Black Hills Ponderosa Pine Woodland and Savanna

This system occurs throughout the northwestern Great Plains along areas that border the Rocky Mountains. The expansion of this system within the central Great Plains may be due to fire suppression. These can be physiognomically variable, ranging from very sparse patches of trees on drier sites, to nearly closed-canopy forest stands on north slopes or in draws where available soil moisture is higher. This system occurs primarily on gentle to steep slopes along escarpments, buttes, canyons, rock outcrops or ravines and can grade into one of the surrounding prairie systems or the Great Plains canyon system. Soils typically range from well-drained loamy sands to sandy loams formed in colluvium, weathered sandstone, limestone, scoria or eolian sand. This system is primarily dominated by Pinus ponderosa but may include a sparse to relatively dense understory of Juniperus scopulorum, Thuja, or Cercocarpus with just a few scattered trees. Deciduous trees are an important component in some areas (western Dakotas, Black Hills) and are sometimes codominant with the pines, including Fraxinus pennsylvanica, Betula papyrifera, Quercus macrocarpa, Ulmus americana, Acer negundo, and Populus tremuloides. Along the Missouri Breaks in north-central Montana, woodlands dominated by Pseudotsuga menziesii are in similar ecological settings as Pinus ponderosa in the Great Plains and are included in this system. In the breaks where it occurs, Pseudotsuga menziesii has a very open canopy over grassy undergrowth, predominantly composed of Pseudoroegneria spicata, with little to no shrubs present. Important or common shrub species with ponderosa pine can include Arctostaphylos uva-ursi, Mahonia repens, Yucca glauca, Symphoricarpos spp., Prunus virginiana, Juniperus communis, Juniperus horizontalis, Amelanchier alnifolia, Rhus trilobata, and Physocarpus monogynus. The herbaceous understory can range from sparse to a dense layer with species typifying the surrounding prairie system, with mixedgrass species common, such as Andropogon gerardii, Bouteloua curtipendula, Carex inops ssp. heliophila, Carex filifolia, Danthonia intermedia, Koeleria macrantha, Nassella viridula, Oryzopsis asperifolia, Pascopyrum smithii, Piptatheropsis micrantha, and Schizachyrium scoparium. Timber cutting and other disturbances have degraded many examples of this system within the Great Plains. However, some good examples may occur along the Pine Ridge escarpment and Pine Ridge district of the Nebraska National Forest in Nebraska.

This system is primarily dominated by Pinus ponderosa but may include a sparse to relatively dense understory of Juniperus scopulorum, Thuja, or Cercocarpus with just a few scattered trees. Deciduous trees are an important component in some areas (western Dakotas, Black Hills) and are sometimes codominant with the pines, including Fraxinus pennsylvanica, Betula papyrifera, Quercus macrocarpa, Ulmus americana, Acer negundo, and Populus tremuloides. Along the Missouri Breaks in north-central Montana, woodlands dominated by Pseudotsuga menziesii are in similar ecological settings as Pinus ponderosa in the Great Plains and are included in this system. In the breaks where it occurs, Pseudotsuga menziesii has a very open canopy over grassy undergrowth, predominantly composed of Pseudoroegneria spicata, with little to no shrubs present. Important or common shrub species with ponderosa pine can include Arctostaphylos uva-ursi, Mahonia repens, Yucca glauca, Symphoricarpos spp., Prunus virginiana, Juniperus communis, Juniperus horizontalis, Amelanchier alnifolia, Rhus trilobata, and Physocarpus monogynus. The herbaceous understory can range from sparse to a dense layer with species typifying the surrounding prairie system, with mixedgrass species common, such as Andropogon gerardii, Bouteloua curtipendula, Carex inops ssp. heliophila, Carex filifolia, Danthonia intermedia, Koeleria macrantha, Nassella viridula, Oryzopsis asperifolia, Pascopyrum smithii, Piptatheropsis micrantha (= Piptatherum micranthum), and Schizachyrium scoparium.

The ponderosa pine system is found in a matrix of northwestern Great Plains grassland systems along escarpments and in foothills and mountains in the Black Hills. It is often surrounded by mixedgrass or tallgrass prairie, in places where available soil moisture is higher, or soils are more coarse and rocky. Some stands are found adjacent to major creek bottoms and the lower toeslope and footslope positions. In some cases, these woodlands or savannas may occur where fire suppression has allowed trees to become established (in areas where deciduous trees are more abundant) (Girard et al. 1987). These are typically not in the same setting as Rocky Mountain ponderosa pine, where ponderosa pine forms woodlands at lower treeline and grades into mixed montane conifer systems at higher elevations. These are physiognomically variable woodlands, ranging from very sparse patches of trees on drier, often rocky sites, to nearly closed-canopy forest stands on north slopes or in draws where available soil moisture is higher. This system occurs primarily on gentle to steep slopes along escarpments, buttes, canyons, rock outcrops or ravines and can grade into the Great Plains canyons the surrounding mixedgrass prairie systems (Hoffman and Alexander 1987). Soils typically range from well-drained loamy sands to loams formed in colluvium, weathered sandstone, limestone, calcareous shales, scoria or eolian sand (Hoffman and Alexander 1987, Hansen and Hoffman 1988).

Marriot and Faber-Langendoen (2000) report different fire regimes for ponderosa pine communities in the Black Hills, with their "Dry Group" more typically having frequent surface fires and the "Mesic Group" having infrequent catastrophic fires (every 100-200 years). The Dry Group of associations includes lower elevation foothill savanna associations, and the mesic group somewhat higher elevation, north-slope, swale associations. K. Kindscher (pers. comm. 2007) believes that almost all the stands in Nebraska were there at the time of settlement and are not a result of pine expansion due to fire suppression; in addition, at least some have disappeared, such as the one in southern Nebraska (Franklin County). It is possible, however, that some areas of this system have expanded in size due to fire suppression, but this needs substantiation.

LANDFIRE developed several a state-and-transition vegetation dynamics VDDT models for this system for different map zones and savanna vs low elevation woodland stands. Shone in the grassland model for Map Zone 29 which has five classes in total (LANDFIRE 2007a, BpS 2911792). These are summarized as:

A) Early Development 1 All Structures (5% of type in this stage): This community is dominated by herbaceous and woody species, including the graminoids needlegrasses, western wheatgrass, bluebunch wheatgrass, sedges, Idaho fescue and little bluestem in moister areas, and various shrubs including skunkbush and snowberry. Ponderosa pine seedlings are scattered and found in small clumps. Little bluestem will also be an indicator species. Number of years in this class is variable depending on climatic patterns and fire disturbances. This class typically ends at 30 years in this model. Without fire for 25 years, this class can move to a mid-closed stage.

B) Mid Development 1 Closed (2% of type in this stage): Tree cover is 0-50%. Multi-story stand of small and medium trees with saplings and seedlings coming in as clumps. Understory is sparse. Some juniper might be present - could be an outlier. Grasses and shrubs are shaded out. This class lasts approximately 70 years, then moves to a late-closed stage. Low-severity surface fires occur every 15 years and move this stage to a mid-open stage. Replacement fires occur infrequently, approximately every 300 years. Insect/disease was modeled at approximately occurring every 50 years, not causing a transition.

C) Mid Development 1 Open (8% of type in this stage): Tree cover is 0-50%. Predominantly single-story stands with a few pockets of regeneration. Low shrubs such as snowberry and skunkbush and poison ivy are dominant as well as grasses and forbs. Graminoids could have up to 70-80% cover. Rocky Mountain juniper present in patches (Rocky Mountain juniper is not common on the Pine Ridge in Nebraska). Carex spp. and little bluestem will also be indicator species. This class lasts approximately 50 years then goes to a late-open stage. Without fire for 40 years, this could transition back to a mid-closed stage. Low-severity surface fires occur every 15 years, maintaining this class. Replacement fires occur very infrequently (modeled at 0.0015 probability).

D) Late Development 1 Open (80% of type in this stage): Tree cover is 0-50%. Predominantly single-story stands of large ponderosa pine with pockets of smaller size classes (replacement). Snowberry, skunkbush and patches of Rocky Mountain juniper. Understory is dominated by shrub species and grasses and poison ivy. Graminoids could have up to 70-80% cover. Carex spp. and little bluestem will also be indicator species. It is thought that class D, the late-open stage, should occupy approximately 80% of the historical landscape. Low-severity fires occur every 15 years and maintain this stage. Replacement fires occur very infrequently (0.0015 probability). If no fire occurs after 40 years, this class could transition to the late-closed stage. Insect/disease occurs every 50 years and maintains this stage.

E) Late Development 1 Closed (5% of type in this stage): Tree cover is 51-100%. This is a somewhat uniform late-development stage, multi-story stands of large, medium, small and seedling ponderosa pine. Shrubs and grasses are sparse. This type generally exceeds 70% canopy cover. dbh is less in this class than late-open. Low-severity surface fires occur every 15 years and cause a transition back to the late-open stage. Replacement fires occur every 300 years. Insect/disease occurs every 250 years, causing a transition back to the late-open stage. Drought can also occur - every 500 years, causing a transition to the late-open stage.

Generally, the fire regime is characterized by frequent fire-return interval of low-severity surface fire. The presence of abundant fire-scarred trees in multi-aged stands supports a prevailing historical model for ponderosa pine forests in which recurrent surface fires affected heterogeneous forest structure (Brown 2006). Mixed-severity fire occurs in closed-canopy conditions and stand-replacement fire is very infrequent (300+ years) (LANDFIRE 2007a, BpS 2911792). Low-severity fires are frequent and range from <10 years to more than 20 years (Fischer and Clayton 1983, Brown and Sieg 1999), but probably not more than 40 years at the high end (3-70 years range). The MFRI is approximately 12-15 years for low-severity fires (LANDFIRE 2007a, BpS 2911792).

There is considerable debate over the role of mixed-severity and surface fires in the historical range of variability in this and other ponderosa pine forests in the northern and central Rockies (Veblen et al. 2000, Baker and Ehle 2001, 2003, Barrett 2004a, b). However, Brown (2006) argues that surface fire was the dominant mode of fire disturbance and that the role of mixed-severity fires is overstated. For MZs 29 and 30, it was suggested that mixed fire be removed from this savanna model; reviewers agreed, and therefore mixed fire is not in this model (LANDFIRE 2007a, BpS 2911792).

Variation in precipitation and temperature interacting with fire, tip moths and ungulate grazing affects pine regeneration. Windthrow, storm damage and mountain pine beetles were minor disturbances in this type unless stands reach high densities. The interactions among drought, insects and disease are not well understood (LANDFIRE 2007a, BpS 2911792). Pinus ponderosa - Juniperus scopulorum savanna in the southern Black Hills has lots of rock exposure or sparsely grassed soils, which probably protected some of the juniper seed trees from being wiped out by fire (LANDFIRE 2007a, BpS 2911792).

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, which maintained a savanna or open woodland structure. 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. Restoration to open woodland or 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.

This system is found in central and eastern Montana, the western Dakotas, eastern Wyoming (east of the Bighorns), the Black Hills, and south into the Sand Hills of Nebraska and northeastern Colorado (north of Pawnee National Grasslands to Cedar Point near Limon and south). In Montana, it occurs along the Missouri River breaks, around the Little Belts and Snowy mountains, in south-central Montana between the Bighorns and the Black Hills (along the Tongue and Powder rivers), and other areas of eastern Montana. In Wyoming, it is found around the Black Hills and Bear Lodge Mountains, and in isolated areas of eastern Wyoming on bluffs and rock outcrops, and along "breaks." Whether this system occurs in Kansas is uncertain.