Arizona pine can withstand extensive scorching as long as buds and twigs, which tolerate higher temperatures than needles, are not badly scorched [43,115,145]. It may recover from as much as 90% scorching as long as 50% of buds and twigs survive to maintain shoot growth on defoliated branches [145]. Extensive scorching of Arizona pine crowns may cause mortality within 3 postfire years [60,72,91]. Generally, recovery of interior ponderosa pine forests is best after dormant-season scorching; trees scorched in the growing season show poorer survivorship [60,65]. After a growing-season (July) wildfire in northern Arizona, Herman [72] noted at least 65% survival for Arizona or ponderosa pines greater than 8 inches (20 cm) DBH that had less than 60% crown scorch. Dieterich [43] 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 [41] reported that more than 75% of 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 Arizona pine survival after burning [60,137,138]. Wagener [138] and Harrington [65] 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 [65] found significant (P=0.05) differences in mortality of scorched interior Arizona 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 pines 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 [65].
Arizona pine is a native tree. It reaches 75 to 100 feet (23-30 m) in height and 30 to 50 inches (75-130 cm) in diameter at maturity. The branches are stout, thick, and self pruning. The trunk is usually devoid of branches for half or more of its length. The crown is typically open and varies from short and conical to rounded or flat [83,89,94,109]. Bark of mature trees is about 2 inches (5 cm) thick. Needles are 4 to 9.5 inches (10-24 cm) long. The majority of needles are 4- or 5-fascicled, but some trees are 2- or 3-fascicled [45,50,94]. Average female cone length (2.5 inches (6.3 cm)) is shorter than other varieties of ponderosa pine. However, there is great between-population variation in length of Arizona pine cones [45]: they range from 1.5 to 2.4 inches (5-8 cm) long. Seeds are winged, with the seed body ranging from 3 to 4 mm in length and the wing extending to 15 mm [50,94]. Mature trees have deep, extensive root systems [29]. Depending on the substrate, roots may penetrate to depths of 33 to 40 feet (10-12 m). Lateral root development is closely related to crown width and varies with tree density. Lateral roots may extend 100 feet (30 m) in open stands [118].
Arizona pine and interior ponderosa pine can be very similar in appearance. In a study comparing morphological traits used to distinguish Arizona and interior ponderosa pines, Dodge [45] found considerable overlap between the 2 taxa. He concluded that differences between the taxa were "minor," and that between-population morphological differences in Arizona pine were at least as pronounced as interspecies differences in morphology. Distinguishing the 2 taxa can be further confounded by their tendency to hybridize. Arizona pine is morphologically distinguished from interior ponderosa pine by having shorter needles with a majority of 4- or 5-needled (vs. 2- or 3-needled) fascicles; smaller cones with incurved (vs. reflected) prickles; thinner bark; a rounder, more open crown; and being shorter and less broad at the base at maturity [45,50,79]. The Flora of North America provides botanical descriptions and dichotomous keys for Arizona pine and other ponderosa pine.
Fire adaptations: Arizona pine is adapted to survive frequent low- to moderate-severity surface fires. Mature trees have thick bark, insulated buds, and a high capacity to recover from crown scorch, all of which confer resistance to surface fires [51,80,95]. Arizona pine is self pruning, which discourages torching. It has a more open crown than ponderosa pine, also reducing the likelihood of torching [145].
Arizona pine seedlings establish on burns from on-site seed, dropped from the crowns of surviving and fire-killed trees [123], and from off-site seed borne by wind [102].
FIRE REGIMES: The sky islands of southeastern Arizona and southwestern New Mexico have among the highest incidences of lightning-caused fires in the United States [82,116]. 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 [11]. 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 [98]. 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 [16]. By July or August, summer rains usually render fuels too moist to burn well [16,21,98].
Historically, Arizona pine forests had mostly frequent surface fire, with occasional moderate-severity and stand-replacement fires [13,31,68,74,119,124,140]. Arizona and ponderosa pine stands have undergone a shift in physiognomy since the late 19th and early 20th centuries. Prior to that time, they were reported as being open and parklike, with thick grass understories [31,77,142]. Fire-return intervals for Arizona pine generally ranged from 2 to 12 years for xeric sites, and up to 15 years for mesic sites [13,31,32,34,74,77,119,142]. Dendrochronological studies in mixed Arizona-Apache-Chihuahua pine gallery forests the Chiricahua Mountains show a fire-return interval range of 1 to 9 years (x̄=4.2 years) [77]. In the Arizona pine and mixed-conifer belts (7,260-8,910 ft (2,200-2,700 m)) of a Chiricahua watershed (Mormon Canyon), fire-return intervals ranged from 1 to 17 years (x̄=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-conifer belt, where high-severity, stand-replacement fires mixed with low- and moderate-severity surface fires [98]. These fires were probably mostly lightning-ignited, with some human ignitions. Prior to European-American settlement, Americans Indians ignited fires to drive game, for warfare, and to bring rain [31,98]. Reconstructive studies show presettlement densities of Arizona and ponderosa pine forests in Arizona and New Mexico ranging from 3 to 66 trees per acre (7-166/ha) [34].
Fire frequency in Rocky Mountain ponderosa pine-Arizona pine forests in southeastern Arizona was greatly reduced after Euro-American settlement in the 1870s [111,119]. The explanations for this change include livestock grazing that removed grassy fuels, fire exclusion, and climatic factors [13,31,31,34,68,74,77,111,114,119,124,140,142]. It is likely no single factor is responsible. In Arizona pine communities 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 [51].
Today, many acres of Arizona and ponderosa pine forests are overstocked, stagnant, and accumulating large quantities of litter at the expense of the grassy understory [31,126,127]. Fire exclusion has led to the build-up of fuels and led to severe crown fires in low-elevation pine forests. These forests contain an understory of young Arizona and/or ponderosa pines, Rocky Mountain Douglas-fir, southwestern white pine, and Gambel oak: species that are less fire-resistant and more shade-tolerant than Arizona and ponderosa pines [12]. Bahre [11,12] 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 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 [12].
Climate and grazing: Grazing appears to reduce fire frequency in Arizona pine forests by removing grassy understory fuels. Interactive effects of grazing and climate on fire frequency are difficult to determine. Savage and Swetnam [114] suggest that climatic factors play the larger role in determining both fire frequency and Arizona stand structure. They attribute a pulse of 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, a pulse of Arizona pine regeneration occurred on 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 (x̄=4.2 years before 1830; <3 fires recorded from 1830-1950) on heavily grazed sites and on sites that had not experienced grazing for several decades. Likewise, a strong pulse of pine establishment occurred on both grazed and ungrazed sites [114].
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 [136]. Similarly, fire occurrence tends to increase 2 to 3 years after above-average precipitation in winter-spring [13,126]. Climatic variation that produces widespread, stand-replacing fire has been associated with southern oscillation events. El Niño is associated with greater soil moisture and herbaceous fuel production in spring, with fire occurrence peaking several years after El Niño events. La Niña events are associated with dry springs, with fire occurrence peaking in the same year [136]. A decline in fire frequency in interior ponderosa pine forests of the Southwest coincided with reduced El Niño-La Niña events between 1780 and 1830 [129,136]. Alternating wet and dry years resulting from El Niño-La Niña events in the mid- to late 1800s increased fire frequency [136].
Fire histories: Prior to the 1880s, surface fires burned through Arizona sky islands populated with interior ponderosa pines once or twice a decade. Fires were somewhat less frequent in higher-elevation, mixed-conifer forests [126]. 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 [13].
Historical fire frequency at the pine-oak woodland interface has been documented at 1 fire or more per decade [39,77]. In a fire history study of Madrean oak-mixed pine gallery forest in Chiricahua National Monument, Arizona, Swetnam and others [127] found an historical (1620-1890) fire regime of frequent surface fires at intervals ranging from 1 to 38 years. Mean fire-return 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 Arizona/ponderosa pine, 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 [127].
Fule and Covington [52] 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 [52].
FIRE REGIMES where Arizona pine is a dominant or important member of the community are summarized below. Find further fire regime information for the plant communities in which this species may occur by entering "Arizona pine" in the FEIS home page under "Find FIRE REGIMES".
Community or Ecosystem Dominant Species Fire Return Interval Range (years) Engelmann spruce-subalpine fir Picea engelmannii-Abies lasiocarpa 35 to >200 pine-cypress forest Pinus-Hesperocyparis spp. <35 to 200 [9] pinyon-juniper Pinus-Juniperus spp. <35 [105] Mexican pinyon Pinus cembroides 20-70 [97,127] Colorado pinyon Pinus edulis 10-49 [105] interior ponderosa pine* Pinus ponderosa var. scopulorum 2-10 [9] Arizona pine Pinus ponderosa var. arizonica 2-15 [13,31,119] Rocky Mountain Douglas-fir* Pseudotsuga menziesii var. glauca 25-100 [9] *Fire-return interval varies widely; trends in variation are noted in the Species Reviews.Current fire-return intervals are greater than the historical range of variability for interior ponderosa pine forests. This trend is stringer at low than high elevations [135]. The expense of excluding fire from Arizona and 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 stand structure and ecological processes are modest in cost [8,120].
Thinning to remove small-diameter trees, accompanied by prescribed fire, has been suggested as a means of restoring structure and function to degraded Arizona and ponderosa pine forests [7,23,33,47,59,64,113,141]. Frequent low-severity surface fires restore ecosystem function by thinning dense stands and reducing woody debris and other organic matter on the forest floor. This can result in increased soil moisture, increased soil temperature (with accompanying rates of increased litter decomposition, soil nutrient cycling, and fine root growth), increased productivity of understory herbs and shrubs, increased basal diameter growth of overstory Arizona pine, and favorable seedbeds [3,4,35,36]. Fire pruning of lower pine branches opens the canopy [57]. Frequent prescribed fires reduce fire hazard without damaging overstory pines [3,4]. Biswell [23] listed several ways in which prescribed burning reduces wildfire hazard in pine forests:
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 [33]. Harrington [65] recommends growing-season (spring or summer) burning Arizona 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 Arizona pine [68], and a logistic regression model predicting probability of Arizona pine mortality by tree size, scorch class, and season of injury are available [60].
Allen and others [5] provide ecologically based recommendations for restoring interior ponderosa pine forests. They stress that restoration programs should include natural variability in interior ponderosa pine stands and the reestablishment of natural processes. Managers are encouraged to fully review their recommendations. A synopsis of their principles for restoration follows [5]:
Prescriptions for mid-summer burning in Arizona pine are available in Harrington [61]. He recommends burning in summer rather than fall to achieve more complete combustion of fuels [61]. 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 [21]. 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 [61]. Beaufait [20] 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 [130] recommend allowing large (>3,000 acres (1,200 ha)) prescribed natural surface fires in Arizona and ponderosa pine forests 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 moderate-severity fires in approved areas, subject to the limitations of wilderness boundaries, visitor safety, and management and suppression capabilities [130].
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 [62] presents a model for estimating moisture of fuels in Arizona pine.
In a comparison of fuel loads on several Arizona pine sites, Harrington [64] 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 interior ponderosa pine forests based upon moisture content of the H surface soil layer [66].
Mortality: McHugh and Kolb [96] found a model using total crown damage by fire (scorch + consumption) and bole char severity as independent variables gave the best 2-way variable model for predicting individual tree mortality for prescribed and wildfires in northern Arizona.
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 Arizona pine snags than they created. Density of 2 species, the northern flicker and the violet-green swallow, was reduced after fires [74]. More information on this study is available in the Research Project Summary Effects of understory fire on cavity-nesting birds in Arizona pine forests.In the United States, Arizona pine is often poorly accessible. It occupies slopes, canyons, rims, and tablelands [50]. Slopes may be steep: Arizona pine sites in the Santa Catalina Mountains of southeastern Arizona ranged from 10 to 36o. In Arizona, Arizona pine soils are mostly derived from limestone, sandstone, and quartzite overlaying schist and granite [45]. Arizona pine is most common between 7,000 and 7,500 feet (2,100-2,300 m), while ponderosa pine is more common above 9,000 feet (2,700 m). Elevations between 7,500 and 9,000 feet are occupied by both Arizona and ponderosa pine, and their hybrids [21,45]. Overall elevational range of Arizona pine by state is:
Arizona 6,200 to 9,840 feet (1,900-3,420 m) [12,18,50,101]
New Mexico 6,900 to 8,000 feet (2,100-2,500 m) [50]
Sonora 7,000 to 9,000 feet (2,000-3,000 m) [45]
Climate is semiarid to arid with bimodal rainfall. Winter rains occur from December through March and are followed by a dry season extending to June. Monsoonal rains occur from July to September, with greater than 50% of the mean annual rainfall occurring in August [17,21,26]. Total annual rainfall is highly variable, and prolonged drought is common [26].
Arizona pine seeds are valuable food for a variety of birds, rodents, and lagomorphs [93,131]. Abert's squirrel, which have been introduced into Arizona and ponderosa pines' range, are ecologically dependent upon ponderosa pine [81]. Common porcupines and smaller rodents eat Arizona pine bark and wood [93]. Deer eat the buds in early spring, when elongation begins, and they occasionally eat the woody shoots [106].
Arizona pine forests are critical habitat for many bird species, supporting, for example, breeding populations of Mexican junco and Mexican chickadee. Sky island populations of Arizona pine play important biogeographic roles in the distribution and survival of neotropical birds [42]. Cavity-nesting birds use interior Arizona pine snags for foraging and roosting as well as nesting [15,35]. Reintroduced and rare in southeastern Arizona, thick-billed parrots are ecologically dependent on Arizona, ponderosa, and Chihuahua pines for food and shelter. Pine seeds are their primary diet item, and the parrots nest in pine snag cavities [73]. Many bird species use Arizona pine needles for nesting material [93].
Arizona pine dominates ponderosa pine and pine-oak (Pinus-Quercus
spp.) communities of southeastern Arizona, southwestern New Mexico, and northern
Mexico. It is dominant or codominant in higher-elevation, montane
mixed-conifer forests. Madrean oak
woodland species are common plant associates in lower-elevation (<about 6,000 feet (1,800
m)) portions of Arizona pine forest. At higher elevations of Arizona pine
forest, Rocky Mountain Douglas-fir (Pseudotsuga menziesii var. glauca),
Rocky Mountain white fir (Abies concolor var. concolor), and Mexican white pine (Pinus ayachuite) are common overstory associates.
Arizona pine forest ascends to interior Douglas-fir-white fir forest around 7,500
feet
(2,300 m) [49,101,104], and fingers into
Madrean oak and pinyon-juniper (Pinus-Juniperus spp.) woodlands and
riparian communities at Arizona pine's lower elevational limits [104].
In southeastern Arizona, Arizona pine is common in and just above Madrean oak
and pine-juniper woodlands. Madrean hardwood associates include Emory oak
(Q. emoryi), Arizona white oak (Q. arizonica), silverleaf oak (Q.
hypoleucoides), Arizona madrone (Arbutus arizonica), and Arizona
sycamore (Platanus wrightii). Conifer associates include alligator juniper
(Juniperus deppeana), Apache pine (Pinus engelmannii), and Chihuahua pine
(P. leiophylla var. chihuahuana) [21]. Pointleaf manzanita (Arctostaphylos
pungens) and longtongue muhly (Muhlenbergia longiligula) are common
in the understory [26].
In the Chiricahua Mountains of southeastern Arizona, Arizona pine forest lies on elevation and moisture gradients between lower-elevation,
drier
Mexican pinyon (P. cembroides), Chihuahua pine, and Apache pine communities, and moister, higher-elevation
southwestern white pine (P. strobiformis) communities [18].
Southwestern ponderosa pine sometimes codominates with Arizona pine. Arizona
pine is generally more common
at lower elevations, with interior ponderosa pine occupying upper portions of
ponderosa pine forest [104]. Brady and Bonham [26] found that in the Huachuca Mountains, which span the
Arizona-Mexico border, Arizona pine assumed dominance at 7,498 feet (2,285 m). It
codominated with silverleaf oak down to 6,989 feet (2,130 m) elevation, where
silverleaf oak became dominant. Arizona pine became
subdominant to Rocky Mountain Douglas-fir
above 8,038 feet (2,450 m), and was not reported above 8,531 feet (2,600 m)
elevation. Along with Rocky Mountain Douglas-fir, corkbark fir (Abies lasiocarpa var. arizonica)
and/or Engelmann spruce (Picea engelmannii) may cooccur with Arizona
pine in high elevation, mixed-conifer sites [98].
In Madrean oak woodland of northern Mexico, Arizona pine
is usually subdominant to oaks. Netleaf oak (Quercus rugosa) and
Chihuahuan oak (Q. chihuahuensis) are the most
common community dominants; Chihuahua and Apache pines share subdominant
status with Arizona pine [53]. At midelevations (5,440-7,250 ft (1,650-2,200
m)), Madrean oak-pine woodland ascends to higher-elevation montane
forest. Handbasin (Q. pennivenia), Mexican white (Q. epileuca), silverleaf, netleaf, and/or Chihuahuan oaks join a
pine-dominated, mixed formation with Arizona, Chihuahua, Apache, Durango (P. durangensis), and/or
weeping (P. lumholtzii) pines. Arizona cypress (Hesperocyparis arizonica),
Arizona madrone, and Texas madrone (A. texana)
are characteristic community components [27]. Pure to nearly pure stands of
Arizona pine occur further south in Mexico [110].
Publications describing plant communities dominated by
Arizona pine are listed below.
Fire prepares a favorable mineral seedbed for Arizona pine establishment. Germinants require mineral soil so the emerging root radicle immediately contacts soil moisture [145]. Seedling density may be great in years when favorable precipitation follows fire, resulting in "doghair" thickets if further fire does not reduce the stand [63]. Thinning by fire results in increased stem growth in remaining trees [91,99,107,134]. Removal of shrubs in interior ponderosa pine forests results in an increase in biomass production of overstory Arizona pines [103].
Sutherland and others [125] present a linear regression model to predict postfire radial growth of Arizona and ponderosa pines after prescribed fire.
Arizona pine reproduces from seed. It is monoecious, with pollen dispersed by wind. Ponderosa pine 1st produces cones at 10 to 20 years of age [83]. Large Arizona pine seed crops are produced every 2 to 3 years [55]. High temperatures during strobili formation have been correlated with large Arizona pine cone crops [40,92]. The seeds are mostly wind dispersed and do not usually carry more than 120 feet (37 m) from the parent tree [55]. Arizona pine seed (collected in Arizona and germinated in the greenhouse) showed mean viability of 75% [83]. Seeds germinate with the onset of mid-summer rains; germination continues until onset of fall drought. Mid-summer germinants show best survivorship, probably because they gain the most growth before onset of winter dormancy and succeeding spring drought [86].
Best Arizona pine seedling establishment occurs in early-seral communities, and establishment is somewhat rare [32,114]. Several conditions are necessary for successful regeneration of Arizona pine: 1) an adequate seed source, 2) mineral soil with lowing stocking of competing vegetation, 3) open light, 4) adequate moisture at the right time, and 5) a low rate of herbivory on seedlings. Cooccurrence of these conditions is irregular and uncommon [14,75,90,114]. A warmer than average spring results in good cone crops [29,40,92]. A disturbance such as fire is required to prepare a bare mineral seedbed and create open-light conditions [90]. Twenty-seven months after cone initiation, a wetter than average summer and fall produces good seed germination [29]. Seeds appear to require continually moist conditions for at least 7 days at temperatures above 55 degrees Fahrenheit (13 oC) [117]. Above-average precipitation the following spring promotes seedling survival [29]. A pulse of Arizona pine regeneration occurred from 1910-1930, when these conditions coincided [114].
Arizona pine is shade intolerant [14,75], and is often a dominant tree in southwestern sky island plant communities [98,100]. Muldavin and others [100] have identified several Arizona pine habitat types.
Successional pathways in Arizona pine communities are poorly understood, and further research is needed in this area. Arizona pine is occasionally seral in white fir habitats [98]. Arizona pine may be a climax species on dry, midelevation (6,800 to 8,500 feet (2,000-2,600 m)) sites [69,98,100,119,122]. Arizona pine types also occur on high-elevation (<9,300 feet (2,800 m)), relatively moist north- and east-facing slopes [30,100] that are maintained by frequent fire [100]. These sites may proceed to mixed-conifer forest in the absence of fire, with Arizona pine being successionally replaced by Rocky Mountain Douglas-fir and/or white fir [30,98]. At low elevations (<6,500 feet (2,000 m)), Arizona pine understories are characterized by Arizona pine and Rocky Mountain Douglas-fir seedlings and saplings, oaks, and junipers [87].
The scientific name of Arizona pine is Pinus arizonica Engelm. (Pinaceae) [78,109,132]. There are 2 varieties of Arizona pine [78,132]:
Pinus arizonica Engelm. var. arizonica
Pinus arizonica Engelm. var. stormiae MartÃÂnez
Arizona pine is closely related to ponderosa pine (Pinus ponderosa), and it is considered part of the ponderosa pine complex. Arizona pine and varieties of ponderosa pine
show differences on broad latitudinal clines. Dodge [45] described the complex
as "a continuation of a large group of populations from the Central
American Highlands to British Columbia." There are morphological and
distributional overlaps among Arizona pine and varieties of ponderosa pine, and disagreement among authorities
regarding the geographical boundaries of
Arizona pine and ponderosa pine [37,50,85,109]. Ponderosa pine populations that extend northward
into western Texas from Coahuila are classified either as a variety of Arizona pine (P. arizonica var. stormiae) [76,78,109] or as Rocky Mountain ponderosa pine (P. p. var. scopulorum) [50]. Varieties of ponderosa pine are:
Follow the links to FEIS reviews of ponderosa pine varieties. Information on ponderosa pine in Texas is included in the FEIS review of southwestern ponderosa pine.
Hybrids: Interspecies hybridization and introgression occur
between Arizona and southwestern ponderosa pines. Arizona and southwestern ponderosa
pines also hybridize and introgress with Apache pine (P. engelmannii);
3-taxa hybrids (southwestern ponderosa × Arizona ÃÂ Apache pine) occur occasionally
[50,108].
Pinus arizonica, commonly known as the Arizona pine, is a medium-sized pine in northern Mexico, southeast Arizona, southwest New Mexico, and western Texas in the United States. It is a tree growing to 25–35 m tall, with a trunk diameter of up to 1.2 m (3 ft 11 in). The needles are in bundles of 3, 4, or 5, with 5-needle fascicles being the most prevalent. This variability may be a sign of hybridization with the closely related ponderosa pine (Pinus ponderosa). The cones are single, paired, or in whorls of three, and 5–11 cm long.
The Arizona pine was commonly thought to be a variant of Ponderosa pine, but since at least 1997 it is now recognized as a distinct species by most authorities.[2]
Three varieties are described;
This pine is a source of construction timber, and is heavily harvested for firewood. Extensive cutting has reduced the formerly widespread Arizona pine forests, particularly in Mexico.
Pinus arizonica, commonly known as the Arizona pine, is a medium-sized pine in northern Mexico, southeast Arizona, southwest New Mexico, and western Texas in the United States. It is a tree growing to 25–35 m tall, with a trunk diameter of up to 1.2 m (3 ft 11 in). The needles are in bundles of 3, 4, or 5, with 5-needle fascicles being the most prevalent. This variability may be a sign of hybridization with the closely related ponderosa pine (Pinus ponderosa). The cones are single, paired, or in whorls of three, and 5–11 cm long.
