Southern Rockies Ponderosa Pine Savanna

This ecological system is found predominantly in the Colorado Plateau region, west into scattered locations in the Great Basin, and north along the eastern front of the southern Rocky Mountains into southeastern Wyoming. These savannas occur at the lower treeline/ecotone between grassland/or shrubland and more mesic coniferous forests typically in warm, dry, exposed sites. Elevations range from less than 1900 m in central and northern Wyoming to 2800 m in the New Mexico mountains to well over 2700 m on the higher plateaus of the Southwest. It is found on rolling plains, plateaus, or dry slopes usually on more southerly aspects. This system is best described as a savanna that has widely spaced (<25% tree canopy cover) (>150 years old) Pinus ponderosa (primarily var. scopulorum and var. brachyptera) as the predominant conifer. It is maintained by a fire regime of frequent, low-intensity surface fires. A healthy occurrence often consists of open and park-like stands dominated by Pinus ponderosa. Understory vegetation in the true savanna occurrences is predominantly fire-resistant grasses and forbs that resprout following surface fires; shrubs, understory trees and downed logs are uncommon. Important and often dominant species include Festuca arizonica, Koeleria macrantha, Muhlenbergia montana, Muhlenbergia straminea, and Pseudoroegneria spicata. Other important grasses, such as Andropogon gerardii, Bouteloua gracilis, Elymus elymoides, Festuca idahoensis, Piptatheropsis micrantha, and Schizachyrium scoparium, dominate less frequently. A century of anthropogenic disturbance and fire suppression has resulted in a higher density of Pinus ponderosa trees, altering the fire regime and species composition. Presently, many stands contain understories of more shade-tolerant species, such as Pseudotsuga menziesii and/or Abies spp., as well as younger cohorts of Pinus ponderosa. Northern Rocky Mountain Ponderosa Pine Woodland and Savanna (CES306.030) in the eastern Cascades, Okanogan, and Northern Rockies regions receives winter and spring rains, and thus has a greater spring "green-up" than the drier woodlands in the Central Rockies.

This system is best described as a savanna that has widely spaced (<25% tree canopy cover) (>150 years old) Pinus ponderosa (primarily var. scopulorum and var. brachyptera) as the predominant conifer. It is maintained by a fire regime of frequent, low-intensity surface fires. A healthy occurrence often consists of open and park-like stands dominated by Pinus ponderosa. Understory vegetation in the true savanna occurrences is predominantly fire-resistant grasses and forbs that resprout following surface fires; shrubs, understory trees and downed logs are uncommon. Important and often dominant species include Festuca arizonica, Koeleria macrantha, Muhlenbergia montana, Muhlenbergia straminea (= Muhlenbergia virescens), and Pseudoroegneria spicata. Other important grasses, such as Andropogon gerardii, Bouteloua gracilis, Elymus elymoides, Festuca idahoensis, Piptatheropsis micrantha (= Piptatherum micranthum), and Schizachyrium scoparium, dominate less frequently. A century of anthropogenic disturbance and fire suppression has resulted in a higher density of Pinus ponderosa trees, altering the fire regime and species composition.

These savannas occur at the lower elevation ecotone between pinyon conifer woodlands, grassland/or shrubland and upper elevation, more mesic coniferous forests typically in warm, dry, exposed sites. Elevations range from less than 1900 m in central and northern Wyoming to 2800 m in the New Mexico mountains to well over 2700 m on the higher plateaus of the Southwest. It is found on rolling plains, plateaus, or dry slopes usually on more southerly aspects; however, it can occur on all slopes and aspects. Stands occur on soils derived from igneous, metamorphic, and sedimentary material, including basalt, andesite, intrusive granitoids and porphyrites, and tuffs (Youngblood and Mauk 1985). Characteristic soil features include good aeration and drainage, coarse textures, circumneutral to slightly acidic pH, an abundance of mineral material, and periods of drought during the growing season. Surface textures are highly variable in this ecological system ranging from sand to loam and silt loam. Exposed rock and bare soil consistently occur to some degree in all the associations. Annual precipitation is 25-60 cm (8-24 inches), mostly through winter storms and some monsoonal summer rains. Typically, a seasonal drought period occurs throughout this system distribution as well.

Pinus ponderosa is a drought-resistant, typically open-grown conifer, which usually occurs at lower treeline in the major ranges of the western United States. Mature trees have thick bark that protects the cambium layer from fire. Historically, fires and drought were influential in maintaining open-canopy conditions in these woodlands. Low-intensity surface fire would burn through these stands every 5-15 year, killing young trees, but not the fire-resistant mature ponderosa pine trees or grass understory maintaining an open park-like stand (Harrington and Sackett 1992, Mehl 1992, Swetnam and Baisan 1996). Infrequent stand-replacement fire on the order of a few hundred years (300-500 years) is possible (LANDFIRE 2007a). Drought and other weather events (e.g., blowdown), parasites and disease may play a minor role, and have very long rotations (LANDFIRE 2007a). Impacts from insects such as mountain pine beetles (Dendroctonus ponderosae) may be significant during outbreaks, but infrequent in occurrence (LANDFIRE 2007a). Beetles attack less vigorously growing trees, e.g., old, crowded, diseased, damaged, or growing on poor sites) especially during droughts (Leatherman et al. 2013). Winter mortality of beetles is a significant factor; however, a severe freeze of at least -30 degrees F is necessary for at least five days during midwinter (Leatherman et al. 2013).

LANDFIRE developed a state-and-transition vegetation dynamics VDDT model for this system which has five classes in total (LANDFIRE 2007a, BpS 2811170). These are summarized as:

A) Early Development 1 All Structures (Shrub-dominated - 10% of type in this stage): Bunchgrass-dominated (0-49 years). Some ponderosa pine individuals also becoming established.

B) Mid Development 1 Closed (tree-dominated - 5% of type in this stage): Small and medium-sized ponderosa pine (50-149 years), still with high bunchgrass cover. Closed canopy defined as >50%.

C) Mid Development 1 Open (tree-dominated - 20% of type in this stage): Small and medium-sized ponderosa pine (50-149 years), with moderate bunchgrass cover. Open canopy defined as 10-49%.

D) Late Development 1 Open (conifer-dominated - 60% of type in this stage): Large and very large old-growth ponderosa pine, with medium to high cover of bunchgrasses. Old-growth attributes prominent, including downed wood, snags and diseased trees.

E) Late Development 1 Open (conifer-dominated - 5% of type in this stage): Large and very large old-growth ponderosa pine, with medium cover of bunchgrasses. Old-growth attributes prominent, including downed wood, snags and diseased trees.

Mean composite surface fire intervals have been found to be 5-15 years (Swetnam and Baisan 1996a). Infrequent stand-replacement fire on the order of a few hundred years possible (300-500 years?). Drought and other weather events (e.g., blowdown), parasites and disease may play a minor role, and have very long rotations. Insects may be a significant, but infrequent occurrence (LANDFIRE 2007a, BpS 2811170).

Nutrient cycling, specifically carbon cycling, is an important ecological process within many ecological systems. However, biological decomposition in ponderosa pine forests is more limited than biological production, resulting in accumulation of organic materials, especially in the absence of fire (Harvey 1994, Graham and Jain 2005).

With settlement and a century of anthropogenic disturbance and fire suppression, stands now have a higher density of Pinus ponderosa trees, altering the fire regime and species composition. Presently, many stands contain understories of more shade-tolerant species, such as Pseudotsuga menziesii and/or Abies spp., as well as younger cohorts of Pinus ponderosa. These altered structures have affected fuel loads and fire regimes. Presettlement fire regimes were primarily frequent (5- to 15-year return intervals), low-intensity ground fires triggered by lightning strikes or deliberately set by Native Americans. With fire suppression and increased fuel loads, fire regimes are now less frequent and often become intense crown fires, which can kill mature Pinus ponderosa (Reid et al. 1999).

Conversion of this type has commonly come from urban and exurban development especially along the Front Range, water developments and reservoirs. With long-term fire suppression, stands have converted through succession to Southern Rocky Mountain Ponderosa Pine Woodland (CES306.648) or Southern Rocky Mountain Dry-Mesic Montane Mixed Conifer Forest and Woodland (CES306.823). Restoration to savanna is difficult or impossible when adjacent to housing development.

Common stressors and threats include fragmentation from housing and water developments, altered fire regime from fire suppression and indirectly from livestock grazing and fragmentation, and introduction of invasive non-native species (CNHP 2010b). Potential climate change effects could include a change in the current extent of this ecosystem with tree mortality in lower elevation stands converting to Western Great Plains Foothill and Piedmont Grassland (CES303.817), if climate change has the predicted effect of less effective moisture with increasing mean temperature (TNC 2013).

This ecological system is found predominantly in the Colorado Plateau region, west into scattered locations of the Great Basin, and north along the eastern front of the Rocky Mountains of Colorado and Wyoming. Pine woodlands and savannas of the Black Hills and central Montana are now included in Northwestern Great Plains-Black Hills Ponderosa Pine Woodland and Savanna (CES303.650), as are woodlands and savannas in Nebraska and northeastern Colorado.