Saplings	Poles	Sawtimber	All trees
	Site A Site B	Site A Site B	Site A Site B	Site A Site B
Crown damage:
none	0 0	8 10	8 8	7 12
light	20 16	20 37	22 72	22 50
moderate	10 16	20 16	35 11	18 18
severe	70 68	52 37	35 9	53 30
Dead after 2 years:
none	0 0	0 0	0 0	0 0
light	0 0	0 0	0 0	0 0
moderate	0 5	0 6	0 0	0 10
severe	60 68	42 30	25 6	43 20

*saplings are < 5 in. dbh, poles are 5-11 in. dbh, sawtimber is > 11 in. dbh

PLANT RESPONSE TO FIRE:

Fire prepares a mineral soil seedbed favorable to interior ponderosa pine regeneration [131,272,320]. Germinants require mineral soil so the emerging root radicle immediately contacts soil moisture [320]. On the Fort Valley Experimental Forest near Flagstaff, moisture in the upper 2 inches (5 cm) of soil was consistently greater on burned sites than on unburned sites. Plots were seeded to interior ponderosa pine 4 weeks after a November prescribed fire. The 1st postfire growing season was droughty, and although overall seedling establishment was poor, it was significantly greater (p=0.05) on burned than on unburned plots. Of 62 surviving seedlings, 95% (59 seedlings) were on burned plots, and 5% (3 seedlings) were on unburned plots [131]. A study on the Coconino National Forest showed a frequency of 90% for natural regeneration the year following moderate-severity prescribed surface fire. At postfire year 12, frequency of interior ponderosa pine regeneration had dropped to 25% on burned plots. Adjacent unburned plots showed no seedling establishment during the 12 years of the study [109].

Mature interior ponderosa pine may show increased growth following low-severity surface fires. Following prescribed fall burning on the Front Range of Colorado, mean fascicle lengths and bud sizes (length and diameter) were significantly greater (p < 0.01) on trees that received burning treatment than on trees on unburned control sites. Sutherland and others [276] present a linear regression model to predict postfire radial growth of southwestern ponderosa pine after prescribed fire.

DISCUSSION AND QUALIFICATION OF PLANT RESPONSE:

The open canopy created by frequent fire produces physiological and morphological changes that significantly increased interior ponderosa pine's resistance to bark beetles and foliar insects, increased nutrient uptake, and promoted growth on a northern Arizona site. These changes occur even in old growth. A year following thinning and prescribed burning near Flagstaff, resin flow was higher in presettlement trees (established before 1876) on burned plots compared to presettlement trees on untreated control plots, with flows of 12.2 mL/24 h and 3.6 mL/24 h, respectively. Foliar toughness, which  increases resistance to pine sawflies, was greater on burned compared to control plots (76 vs. 69.8 g tension). Trees on burned plots also had greater leaf nitrogen content (1.59 vs. 1.44 g/m²) and showed more basal area growth  (20.7 vs. 14.3 cm²) on burned than on control plots. Thinning without burning produced similar changes in tree physiology and morphology, except that with a mean resin flow of 4.3 milliliters per day, presettlement trees on plots that were thinned but not burned developed less resistance to bark beetles than trees on thinned and burned plots [75,106,107].

FIRE MANAGEMENT CONSIDERATIONS:

Current fire return intervals are greater than the historical range of variability for 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 [295].  Millions of acres of forest burns in the western United States during fire years. Roughly half of those acres are in ponderosa pine. The expense of excluding fire from 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 ponderosa pine structure and ecological processes are modest in cost [17].

Thinning to remove small-diameter trees, accompanied by prescribed fire, has been suggested as a means of restoring structure and function to degraded ponderosa pine ecosystems [16,41,74,101,101,138,145,252,303]. 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 [6,7,75,76]. Fire-pruning of lower pine branches opens the canopy, aiding production of wildlife shrubs such as snowberry and chokecherry [126]. Frequent prescribed fires reduce fire hazard without damaging overstory ponderosa pine [6,7]. Biswell [41] listed several ways in which prescribed burning reduces wildfire hazard in interior ponderosa pine:

• reduces the volume of dead fuel
• thins dense thickets of saplings and pole-sized stands
• keeps shade-tolerant trees out of the understory, thus destroying ladder fuels
• raises the height of green foliage level by needle scorching, making crown fires less likely to occur
• allows pine needles to fall to the ground, where they compact more closely than when draped over understory vegetation and debris that keep them off the ground

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 [74]. Harrington [142] recommends growing-season (spring or summer) burning in interior 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 [147], and a logistic regression model predicting probability of interior ponderosa pine mortality by tree size, scorch class, and season of injury are available [140].

Swetnam and Dieterich [283] 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.

Mixed and stand-replacing fire: | Interior Ponderosa Pine High-elevation interior ponderosa pine forests were historically denser, with lower fire frequencies and greater severities, than lower-elevation forests. Even at low elevations, north-facing slopes and mesic ravines probably supported dense forests that experienced occasional mixed and stand-replacing fires. Evidence of a mixed regime of frequent surface fires and occasional mixed or stand-replacement fires suggests that a single prescription cannot capture historical variability of fire regimes in interior ponderosa pine types [295].

Historical occurrence of large, widespread prehistoric fires in interior ponderosa pine demonstrates the potential for large portions of montane zones to burn during a single year. Interannual climatic variability, particularly El Nino-La Nina events, can greatly increase fuels. Fuels management through thinning and prescribed burning reduces the probability of widespread wildfire in most years, but it is uncertain that low to moderate levels of fuels management will reduce fire hazard enough to prevent stand-replacing fire during years when the weather is exceptionally conducive to fire spread.  It may be helpful for public education programs on fire hazard reduction to emphasize that although frequent surface fires were most common in interior ponderosa pine stands, mixed and stand-replacement fires did occur in prehistoric times, and that during years of high fire hazard, stand-replacement fires may occur again despite fuels reduction programs [295].

Fuels: | Interior Ponderosa Pine Fuel loads in pole-sized thickets of interior ponderosa pine can be high. Ponderosa pine's long needles form a loose litter layer that burns readily [243]. Near Flagstaff, fuels averaged 28.3 tons per acre (63.4 t/ha), 21.3 tons (19.2 t) of which was duff [75].  Eakle and Wagle [98] provide a model, developed on the Fort Apache Reservation, for estimating fine fuels in southwestern ponderosa pine stands. Harrington [143] presents a model for estimating forest floor consumption in southwestern ponderosa pine forest based upon moisture content of the H surface soil layer.

Beaufait [35] 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.

Boldt [49] provides prescriptions for thinning pole-sized "dog-hair" thickets of interior ponderosa pine using sequential treatments.

Range: | Interior Ponderosa Pine Prescribed fire can enhance understory forage production by reducing forest floor depth, tree density, and allelopathic toxins, and by increasing nutrient availability [12,148]. On the Fort Apache Reservation in Arizona, Gambel oak had reached mid-story in an interior ponderosa pine forest. Prescribed surface fires at 5- to 7-year intervals reduced Gambel oak to an understory shrub that was  readily available to browsing animals. Coverage of the dominant grass, mountain muhly, increased under the prescribed burning regime [41].

Pearson and others [231] reported that on the Wild Bill Range of Arizona, an area of interior ponderosa pine that had been thinned to 20 square feet basal area per acre (4.5 m²/ha) did not show a significant reduction in density following a wildfire. The fire crowned and killed the trees in an adjacent unthinned stand, however. Forage production increased as much as 578 pounds per acre (650 kg/ha) from prefire levels on sites were stand-replacement fire occurred, while pre- and postfire forage production were similar on thinned and burned sites.

Wild ungulate foraging, especially that of bighorn sheep, increased after late April prescribed fire in a interior ponderosa pine/sedge-bluegrass (Carex-Poa spp.) community in Custer State Park, South Dakota. Grass production did not increase after the fire, but forb production was significantly increased (p < 0.01) on burned compared to unburned sites [100].

October prescribed burning brought a 2-fold increase in nitrogen content of Arizona fescue and associated grasses on an interior ponderosa pine/Arizona fescue community on the Fort Valley Experimental Forest near Flagstaff. Concentrations of potassium, phosphorus, calcium, and magnesium were also generally greater in forb and grass species on burned plots than in herbs on unburned control plots. Understory biomass was significantly (p < 0.05) greater on burned plots. The 1st fall after burning, understory yield was twice as great on burned plots compared to unburned plots [148].

Overgrazing can greatly reduce fire frequency by removing understory fuels. Fire history studies of southwestern ponderosa pine document the near-cessation of fire in the mid-1800s due to livestock grazing in Arizona and New Mexico [289,290].

Exclusion of the recurrent fires that once swept the interior ponderosa pine-plains grasslands interface has resulted in interior ponderosa pine invasion into the grassland. Expansion is likely to continue without application of prescribed fire [48,270].

Since fire kills interior ponderosa pine seedlings and some saplings, prescribed fire can be used to reduce the density of encroaching pines. Late-April fire in little bluestem-hairy grama-sideoats grama in the Black Hills of South Dakota caused 79% mortality of pine seedlings. Mean density of interior ponderosa pine seedlings was 8,132 per acre (3251/ha) before the fire and 1,669 per acre (4173/ha) in late summer, after burning. The fire reduced ground fuels by 32% [121].

Wildlife: | Interior Ponderosa Pine Fire benefits the majority of wildlife species inhabiting interior ponderosa pine ecosystems. Among bird species, fires in interior ponderosa pine tend to increase guilds that use open stands and snags. On the Prescott National Forest in Arizona, a stand-replacement burn in interior ponderosa pine attracted relatively more granivores, aerial insectivores such as flycatchers, and bark-feeding insectivores such as woodpeckers, while unburned areas attracted more ground and foliage insectivores [45]. A 20-year wildlife study on 4 stand-replacement burns near Flagstaff had similar findings for birds. Most rodent species, except chipmunks, also increased on burned interior ponderosa pine sites relative to adjacent unburned control sites. Wild ungulate use increased greatly on burned sites compared to unburned sites. Mule deer use of burned sites declined at postfire year 1, but  increased to 2.5 times that of unburned sites for the next 19 years. Elk use also dropped at postfire year 1 relative to unburned sites, but increased to 3 times that of controls until postfire year 20, when use of burned and unburned sites was nearly equal. Elk use of the burns peaked at postfire year 7 [201].

Availability of interior ponderosa pines as nesting sites for cavity-nesters, particularly secondary cavity-nesters, is limited due to the scarcity of large, old-growth trees. Brawn and Balda [57] reported that violet-green swallow, pygmy nuthatch, and western bluebird populations increased after artificial nest boxes were placed in open and thinned plots, but not on untreated (dense) plots. Prescribed underburning that leaves large trees may encourage nesting. Prescribed fire in interior ponderosa pine of Wind Cave National Park, South Dakota, significantly  (p< 0.05) increased the number of breeding bird pairs and deer mice on burned sites compared to unburned control sites [47]. Creating small openings with prescribed fire may also promote nesting. Aulenbach and O'Shea-Stone [25] noted differential songbird use of burned sites created by a small, crowning wildfire compared to adjacent unburned sites in interior ponderosa pine on the Front Range of Colorado. Several species including red-breasted nuthatch, chipping sparrow, yellow-rumped warbler, and northern flicker occurred only on the burn. American robin, Steller's jay, and dark-eyed junco preferred the burn but used the control, while pygmy nuthatch, downy woodpecker, white-breasted nuthatch, and mountain chickadee occurred only on the unburned site.

Standing time of interior ponderosa pine snags is somewhat predictable. Harrington [144] found that 75% of fire-killed interior ponderosa pine snags on the San Juan National Forest of southwestern Colorado fell within 10 postfire years. Fall rates were not significantly different (p=0.1) among trees 2 to 16 inches (5-41 cm) dbh, but large trees that died quickly after 80% or greater crown scorch were likely to fall more quickly, while large trees that survived 2 or 3 postfire years before succumbing were likely to remain standing for longer periods of time.

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Related categories for SPECIES: Pinus ponderosa var. scopulorum | Interior Ponderosa Pine

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• Botanical and Ecological Characteristics
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