• FIRE ECOLOGY OR ADAPTATIONS
• POSTFIRE REGENERATION STRATEGY

FIRE ECOLOGY OR ADAPTATIONS:

Singleleaf pinyon occurs in many xeric sites with infertile, shallow, rocky soils, where fires are infrequent and unpredictable and depend on exceptional rainfall years that lead to herbaceous growth sufficient to carry fire. In southern California the vegetation structure in singleleaf pinyon communities does not carry fire well, and fire return intervals of several hundred years are considered typical [187,227]. For example, singleleaf pinyon communities in the San Bernardino Mountains have experienced long-interval stand-replacement fires both before and during suppression with an estimated fire interval of 450 years, resulting in a mosaic of small scattered patches within uniform old-growth stands across the landscape [152,227]. Thin bark and lack of self pruning makes singleleaf pinyon very susceptible to intense fire [105], and the sharp ecotone between pinyon-juniper and mixed conifer communities may be due to the inability of singleleaf pinyon to withstand the frequency and intensity of surface fires in mixed-conifer communities [152].

In more mesic areas, north slopes, and canyon bottoms fires may have occurred at 15- to 90-year intervals, some through ignition by Native Americans, maintaining open or patchy stands [1]. This fire frequency favored older stands, since there is little recruitment and seedlings and saplings are more susceptible to fire. Older trees in open canopy structures are able to withstand surface fires that would have occurred with the fine fuel loads presumed to have been present before the huge influx of domestic livestock [85,197]. Higher densities of young trees have been observed at upper elevations, suggesting that singleleaf pinyon may not reach maturity on these sites because of susceptibility to periodic stand-replacement fires [115].

Burning in pinyon-juniper woodlands requires at least 600 to 700 lb/acre of fine fuel [58]. In the absence of fire and the presence of grazing, tree densities have increased and undergrowth is so sparse in many areas that surface fuels do not support fire [29,57,85,139,226]. Increased tree density and subsequent buildup of hot burning fuels causes a shift from low-severity to severe fires burning under extreme conditions of hot, dry weather and strong winds [1,85]. When stands are burned under these conditions, singleleaf pinyon tends to be eliminated from the site and recolonizes very slowly. In some areas, expansion of aggressive annual exotic grasses, such as cheatgrass, serve as fine fuels in a differently structured ecosystem, further altering the fire regime [111].

Fire regimes for plant communities in which singleleaf pinyon occurs are summarized below:

Community or Ecosystem	Dominant Species	Fire Return Interval Range (years)
sagebrush steppe	Artemisia tridentata/Pseudoroegneria spicata	20-70 [24]
basin big sagebrush	Artemisia tridentata var. tridentata	12-43 [176]
mountain big sagebrush	Artemisia tridentata var. vaseyana	20-60 [2,28]
Wyoming big sagebrush	Artemisia tridentata var. wyomingensis	10-70 (40**) [225,245]
saltbush-greasewood	Atriplex confertifolia-Sarcobatus vermiculatus	< 35 to < 100
desert grasslands	Bouteloua eriopoda and/or Pleuraphis mutica	5-100
cheatgrass	Bromus tectorum	< 10 [24]
curlleaf mountain-mahogany*	Cercocarpus ledifolius	13-1000 [3,179]
mountain-mahogany-Gambel oak scrub	Cercocarpus ledifolius-Quercus gambelii	< 35 to < 100
blackbrush	Coleogyne ramosissima	< 35 to < 100
Arizona cypress	Cupressus arizonica	< 35 to 200
western juniper	Juniperus occidentalis	20-70
creosotebush	Larrea tridentata	< 35 to < 100
pine-cypress forest	Pinus-Cupressus spp.	< 35 to 200
pinyon-juniper	Pinus-Juniperus spp.	< 35 [24]
Mexican pinyon	Pinus cembroides	20-70 [155,195]
Colorado pinyon	Pinus edulis	10-49
Jeffrey pine	Pinus jeffreyi	5-30
Rocky Mountain ponderosa pine*	Pinus ponderosa var. scopulorum	2-10
Arizona pine	Pinus ponderosa var. arizonica	2-10 [24]
quaking aspen (west of the Great Plains)	Populus tremuloides	7-120 [24,84,145]
California oakwoods	Quercus spp.	< 35
oak-juniper woodland (Southwest)	Quercus-Juniperus spp.	< 35 to < 200
canyon live oak	Quercus chrysolepis	<35 to 200 [24]

*fire return interval varies widely; trends in variation are noted in the species summary
**mean

POSTFIRE REGENERATION STRATEGY [189]:

Related categories for SPECIES: Pinus monophylla | Singleleaf Pinyon