that the hottest and driest areas within the burned area would be most likely to convert from forest to a non-forest ecosystem type, because the post-fire areas are recovering in a climate period that is warmer than when the forest established. To test this, I created a spatially explicit landscape model based on topographic variables: elevation, slope position, and aspect. I developed the model using empirical field data collected from nine areas in Arizona and New Mexico. The results show low elevation, xeric sites are more likely to type convert than higher elevation sites with more mesic conditions, indicating a shift toward post-fire climate conditions that limit pine regeneration. Additionally, areas far away from a viable seed source have a high probability of type conversion, even if those areas are favorable for pine germination. This model provides a spatially explicit illustration of landscape vulnerability to type conversion. The field survey shows a wide variety of post-fire ecological responses following stand-replacing crown fire. Together, these research results provide a greater understanding of landscape change in the face of climate warming, and I hope that they will contribute meaningfully to informed land management decisions regarding adaptation or mitigation strategies.