Abstract
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.
Highlights
IntroductionScopulorum Engelm.) communities in the American Southwest grew primarily in open, park-like savannas, maintained by low intensity, frequent fire (Cooper 1960, Barrett et al 1980, Madany 1981, Harrington and Sackett 1992)
These goals aligned with the Leopold Report (Leopold et al 1963), which recommended that a primary goal of the National Park Service (NPS) should be the maintenance or re-creation, as necessary, of the biotic communities within each park to the condition that was present prior to Euro-American settlement
Burn units were treated with prescribed fire to replicate the natural disturbance regime; fire monitoring plots within each unit were considered burned at that time even if the spatial pattern of the fire precluded a plot from actual burning
Summary
Scopulorum Engelm.) communities in the American Southwest grew primarily in open, park-like savannas, maintained by low intensity, frequent fire (Cooper 1960, Barrett et al 1980, Madany 1981, Harrington and Sackett 1992) Beginning in the latter half of the 1800s, cattle and sheep grazing, timber harvesting, and fire exclusion and suppression activities led to increases in tree and woody shrub density, decreases in understory herbaceous cover, and shifts in understory species composition, culminating with an increase in potential fire severity (Covington and Moore 1994, Allen et al 2002, Fulé et al 2004). This poses a management challenge as prescribed burning alone may not completely meet management objectives (Arno et al 1995, Ryan et al 2013)
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