Restoration benefits of re-entry with resource objective wildfire on a ponderosa pine landscape in northern Arizona, USA

Prescriptions for ecological restoration of ponderosa pine (Pinus ponderosa) forests of the American Southwest are generally aimed at approximating pre-fire-exclusion conditions by reducing tree densities and hazardous fuels, and reintroducing surface fire. Increasingly, land managers are utilizing natural fire ignitions to achieve restoration and hazardous fuels reduction objectives, but to minimize risk of escape and reduce potential for undesirable outcomes fires are commonly managed under weather and fuel moisture conditions that produce lower burn severities. Few studies have examined effectiveness of incremental approaches that attempt to restore these ecosystems by allowing repeated, low-severity wildfires to reburn sites. We analyzed a 12,035-ha landscape on the Kaibab National Forest in northern Arizona and compared forest structure, hazardous fuel loading, and potential fire behavior in areas that had burned once (single fire) to others that had burned twice (re-entry) over the preceding 13 years. Forest structure and hazardous fuels characteristics in re-entry areas tended to be closer to historical ranges of variability (HRV) than in single fire areas; however, we found few statistically significant differences and tree densities in both single and re-entry fires remained higher than HRV by up to 3.5-fold. Similarly, potential fire behavior variables were statistically similar between single and re-entry fire areas. Analysis of burn severity effects suggested that trends toward reduced tree density and canopy fuels in re-entry areas were consistently driven by “cleanup effects”, whereby re-entry fires produced low-severity effects in areas showing unburned/low severity after first-entry fires. Evidence of “additive effects”, whereby restoration and hazardous fuels reduction objectives were being met incrementally by repeated low-severity fire, was difficult to detect on the re-entry fire landscape. Thus, with similar management, several additional fire entries will be required to restore tree densities to within the historical range. Further, our findings suggest that managing resource objective wildfires to allow for more moderate-severity burning, even with a single entry, may be more effective for restoring these forests than repeated low-severity entries.