IMMEDIATE FIRE EFFECT ON PLANT:

Thin bark and lack of self pruning make singleleaf pinyon very susceptible to intense fire [105]. Mature singleleaf pinyon can survive low-severity surface fires but is killed by more severe fires [1]. Most tree seedlings are killed by fire, but cached seeds may survive [35,63].

DISCUSSION AND QUALIFICATION OF FIRE EFFECT:

The ability of singleleaf pinyon to survive fire is dependent on the size of the tree, intensity and severity of fire, and resulting damage to individual trees. Generally, as tree size increases, mortality decreases [115], and as fire severity increases, mortality increases.

Because mature singleleaf pinyon trees are short with open crowns, full of long-lived, highly flammable foliage, and do not self-prune their dead branches, individual trees are susceptible to fire [22]. On Nevada sites with prefire cover of 6 to 29% trees, 23 to 56% shrubs, and trace grasses and forbs, a spring prescribed fire killed all trees in all age classes  [228]. The effectiveness of tree kill depends on fire severity, which is largely determined by the amount of grass that carries the fire [158]. Low-severity fires in a mature pinyon-juniper stand will usually remove trees in the understory and a few overstory trees, while moderate-severity fires remove more of the overstory. In open stands with grass understories, trees less than 4 feet (1.2 m) tall (about 50 years old) are very susceptible to being killed by fire [22,27,58,85,245]. However, low-severity fire in open, pole-size stands may leave scattered individuals [22]. Trees taller than 4 feet (1.2 m) are difficult to kill unless there are heavy accumulations of fine fuel beneath the trees [22,85]. Fire scars have been observed on mature singleleaf pinyons, indicating survival [203], but are uncommon. This indicates that in some fire regimes, singleleaf pinyon is either killed outright or does not live long after being scarred because heart-rotting fungi enters the scar and hastens mortality [82].

Weise [230] found mortality of singleleaf pinyon varied with damage class and site after 2 wildfires in California in 1987. One year after wildfire, mortality of undamaged and slightly damaged trees differed between the 2 sites. It appears that singleleaf pinyon tolerates a small portion of crown scorch (<33%), while all trees with over 66% crown scorch and some crown consumption died within the 1st postfire year. Tree size did not appear to be a factor in initial survival, but may have been critical to long-term survival.

PLANT RESPONSE TO FIRE:

Reestablishment of singleleaf pinyon after fire requires seed survival or seed dispersal by birds or rodents into the burn area. Seedlings then require nurse plants (primarily sagebrush) for establishment, so initial postfire establishment may be delayed for 20 to 30 years until the establishment of the shrub layer [152]. Seedlings have been observed under the canopies of mature trees but they eventually die out unless the overstory is removed. Ward [228] found that singleleaf pinyon seedlings emerging under burned tree canopies died by the 2nd year. No singleleaf pinyon seedlings were recorded on prescribed burns for 5 years following fire in Nevada woodlands [63,69]. Moderate establishment was observed on 24-year-old burns, and singleleaf pinyon density was still low (5-10%) 115 years after fire in the Great Basin [186].

DISCUSSION AND QUALIFICATION OF PLANT RESPONSE:

The response of singleleaf pinyon to fire is a function of initial postfire damage and survival, postfire seed dispersal, and postfire succession of associated plant species. Response also varies by size, season, and type of burn [27,69]. Singleleaf pinyon grows very slowly, so the return to mature woodlands requires more than a century [186,227]. Where stands are closed or dense, the undergrowth declines significantly; fires become less and less common because fuels are scarcer [22] and require extreme conditions of hot, dry weather and strong winds to carry fire [1,85]. When stands are burned under these conditions, singleleaf pinyon tends to be eliminated from the site and recolonizes very slowly.

Perennial grass surface fires in mature pinyon-juniper woodlands result in light to moderate tree mortality and recovery to prefire conditions in less than 5 years, while woodlands with shrub understories commonly experience canopy fires that result in heavy tree mortality and require more than 100 years for recovery to prefire conditions [69,112,227].

Singleleaf pinyon seeds that are cached on the prefire site may survive [35]. Clark's nutcrackers have been observed caching seeds of whitebark pine in recently burned areas [212]; it is possible that pinyon seeds may also be cached in burn areas by these and other birds and rodents. On large burns, return of singleleaf pinyon may be slow due to greater distance from seed sources [12].

Wildfire and prescribed burning in singleleaf pinyon stands result in an initial increase in understory vegetation [5,13,69,200]. The species represented, their relative coverage, and patterns of succession are variable. Early (1-year) postfire succession in the Great Basin is generally dominated by annual and perennial forbs, giving way to shrubs and annual grass dominance in mid-succession (15-17 years), and to tree, shrub, and perennial grass dominance in late succession (22-60 years) [5,112]. In the San Bernardino Mountains in California, postfire succession to mature woodlands proceeds over a span of a century, beginning with the colonization of shrubs that increase in cover and density for 30 to 50 years and are joined by singleleaf pinyon recruits at 25 to 40 years. After 50 years, increases in singleleaf pinyon densities accompany a decline in the shrub layer, and mature woodlands with sparse understories return at 100 to 150 years [227]. Pinyon generally has a lower survival rate and slower establishment than juniper after a fire, but given time and more mesic site conditions, pinyon may dominate after about 60 years [25,27,203].

FIRE MANAGEMENT CONSIDERATIONS:

Prescribed fire may be used in singleleaf pinyon stands to reduce overstory trees and reestablish understory species in the successional cycle [25,60,107,109], to prevent invasion of trees into adjacent grass and shrublands [22,108,186], to reduce the effects of allelopathic chemicals in the litter [60,239,241], to improve big game winter range (mule deer, bighorn sheep, pronghorn, and elk) [108], to create mosaics of woodland and openings [165], to remove slash [58], to reduce fuel loads [85], and to control new tree regeneration [165]. Some primary considerations for prescribed burning in singleleaf pinyon stands are the number and species of trees present, nature of the understory vegetation, and weather conditions [58]. These elements determine if and how a stand will burn, and may help predict the ecological effects of burning.

Burning in singleleaf pinyon woodlands requires an understory adequate to carry a ground fire or extreme conditions to carry a crown fire. Pinyon-juniper stands most likely to burn have small scattered trees with abundant herbaceous fuel between them, or have a high density of mature trees capable of carrying crown fire during dry, windy conditions. More widely spaced trees with little understory are unlikely to burn [22]. It usually requires 600 to 700 pounds/acre (672-784 kg/ha) of fine fuels, or 20% sagebrush with 300 pounds/acre (336 kg/ha) of fine fuels, to carry a fire in the Great Basin [242]. Many singleleaf pinyon stands in the Great Basin are not productive enough to yield this amount of fine fuel, particularly if they have an overstory of trees [22,26]. Periodic growth of annual grasses in response to favorable weather conditions has become the primary fuel source in some rangelands that previously experienced low fire frequencies [111].

Research in the Great Basin suggests prescribed burning is likely to succeed on sites with scattered trees (9-23% cover) and in dense (not closed) stands (24-35% cover) [25]. Blackburn and Tueller [15] found that scattered and dense stands with 9 to 35% canopy cover burned well, whereas closed stands (>35% cover) were difficult to burn out of fire season. Many pinyon-juniper woodlands have advanced to a stage of succession where prescribed fire is no longer a viable option due to low fuel loads and large trees in open sites that are susceptible to weed invasion and erosion. Since pinyon burns more readily than juniper, stands burn better as the proportion of pinyon to juniper increases [22].

Crown fuels are typically around 3.6 tons/acre (8.1 t/ha) for foliage and 1.8 tons/acre (4.0 t/ha) for 0- to 0.25-inch branchwood in Colorado pinyon in Arizona [165]. Fuel biomass estimates in singleleaf pinyon using aerial photography are presented by Meeuwig and others [144]. Heat and ash content values, physical properties of woody fuel particles, and fuel bed characteristics for singleleaf pinyon in the Sierra Nevada are available [221,222].

A detailed site evaluation is critical in predicting the successional effects of fire in singleleaf pinyon stands. Qualitative prediction of postfire response is possible when we consider aspect, elevation, and the known response of individual species in the prefire community, but accurate predictions are difficult because of unknown soil seed reserves, species immigration potential, and postfire weather [60,65,69]. A low increase in forage production may be expected in sites that have greater than 20% canopy coverage of trees, while stands with fewer trees and high amounts of perennial grass cover respond quickly. Stands with high levels of tree coverage are often invaded by annuals following fire if perennials do not quickly occupy the site [26]. Response on grazed and ungrazed sites will be floristically different [61]. Current conditions in many pinyon-juniper stands may have little understory ground cover, seed production and soil seed reserves [60]. Burning a site with few desirable understory species may worsen the ground cover situation and only serve to destroy valuable wood [165]. Trees may be eliminated for 50 years [5], and it may take over 300 years for a climax stand to establish [55]. Prescribed burning to remove slash may damage residual trees and kill seedlings, sterilize soils where slash is piled, and remove valuable nutrients form the site [58]. Postfire seeding appears to be detrimental to many naturally occurring species [112].

Lack of native understory vegetation and presence of cheatgrass or other invasive plants within or adjacent to stands creates new postfire community types following fire in singleleaf pinyon communities [26,173]. Seeding burns with native species may be the only means of restoring the grass successional stage to many overgrazed woodlands, and has had variable success in the past [26,61,173]. Where understory is already composed of annual species, fire may only increase the cover of annuals and thus increase the fire hazard, resulting in more frequent fires that will prevent reestablishment of shrubs and trees [26]. For example, stands with good growth of cheatgrass are at higher risk for large fires that will likely result in increased yields of cheatgrass [12,22]. Several large fires in singleleaf pinyon stands in southern California and a reburn several years after a crown fire suggest that the spread of cheatgrass may be responsible for carrying these fires. This could have serious negative effects by shortening the fire return interval and thereby preventing tree establishment [187]. Severe burning in singleleaf pinyon communities in the San Gabriel and San Bernardino mountains of southern California can convert these woodlands to chaparral [86]. Burning areas invaded by cheatgrass with fires hot enough to remove litter and any stored seed may give managers a window of opportunity the 1st year after burning to plant more desirable perennial species [58].

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Related categories for SPECIES: Pinus monophylla | Singleleaf Pinyon

• Introductory
• Distribution and Occurrence
• Value and Use
• Botanical and Ecological Characteristics
• Fire Ecology
• Fire Effects
• References
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