- Contemporary fire effects are raising concerns about the resistance and resilience of dry mixed-conifer forests to large wildfires. Fire refugia – unburned or low-severity patches within fire perimeters – are understudied components of post-fire mosaics that may be key drivers of forest recovery following high-severity fire. Little is known about the capacity of dry mixed-conifer forests to regenerate forest following high-severity fire in Oregon’s Blue Mountains, and more broadly, there are significant knowledge gaps regarding the function and composition of fire refugia in dry forest ecosystems in the western United States. This thesis took advantage of a large natural experiment resulting from four large fires that burned in the Blue Mountains between 2000 and 2005. The primary objectives were to: (a) quantify post-fire conifer regeneration in stand-replacement patches and determine the influence of local- and landscape-scale refugial seed source pattern on post-fire forest regeneration, and (b) characterize fire refugia structure and composition, and compare understory plant communities in fire refugia to the higher-severity burned matrix.
Dry mixed-conifer forests in the Blue Mountains of Oregon show evidence of resilience to high-severity fire effects, 12 – 17 years post-fire. Seed sources that survived fire in refugia are critical drivers of post-fire forest regeneration in adjacent high-severity burned areas. In contrast to slow or absent post-fire forest recovery reported in dry forests in other regions, regenerating conifer seedlings were generally abundant in our study area: over 80% of plots in stand-replacement patches contained regenerating seedlings and the median seedling density across all plots was 1100 seedlings ha-1. Consistent with previous studies, we found that proximity to surviving seed source is a key driver of post-fire conifer regeneration. In addition, high-resolution maps of landscape fire refugia, developed using 1 meter aerial imagery, allowed us to provide novel insights into the influence of landscape patterns of surviving seed source on post-fire forest regeneration, and the additive effect of multiple seed sources contributing to a site’s capacity to reestablish forest following high-severity fire. Although stand-replacement patches in our study fires have been largely converted from forests to shrublands 12 – 17 years post-fire, we did not find evidence of a competitive interaction between regenerating conifers and shrubs. In addition, many seedlings appear to have recently emerged above the shrub canopy, suggesting these sites have not been permanently “captured” by woody understory species.
Understory plant community composition in fire refugia and the higher-severity burned matrix was similar 12 – 17 years post-fire, despite substantial structural differences between refugia and stand-replacement patches. We found no evidence of differences between fire refugia and stand-replacement patches in species richness, diversity, or invasibility by exotic annual grasses. Although plant community composition was similar between plot types, we did identify several indicator species for fire refugia and stand-replacement patches, suggesting that differences between these plot types strongly influences the abundance of some plant species. Our findings highlight the capacity of understory plant communities in the Blue Mountains to recover following fire, and together with results from our seedling regeneration analysis, these results suggest that dry mixed-conifer forests in our study area are resilient to even high-severity fire effects. We anticipate that in the absence of subsequent disturbance (e.g., reburn), dry mixed-conifer forest in the Blue Mountains will recover following stand-replacement fire, provided adequate seed sources survived fire within refugia. Fire refugia in the Blue Mountains appear to be important primarily as remnant forest structure and as surviving seed sources essential for the reestablishment of trees in high-severity burned areas, rather than as “safe havens” for plant communities otherwise absent from the higher-severity burned matrix.
Understanding the drivers of forest resistance and resilience to landscape-scale disturbance is increasingly important in the context of accelerating global change. This thesis provides new insights into the composition and ecological function of dry mixed-conifer fire refugia in Oregon’s Blue Mountains, as well as the important role fire refugia play in supporting post-fire forest resilience. Studies like this one can contribute to a growing recognition that fire refugia are important, but perhaps underappreciated, components of post-fire landscapes. However, for the concept of fire refugia to gain lasting currency with scientists and managers, more research will be needed to understand the drivers, persistence, composition, and ecological functions of fire refugia across a diversity of forest ecosystems and fire regimes.