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Fuel treatment longevity in a dry mixed conifer forest on the Colorado Front Range




Francis, David, author
Ex, Seth, advisor
Hoffman, Chad, committee member
Martin, Patrick, committee member

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Hazardous fuel treatments have been occurring on an increasingly large scale throughout the western US in response to uncharacteristically severe wildfires in recent decades. These treatments have been shown to be effective in the short term but how long they remain effective, and the factors that affect this, is less clear. As these treatments are often very expensive to implement, knowing when a treated unit will return to pre-treatment fire risk is of critical importance for prioritization of sites and long-term forest planning. The majority of these treatments have occurred in dry mixed conifer forests as they have been the most affected by fire suppression policies, allowing fuels to accumulate and create high fire risk potential, and are often close to human settlement. We examined treatments that used thinning with and without follow-up prescribed fire in mixed stands of ponderosa pine and Douglas-fir. These treatments were examined on north and south aspects at times from 3-12 years after treatment. As hypothesized, we found conifer seedlings densities increased with time since treatment. Our data suggest a density of 500 seedlings ha-1 could be observed on southern aspects 10 years post-treatment. North aspects had twice the seedling density of south aspects at 10 years post-treatment, driven by the presence of Douglas-fir. Ponderosa pine seedling density was found to increase as a result of treatment, with the regeneration rate not being significantly affected by aspect or treatment type. In contrast, Douglas-fir regeneration was not promoted through treatment but was most correlated to the amount of Douglas-fir overstory basal area and found mainly on north aspects. We also found that neither fine or coarse surface fuels nor litter or duff depths had begun showing a significant increasing trend a decade after treatment. Conifer regeneration is diminishing the treatment effectiveness within a decade of treatment and will require retreatment in the future to reduce severe fire potential. While ponderosa pine regeneration can be expected to increase as treatments create more favorable establishment conditions, Douglas-fir regeneration can be expected to be higher when more Douglas-fir is left in a stand during treatment. Advance regeneration was also found to comprise a considerable portion of encountered seedlings and should be removed during treatment to increase treatment longevity.


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