• FIRE ECOLOGY OR ADAPTATIONS
• POSTFIRE REGENERATION STRATEGY

FIRE ECOLOGY OR ADAPTATIONS:

Surface fire often kills interior ponderosa pine seedlings and saplings [48,55,85]; however, the effect is dependent upon fire severity and stand structure. Young trees in open canopies acquire fire-resistant traits rapidly, and 6-year-old saplings often survive low-severity surface fire. Fire is especially damaging in overcrowded young stands: the relatively denser foliage and thinner bark of trees in thick stands reduce resistance to surface fire. Such stands are also prone to crown fire [53].

Fire prepares a favorable seedbed for interior ponderosa pine regeneration. Periodic surface fire removes the heavy litter and duff that accumulate in ponderosa pine forests. Wind-borne seeds falling from the crowns of surviving or fire-killed trees land on a nutrient-enriched mineral seedbed under an open canopy that favors germination and seedling establishment [248,272,320]. Seedling-water relations may be enhanced when fire removes competing vegetation [131].

Fire regimes: | Interior Ponderosa Pine Interior ponderosa pine evolved under a regime of frequent surface fires and infrequent mixed-severity and stand-replacement fires [50,278,304]. Presettlement fires in lower-elevation (<7920 feet (2400 m)) ponderosa pine communities were mostly low- to moderate-severity surface fires that  maintained open-grown, parklike stands [31,59,61,72,75,115,147,275,277,295,302,308]. Prior to the 1900s interior ponderosa pine was perpetuated by surface fires that recurred every 5 to 30 years. Fire return intervals tended to be shorter in the warm, dry forests of the Southwest than in the cool, dry forests of the central Rocky Mountains or the cool, relatively moist forests east of the northern Rocky Mountains [59,75,93,139,196]. For example, Dieterich and Swetnam [94] report a 2-year mean fire return interval for presettlement interior ponderosa pine on the Fort Valley Experimental Forest near Flagstaff; Laven and others [196] report a 45.8-year mean fire return interval (range=20.9-66.0 years) for the Front Range of Colorado; and Brown and Sieg [61] report an average fire return interval of 22 years for presettlement interior ponderosa pine forests of South Dakota. Gruell [129] provides an annotated record of wildfires that occurred throughout interior ponderosa pine's range during the settlement period (1776-1900). Historic fire regimes are summarized by state and region in the ending paragraphs of this section.

Fire history studies show mixed-severity fire regimes for some interior ponderosa pine forests. Many forests experienced infrequent, large stand-replacement fires prior to the European-American settlement period [59]. For example, Laven and others [196] reported a range of 3 to 161 years (mean (m)=45.8) for the central Colorado Front Range. Small fires occurred on average every 20.9 years; large fire occurrence averaged 41.7 years. Higher-elevation  (>7920 feet (2400 m)), relatively mesic mixed-conifer forests with interior ponderosa pine, Rocky Mountain Douglas-fir, and Rocky Mountain lodgepole pine tend to have more mixed-severity fires than lower-elevation interior ponderosa pine forests  [277,312]. This is probably because herbaceous species recover from fire more quickly, and dry out earlier in the season, at low elevations. In the northern Colorado Front Range, mean fire interval for widespread (> 10 trees scarred), mixed-severity fire in higher-elevation forests during 1650 to 1920 ranged from 34 to 43 years; mean fire interval for widespread, mixed-severity fire in lower-elevation interior ponderosa pine forest was 14 to 24 years. During the same period, mean fire interval for localized (2-9 trees scarred) fires in higher- and lower-elevation ponderosa pine forest was 17 to 22 years and 8 to 18 years, respectively [295]. Interior ponderosa pine at 5,633 to 5,919 feet (1707-1804 m) in the Chiricahua Mountains experienced an historical fire return interval ranging from 1 to 15 years (m=6.17 years) compared to a range of 1 to 31 years (m=7.96 years) in higher-elevation (6,801-7,002 feet (2073-2134 m)) mixed-conifer forest in the Chiricahua Mountains. Some higher-elevation mixed-conifer forests show an historical fire regime similar to lower-elevation ponderosa pine, however. Swetnam and Basian [277] suggest that mixed-conifer forests on dry, steep slopes, where fire can easily ignite and spread upslope from many directions, are most likely to experience frequent surface fire.

Native American burning influenced fire regimes in ponderosa pine ecosystems prior to and during European-American settlement, mostly by increasing fire frequency. For example, in studying fire history in the Chiricahua Mountains of southeastern Arizona, Seklecki and others [261] found that southwestern ponderosa pine showed a shorter fire return interval (mean=3.0 years, range=1.0-16.0 years) between 1700 and 1900, when Chiricahua Apaches inhabited the area, than in earlier periods when Apaches did not reside there. Dormant-season (spring) fires were also more frequent during that time period in southwestern ponderosa pine sites of the Chiricahua Mountains compared to southwestern ponderosa pine sites near the Mexican border, which were used as travel corridors but mostly unoccupied at that time [261]. Near the border, most fires occurred in the late May to early June growing season [277]. Fire frequency increased to 1 per year (an uncommonly low number for southwestern ponderosa pine) during the Spanish-Apache wars of 1760 to 1780, when Apaches probably used fire as a method of warfare. Fire occurrence terminated abruptly in the late 1880s, coincident with Apache resettlement to reservations (Geronimo surrendered in 1886) and increased livestock grazing by European-Americans [261,277].

Fire regimes in interior ponderosa pine also affect regimes of adjacent communities. In many cases, fire frequency has been reduced in adjacent communities because ignitions in interior ponderosa pine are suppressed and fire does not spread into adjacent communities. For example, stand structure of a shrub live oak-hairy mountain-mahogany (Quercus turbinella-Cercocarpus montanus var. paucidentatus) community on the Prescott National Forest of Arizona was historically a fire-maintained mosaic of different-aged chaparral. Mean fire frequency in the adjacent interior ponderosa pine/Arizona white oak (Q. arizonica) stand was 2 years. After over 100 years of fire exclusion, the chaparral stand is even-aged and senescent, with heavy accumulations of dead material. Interior ponderosa pine is encroaching into the chaparral [91].

Climate and fire frequency: | Interior Ponderosa Pine Long-term fire history studies on the northern Colorado Front Range show that interannual variability in soil moisture is more conducive to widespread fire than drought alone. Fire occurrence, especially widespread fire, tends to increase 1 to 4 years after above-average moisture availability in spring-summer [295]. Similarly, fire occurrence tends to increase 2 to 3 years after above-average precipitation in winter-spring  [28,278]. Climatic variation that produces widespread, stand-replacing fire has been associated with southern oscillation events. El Nino is associated with greater soil moisture and herbaceous fuel production in spring, with fire occurrence peaking several years after El Nino events. La Nina events are associated with dry springs, with fire occurrence peaking in the same year [295]. A decline in fire frequency in interior ponderosa pine forests of the Southwest coincided with reduced El Nino-La Nina events between 1780 and 1830 [281,295]. Alternating wet and dry years resulting from El Nino-La Nina events in the mid- to late 1800s increased fire frequency [295].

Fire exclusion: | Interior Ponderosa Pine The ecological changes that have occurred in ponderosa pine forests over the last century have been well documented by a number of researchers [21,72,93,115,221,303,308,309]. Frequent, mostly light-severity surface fires thinned small trees, especially the less fire-resistant Rocky Mountain Douglas-fir and firs. The combined effects of 60 to 80 years of fire exclusion, logging that removed many overstory pines, heavy livestock grazing, and climate change have created closed-canopy stands with dense understories and ladder fuels [55,60,61,74,75,115,253,278,289].  These changes have been documented throughout interior ponderosa pine's range [74,207,268], and have also occurred in interior ponderosa pine/Douglas-fir and mixed-conifer types [176,295]. A fire history study of interior ponderosa pine stands near Flagstaff, Arizona, documented changes in stand structure over a 116-year period [75].

When wildfire burns these dense interior ponderosa pine stands under dry conditions, the abundant fuel quickly allows it to develop a high intensity and to spread into tree crowns. Severe, stand-replacing fires were infrequent in interior ponderosa pine forests in the past; now they are common [20,53,60]. Abundant litter and living and dead woody fuels feed explosive wildfires of intensities and sizes that have not occurred for many centuries, if ever [277]. The increasingly frequent occurrence of large, crowning wildfires in interior ponderosa pine may indicate a shift to a fire regime characterized by very large (> 100 000-acre (4000 ha)) crown fires [74,252]. Data in Sackett and others [252] show a great increase in the number of acres burned by wildfire in Arizona and New Mexico since 1970. Over 100,000 acres (40 000 ha) burned from 1915 to 1990, with 70% of the fires occurring after 1970. Before 1970, total acreage burned per year never exceeded 130,000 acres (52 000 ha). After 1970 there were 8 years in which total acreage burned exceeded 119,000 acres (47 600 ha), with nearly 500,000 acres (200 000 ha) burning in 1989. On the Mexican side of the international border, where fire exclusion is not practiced, frequent, widespread surface fires have persisted in southwestern ponderosa pine and mixed-conifer forests [277].

Besides unprecedented, large-acreage severe fires, other ecological consequences of fire suppression in interior ponderosa pine ecosystems include [75,77,309]:

Organisms within interior ponderosa pine ecosystems have evolved with fire, and frequent fires are probably required to maintain ecosystem health [52,74,119,218,219]. Some researchers have questioned whether ponderosa pine ecosystems are sustainable under current conditions [21,75,262,277].

Fuels: | Interior Ponderosa Pine Even within the same provenance, fuel loadings in ponderosa pine stands may vary greatly depending upon age class, stand structure, and understory composition [249]. Prediction equations for fuel loads in ponderosa pine are available  [3,108,109,110,141,245]. Mean fuel loadings (tons/acre, 0-1 inch and > 1-inch fuels) have been calculated for ponderosa pine stands on 3 Reservations, 2 National Parks, and 8 National Forests of Arizona and New Mexico. The study involved 62 sites: mean forest floor loading for the entire 62 stands was 12.5 tons/acre (4.1 t/ha) [246].

Absence of fire in interior ponderosa pine and mixed-conifer forests has led to uncharacteristically large accumulations of surface and ground fuels [27,173,249]. Structurally, fire exclusion has led to vertical continuity, with Douglas-fir, firs, and other shade-tolerant, less fire-resistant species in the understory. These late-successional species become ladder fuels that encourage crown fires in interior ponderosa pine and mixed-conifer forests [27,53]. In the dry southwestern climate, the natural accumulation of pine needles and woody fuels is exacerbated by slow decomposition [147].

State and regional fire regime studies | Interior Ponderosa Pine
Black Hills and interior Northwest: Fire season on the interior ponderosa pine-grassland savannas of eastern Montana and the Dakotas peaks in July and August, when the majority (73%) of lightning-strike ignitions occur. Wildfire season generally extends from April to September [159]. Frequent surface fires historically burned litter and killed young interior ponderosa pine and other non-sprouting woody species encroaching into grasslands [48,51]. Interior ponderosa pine in the Black Hills was historically characterized by 2 communities: interior forest and savanna. These treed landscapes were described as "islands" surrounded by plains grasslands. Brown and Sieg [61] found that in interior forest sites at Jewel Cave National Monument, South Dakota, fire return intervals from the 1500s to the late 1800s averaged 20 to 24 years, with a range of 1 to 93 years. Fire return intervals at savanna sites in Wind Cave National Park, South Dakota, averaged 10 to 12 years, with a range of 2 to 23 years. The fire return interval of interior ponderosa pine savannas is the shortest documented for northern ponderosa pine ecosystems, and is similar to the short-return intervals of interior ponderosa pine forests in the Southwest.

Invasion of interior ponderosa pine onto grasslands, and increased tree density in formerly open savanna, is thought to be largely attributable to reduced fire frequency, although grazing has probably contributed to increased interior ponderosa pine density on forest and grassland margins [48,51]. A little less than one-half of interior ponderosa pine in the Black Hills are single-storied, even-aged stands that developed after crown fires or mountain pine beetle epidemics [7].  "Dog-hair" interior ponderosa pine thickets are common on many sites [48].

Brown and others [60] report the following measures of fire frequency on interior ponderosa pine sites less than 50 acres (20 ha) in size:

Colorado: A fire history study of a 10,000-acre (4000 ha) interior ponderosa pine-Rocky Mountain Douglas-fir site in central Colorado showed a pattern of frequent surface fires from 1197 to 1851. Large stand-replacement fires were rare, but several landscape-level fires are documented. Intervals between fire years ranged from 1 to 128 years at  the landscape scale and from 1 to 58 years for individual stands. Fires occurred throughout the growing season. Fire size varied across time; for example, numerous small fires occurred in the 1500s, while landscape-level fires occurred in 1631, 1696, and 1723. After 1723, there were few fires until 1851; that fire was a stand-replacement, mixed crown and severe surface fire that covered most of the landscape. There have been no extensive fires in the study area since 1851, and most stands have not experienced fire for nearly 100 years [59].

Utah: Fire history studies show a range of 3 to 47 years for interior ponderosa pine in southern Utah [62,207,269]. Studies in Zion National Park show a presettlement fire frequency range of 3 to 12 years for interior ponderosa pine and mixed-conifer forests. All study sites had experienced fire at least once a decade [62,207]. Large fires that burned more than 1,000 acres (400 ha) occurred nearly every 3 years on the Horse Pasture Plateau. Fire return intervals declined greatly in the Park beginning in the  late 1800s, when the study sites were subject to intense livestock grazing and grassy fuels were sparse. Although grazing has not been practiced in the Park for over half a century, fire events remained infrequent under fire exclusion [207].

Nebraska: Bragg [54] evaluated fire frequency for upland interior ponderosa pine-bur oak forest adjacent to sandhills prairie. He found a mean fire return interval of 3.5 years from 1851 to 1900, and a mean of 8.5 years for 1901 to 1950. Without the frequent fires that killed the seedlings, upland interior ponderosa pine has invaded sandhills prairie communities [174].

Texas: A fire history study in mixed interior ponderosa pine-Rocky Mountain Douglas-fir-southwestern white pine in Guadalupe Mountain National Park showed a mean return interval between major fires of 17 years prior to the 1920s; mean return interval for all fires detected between 1696 and 1922 was 4.7 years. Evidence of fire scarring was not found after 1922. Portions of the montane forest have a 2-tiered structure of overstory interior ponderosa pine and Rocky Mountain Douglas-fir, and dog-hair thickets of ponderosa pine and Douglas-fir saplings in the understory [1,2].

New Mexico: Swetnam and Dieterich [283] found that extensive surface fires were common in the Gila Wilderness of southwestern New Mexico before 1900. Mean fire return interval from 1633 to 1900 was 4 to 8 years, with a range of 1 to 26 years. Most fires burned throughout the 40.5-hectare study sites, although a few fires appeared to be smaller. Fire history was similar in northern New Mexico, with extensive high-frequency, low-severity surface fires in low-elevation (< 5,545 feet (1690 m)) interior ponderosa pine forests and less frequent surface fires, along with some patchy crown fires, in higher-elevation mixed-conifer Rocky Mountain Douglas-fir-interior ponderosa pine-white fir forest. Across sites, the minimum fire interval for major fires (> 10% of trees scarred) in interior ponderosa pine before 1900 ranged from 1 to 9 years; maximum fire interval for major fires ranged from 16 to 51 years (m=9.4 years). In mixed-conifer forests where interior ponderosa pine was codominant, minimum and maximum fire interval ranges were 1 to 4 and 21 to 46 years, respectively (m=10.4 years). Fires were rare in interior ponderosa and mixed-conifer forest after the 1850s. The authors attribute the initial drop in fire frequency to domestic sheep grazing by the Navajos, and by the 1900s, to fire exclusion by European-Americans [289].

Arizona: The sky islands of southeastern Arizona have among the highest incidences of lightning-caused fires in the United States [183,257]. The lightning fire season begins in late April, peaks in June, and runs into October. Maximum lightning fire incidence is above 6,000 feet (1800 m): directly within the interior ponderosa pine zone [27]. Prior to the 1880s, surface fires burned through ponderosa pine sky islands once or twice a decade, but occurred at highly variable intervals. Fires were somewhat less frequent in higher-elevation, mixed-conifer forests [278]. Swetnam and others [279] reported a 9- to 22-year range (m=14.6) for presettlement fire return intervals in interior ponderosa pine-Douglas fir in Chiricahua National Monument. Fire frequency in interior ponderosa pine-Arizona pine forests in southeastern Arizona was greatly reduced after Euro-American settlement in the 1870s. Intense livestock overgrazing coupled with fire exclusion caused both the number of fires and the area burned to decline dramatically [235].

Historical fire frequency in the interior ponderosa pine-encinal oak woodland interface has been documented at least 1 fire per decade [83,172]. In interior ponderosa pine-oak woodlands of the Rincon Mountain Wilderness, fire regime was dominated by large-scale (> 500 acre (200 ha)), early-season (May-July) surface fires. Mean fire return interval from 1657 to 1893 was 6.1 years, with a range of 1 to 13 years. From 1748 to 1996, mean fire return interval in the high-elevation mixed-conifer type was 9.9 years, with a range of 3 to 19 years [28].

There are many western plant communities and ecosystems in which interior ponderosa pine is either dominant, an important component of the vegetation, or an invader. Historic fire return intervals for these communities and ecosystems are summarized below. Please refer to the Fire Effects Information System report on the dominant species listed here for further information on fire regimes in these communities and ecosystems.