Evaluating Prescribed Fire Effectiveness Using Permanent Monitoring Plot Data: A Case Study

Since Euro-American settlement, ponderosa pine forests throughout the western United States have shifted from high fire frequency and open canopy savanna forests to infrequent fire and dense, closed canopy forests. Managers at Zion National Park, USA, reintroduced fire to counteract these changes and decrease the potential for high-severity fires. We analyzed existing permanent monitoring plot data collected between 1995 and 2010 to assess achievement of management objectives related to prescribed fire in ponderosa pine forests. Following first entry fire, ponderosa pine (Pinus ponderosa C. Lawson var. scopulorum Engelm.) and Gambel oak (Quercus gambelii Nutt.) overstory and midstory densities declined between 10 % and 45 % and effectively shifted the Gambel oak diameter distribution toward larger trees. Second entry fires had a greater effect, reducing ponderosa pine and Gambel oak overstory and midstory densities between 24 % and 92 %. Diameter distributions of both species shifted toward fewer, larger trees following second entry fires. Total fuel load was reduced by <20 % in first entry fires and by half in second entry fires. Several objectives identified by the National Park Service (e.g., overstory ponderosa pine reduction) were not achieved with either fire entry; however, power analysis indicated that sample sizes were not adequate to fully detect long term changes following first entry fires. First entry, low intensity prescribed fire alone may not meet management objectives in southwestern ponderosa pine forests. We recommend using multiple fire entries or increased fire intensity if mechanical treatments are not also being utilized concurrently. Long term fire effects data is critical to adaptive management in national parks; however, the utility of this data could be improved through increased sample sizes, consistent data archiving, and regional scale analyses.