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Wildlife, Animals, and Plants |
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FIRE EFFECTS
IMMEDIATE FIRE EFFECT ON PLANT:Low-severity surface fires usually kill southwestern ponderosa pine less than 3 to 5 years of age or less than 6 inches (15 cm) dbh. Mortality in the 6- to 30-inch (15-76 cm) dbh class is not unusual. Trees in dense stands and trees infected with southwestern dwarf-mistletoe are most susceptible to mortality, particularly in the smaller size classes. Pole-sized and larger trees are resistant to low-severity surface fires. Severe surface or crown fires generally kill ponderosa pine of all size classes [3,4,39,65,141,142], although some "sawtimber-sized" trees may survive severe surface fire [26]. Heavy accumulations of litter at the base of trees increase the duration and intensity of fire, making trees more susceptible to scarring. Resin deposits around an old "cat-face" may increase bark flammability and promote further injury [23]. DISCUSSION AND QUALIFICATION OF FIRE EFFECT:Ponderosa pine can withstand extensive scorching as long as buds and twigs, which tolerate higher temperatures than needles, are not badly scorched [41,112,142]. Ponderosa pine may recover from as much as 90% scorching as long as 50% of buds and twigs survive to maintain shoot growth on defoliated branches [142]. Extensive scorching of ponderosa pine crowns may cause mortality within 3 postfire years [58,70,89]. Generally, southwestern ponderosa pine recovery is best after dormant-season scorching; trees scorched in the growing season show poorer survivorship [58,63]. After a growing-season (July) wildfire in northern Arizona, Herman [70] noted at least 65% survival for ponderosa pine greater than 8 inches (20 cm) dbh that had less than 60% crown scorch. Dieterich [41] observed 89% recovery of 6 to 14-inch (15-36 cm) dbh trees that had been up to 90% scorched by dormant-season (November) wildfire on the Coconino National Forest. Studies of postfire survivorship after scorching show mixed results, however. Davis and others [39] reported that more than 75% of ponderosa pines (5- to 11-inch (13-28 cm) dbh class) scorched more than 67% died within 2 years following a dormant-season (October) prescribed fire on the Coconino National Forest. Dormant-season studies indicate that bud kill, which is related to fire season, is more important than foliage kill in determining chances of southwestern ponderosa pine survival after burning [58,134,135]. Wagener [135] and Harrington [63] found the minimum requirement for ponderosa pine survival was 90% or less scorch with 50% or more of buds and twigs remaining. Five years after prescribed burning on the San Juan National Forest of Colorado, Harrington [63] found significant (p=0.05) differences in mortality of scorched interior ponderosa pine, depending upon season of burning. Mortality was lowest for fall-scorched trees (5%), and spring-scorched trees showed less mortality than summer-scorched trees (17 vs. 21%, respectively). Ninety percent of fire-damaged ponderosa pine that died had done so by postfire year 4. Most trees greater than 7.2 inches (18 cm) diameter survived fall burning even with 90% scorching. With spring and summer burning, trees less than 4 inches (10 cm) diameter died with greater than 50% scorching, while at least 90% scorching was required before trees larger than 4 inches (10 cm) in diameter were killed by spring or summer fire. PLANT RESPONSE TO FIRE:Fire prepares a favorable mineral seedbed for Arizona pine establishment. Germinants require mineral soil so the emerging root radicle immediately contacts soil moisture [142]. Seedling density may be great in years when favorable precipitation follows fire, resulting in "doghair" thickets if further fire does not reduce the stand [61]. Thinning by fire results in increased stem growth in remaining trees [89,96,104,131]. Removal of shrubs in southwestern ponderosa pine forests results in an increase in biomass production of overstory Arizona pines [100]. Sutherland and others
[122] present a linear regression
model to predict postfire radial growth of southwestern ponderosa pine after
prescribed fire.
DISCUSSION AND QUALIFICATION OF PLANT RESPONSE:Watershed: Improved water availability after fire may contribute to postfire growth of Arizona pine [56]. Alain-Morales [1] studied the effects of prescribed burning on watersheds in an Arizona pine forest in northwestern Chihuahua, Mexico. Comparing fire effects under fall drought burning, 2nd-year reburning under fall drought, winter burning, and no burning treatments, he found the most significant change in water flow occurred with the fall reburn treatment. The fall reburn showed a large increase in surface water flow relative to the no burn control. FIRE MANAGEMENT CONSIDERATIONS:Current fire return intervals are greater than the historical range of variability for southwestern ponderosa pine forests. Magnitude of fire decline is greater at lower than higher elevations, which may aid managers in determining where management actions to reduce fuels and restore more natural fire regimes might be of highest priority [132]. The expense of excluding fire from southwestern ponderosa pine forests in an active fire year can easily exceed a billion dollars, and these costly attempts at fire suppression are not always successful. In comparison, treatments to restore southwestern ponderosa pine structure and ecological processes are modest in cost [6,117]. Thinning to remove small-diameter trees, accompanied by prescribed fire, has been suggested as a means of restoring structure and function to degraded southwestern ponderosa pine ecosystems [5,21,31,45,57,62,110,138]. Frequent low-severity surface fires restore ecosystem function by reducing organic matter in woody debris, increasing soil moisture, increasing soil temperature (with accompanying rates of increased litter decomposition, soil nutrient cycling, and fine root growth), increasing productivity of understory herbs and shrubs, increasing basal diameter growth of overstory ponderosa pine, and preparing seedbeds [3,4,33,34]. Fire pruning of lower pine branches opens the canopy [55]. Frequent prescribed fires reduce fire hazard without damaging overstory ponderosa pine [3,4]. Biswell [21] listed several ways in which prescribed burning reduces wildfire hazard in ponderosa pine:
If several fire cycles have been missed, thinning presettlement trees and manually removing heavy fuels from the base of large trees may be necessary to in order to protect old growth from severe scorching or death [31]. Harrington [63] recommends growing-season (spring or summer) burning in southwestern ponderosa pine forests if the management objective is thinning from below, and fall prescribed burning if stand losses must be minimized. Weather parameters for prescribed burning in southwestern ponderosa pine [66], and a logistic regression model predicting probability of interior ponderosa pine mortality by tree size, scorch class, and season of injury are available [58]. Prescriptions for mid-summer burning in Arizona pine are available in Harrington [59]. He recommends burning in summer rather than fall to achieve more complete combustion of fuels [59]. This may also reduce effects of burning on other plants and animals, since natural fires occurred more often in June and July than in fall [19]. Prescriptions for prescribed burning in both open and closed stands of Arizona pine require measurement of litter and humus layer moisture content, relative humidity, and wind speed ranges for safe, effective burning. Recommended prescriptions for summer fires use downslope, backing fires for initial fuel reductions [59]. Beaufait [18] found that backfires in ponderosa pine needles spread more slowly and had less flame depth, longer residence time, and a higher rate of energy release than headfires. Swetnam and Dieterich [127] recommend allowing large (> 3000 acres (1200 ha)) prescribed natural surface fires in southwestern ponderosa pine in wilderness areas such as the Gila Wilderness. Based upon their fire history research, which showed evidence of mostly extensive but also small fires, they also recommend allowing small and patchy mixed-severity fires in approved areas, subject to the limitations of wilderness boundaries, visitor safety, and management and suppression capabilities. Models: Fuel moisture ratings for Arizona pine stands estimated using National Fire Danger Ratings showed good correlation at the driest levels but showed differences with increasing precipitation. Empirically derived equations permit adequate estimates of actual fuel moisture for burning projects. Harrington [60] presents a model for estimating moisture of fuels in Arizona pine. In a comparison of fuel loads on several Arizona pine sites, Harrington [62] found large differences between the relationship of forest floor depth to fuel loading, and cautioned managers against using forest floor depth:fuel loading regression models to assess fuels without some site-specific testing. Harrington has developed a model for estimating forest floor consumption in southwestern ponderosa pine forest based upon moisture content of the H surface soil layer [64]. Wildlife: Prescribed spring or fall burning on the Coronado National Forest of southeastern Arizona had no effect on the density of nesting pairs of 14 species of cavity-nesting birds, despite the fact that the fires destroyed more large, dead ponderosa pine snags than they created. Density of 2 species, the northern flicker and the violet-green swallow, was reduced after fires [72].
Related categories for SPECIES: Pinus ponderosa var. arizonica | Arizona Pine |
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