Graduate Thesis Or Dissertation
 

Relationship of Soil Type and Burn Severity to Post-Fire Vegetation Response

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  • Public lands in the Pacific Northwest are managed for multiple uses including timber production, recreation and aesthetic value, maintaining wildlife habitat, conserving native species, and carbon storage. Wildfires impact large areas encompassing broad environmental conditions. Interactions among underlying environmental gradients and alteration of overstory competition by fire of varying severities often leads to a diverse vegetation response in the post-fire environment. Plant community succession following a fire event is influenced by biotic and abiotic factors including the pre-fire community, burn severity, burn intensity, distance to nearest seed source, previous fires, and site specific topographic and soil characteristics. Many studies investigate vegetation response following wildfires within a few years of fire occurrence, leaving a knowledge gap about how conditions following a fire lead to more persistent vegetation communities. The 2003 B&B Fire Complex burned 36,000 hectares within the Metolius Basin on the Sisters Ranger District of the Deschutes National Forest. A unique feature of this particular landscape are the soils. These forest soils are highly irregular as a result of volcanic deposits from nearby Mount Mazama, Sand Mountain, and Blue Lake. We hypothesized that soil type, and its interaction with burn severity, is strongly correlated with the observed vegetation response. We found differences in the physical and chemical soil characteristics comparing burned and unburned sites 15 years post-fire. Notably, mean soil pH was higher in burned sites compared to the unburned sites. We also found higher soil total nitrogen concentrations in the burned sites compared to the unburned sites, which contradicts previous knowledge about the duration of nitrogen increases following fire. We hypothesize this may be a result of post-fire Ceanothus spp. colonization. We found a non-linear response of seedling density and fire intensity with the highest seedling density observed in the low severity sites. Elevation was also positively correlated to seedling density, likely due to the increase of precipitation across the elevation gradient. We did not observe an effect of total shrub cover, maximum shrub height, or overstory basal area on seedling density. Soil characteristics negatively correlated with seedling density included organic matter content, soil carbon content, and manganese concentration. Conversely, we observed a positive correlation of seedling density with pH and surface bulk density. We also found a positive relationship between seedling density and interaction terms of pH with C:N ratio and iron concentration. This study revealed differences in abiotic site conditions related to conifer regeneration 15 years post-fire. However, fire intensity remained the strongest influence on post-fire conifer regeneration.
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