• FIRE ECOLOGY OR ADAPTATIONS
• POSTFIRE REGENERATION STRATEGY

FIRE ECOLOGY OR ADAPTATIONS:

Ponderosa pine seedlings establish on burns from on-site seed, dropped from the crowns of surviving and fire-killed trees [120], and from off-site seed borne by wind [99].

Fire regime: The sky islands of southeastern Arizona and southwestern New Mexico have among the highest incidences of lightning-caused fires in the United States [80,113]. The lightning fire season begins in late April, peaks in June, and runs into October. Maximum lightning fire incidence is above 6,000 feet (1,800 m), directly within the Arizona pine zone [9]. The fire season occurs in late spring and early summer in southeastern Arizona and southwestern New Mexico. Increasing temperatures and sparse rainfall create extremely dry conditions in spring. By June, weak storm systems typically bring lightning but little rain [95]. Few actual lightning ignitions occur, but these typically result in large burns. July has a high incidence of lightning-ignited fires, but total area burned is less compared to June [14]. By July or August, summer rains usually render fuels too moist to burn well [14,19,95].

The historical fire regime in Arizona pine forest was primarily frequent surface fire, with occasional mixed-severity and stand-replacement fires [11,29,66,72,116,121,137]. Southwestern ponderosa pine forests have undergone a shift in physiognomy since the late 19th and early 20th centuries. Prior to that time, southwestern ponderosa pines were reported as occurring in open, parklike stands with  thick grass understories [29,75,139]. Fire return intervals for Arizona pine generally ranged from 2 to 12 years for xeric sites, and up to 15 years for mesic sites [11,29,30,32,72,75,116,139]. Dendrochronological studies in mixed Arizona-Apache-Chihuahua pine gallery forests the Chiricahua Mountains show a fire interval range of 1 to 9 years (µ=4.2 years) [75]. In the Arizona pine and mixed-conifer belts (7,260-8,910 ft (2,200-2,700 m)) of 1 Chiricahua watershed (Mormon Canyon), fire return intervals ranged from 1 to 17 years (µ=3.7 years), with a tendency for wet years to precede fire years. Lower-elevation, mostly low-severity surface fires were an important source of ignition in the mixed-confer belt, where high-severity, stand-replacement fires mixed with low- and moderate-severity surface fires [95]. These regimes were probably largely driven by lightning, but it is reported that prior to European-American settlement, Native Americans ignited fires to drive game, for warfare, and to bring rain [29,95]. Reconstructive studies show presettlement densities of ponderosa pine forest in Arizona and New Mexico ranging from 3 to 66 trees per acre (7-166/ha) [32].

Fire frequency in Rocky Mountain ponderosa pine-Arizona pine forests in southeastern Arizona was greatly reduced after Euro-American settlement in the 1870s [108,116]. The explanations for this change include livestock grazing that removed grassy fuels, fire exclusion, and climatic factors [11,29,29,32,66,72,75,108,111,116,121,137,139]. It is likely no single factor is responsible. In ponderosa pine ecosystems of northern Mexico, where fire suppression is rare, mean fire return interval is less than 5 years. On an Arizona pine site in Durango, for example, widespread fires averaging every 9 years scarred at least 25% of sampled trees [49].

Today, many acres of southwestern ponderosa pine forests are overstocked, stagnant, and accumulating large quantities of litter at the expense of the grassy understory [29,123,124]. Fire exclusion has led to the build-up of fuels and led to severe crown fires in southwestern ponderosa pine and mixed-conifer forests. These forests contain an understory of young southwestern ponderosa pine, Rocky Mountain Douglas-fir, southwestern white pine, and Gambel oak: species that are less fire-resistant and more shade-tolerant than southwestern ponderosa pines [10]. Bahre [9,10] notes that fire frequency in ponderosa pine in the Chiricahua Mountains of southeastern Arizona has decreased since European-American settlement. The fire regime has changed from small, frequent surface fires to large, infrequent, stand-replacing crown fires. For example, the 1994 Rattlesnake Peak Fire burned 27,000 acres (10,800 ha) in southeastern Arizona [10].

Climate and grazing: Grazing appears to reduce fire frequency in southwestern ponderosa pine forests by removing grassy understory fuels. Interactive effects of grazing and climate on fire frequency are difficult to determine. Savage and Swetnam [111] suggest that climatic factors play the larger role in determining both fire frequency and Arizona stand structure. They attribute a pulse of southwestern ponderosa pine regeneration in the early 1900's to a favorable climatic sequence, which, when coupled with the lack of fire to thin the stands, resulted in the overstocked, stagnant stands found today. In the Chuskas Mountains on the Arizona-New Mexico border, this pulse of regeneration occurred on both sites with heavy domestic sheep grazing and on sites where livestock grazing had not been practiced for decades. In the Chuskas, mean fire frequency dropped dramatically (µ=4.2 years before 1830; < 3 fires recorded from 1830-1950) on both heavily grazed sites and on sites that had not experienced grazing for several decades. Likewise, a strong pulse of southwestern ponderosa pine occurred on both grazed and ungrazed sites [111].

Long-term fire history studies on the northern Colorado Front Range show that interannual variability in soil moisture, rather than drought alone, is conducive to widespread fire. Fire occurrence, especially widespread fire, tends to increase 1 to 4 years after above-average moisture availability in spring-summer [133]. Similarly, fire occurrence tends to increase 2 to 3 years after above-average precipitation in winter-spring  [11,123]. 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 [133]. 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 [126,133]. Alternating wet and dry years resulting from El Nino-La Nina events in the mid- to late 1800s increased fire frequency [133].

Fire histories: Prior to the 1880s, surface fires burned through Arizona sky island southwestern ponderosa pine forests once or twice a decade. Fires were somewhat less frequent in higher-elevation, mixed-conifer forests [123]. In the Rincon Mountain Wilderness, fire regime in Arizona pine forest was mostly 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. Mean fire return interval in the mixed-conifer type from 1748 to 1996 was 9.9 years, with a range of 3 to 19 years [11].

Historical fire frequency at the ponderosa pine-oak woodland interface has been documented at 1 fire or more per decade [37,75]. In a fire history study of Madrean oak-mixed pine gallery forest in Chiricahua National Monument, Arizona, Swetnam and others [124] found an historical (1620-1890) fire regime characterized by frequent surface fires at intervals ranging from 1 to 38 years. Mean fire interval across the study site was 3.9 years; mean fire return interval for fires that scarred at least 25% of trees on the study site was 13.2 years. Species composition of the forest was southwestern ponderosa, Chihuahua, Mexican pinyon, and Apache pines, and canyon live (Quercus chrysolepis), netleaf, silverleaf, and other oaks. The authors hypothesized that the oak-pine gallery served as a conduit that allowed fire spread across elevational gradients.

Fule and Covington [50] found that in Durango, fire exclusion in Arizona pine-Durango pine-Apache pine-oak (Quercus spp.) woodland favored sprouting oaks, alders (Alnus spp.), and madrones (Arbutus spp.) over Arizona and other pines. On sites where fire exclusion was practiced, a larger proportion of pines was killed by uncontrollable wildfires due to higher fire severities compared to sites with uninterrupted fire regimes.

Fire regimes where Arizona pine is a dominant or important member of the community are summarized below.

POSTFIRE REGENERATION STRATEGY [120]:

Related categories for SPECIES: Pinus ponderosa var. arizonica | Arizona Pine