Ecological dynamics
Black Hills vegetation types consist of a mixture of forest and grasslands resulting from the varied topography, geology, soils, climate and natural disturbances. Frequent fires, periodic drought, and episodic infestations of mountain pine beetles all contribute to the maintenance of large, open grasslands scattered throughout the Black Hills. Ponderosa pine is the dominant tree species in the Black Hills. It is a fire-adapted species that coexists with frequent, low-intensity fires that consume small seedlings, prune lower branches from larger trees, and reduce fuel loads.
This site developed under Black Hills climatic conditions with short-term weather variations, light to severe grazing by bison, elk, and small mammals, insects, sporadic natural or man-caused wildfire (often of light intensities), and other biotic and abiotic factors that typically influence soil/site development. The natural fire regime maintained this site as a grassland and the plant communities were free of non-native cool-season grasses.
Fire, or the lack of fire, grazing, and pine encroachment are major drivers that shape this site as well as adjacent ecological sites. Between 1388 and 1900, fire intervals in the Black Hills ranged from 16 to 20 years (Brown, 1996). In the absence of fire, encroachment of ponderosa pine is likely to occur on this site. The Shirttail series appears to be more susceptible to pine encroachment and establishment than the Hilger soils. This is probably due to the parent materials of schist or granite.
Changes will occur in the plant communities due to short-term weather variations, impacts of native and/or exotic plant and animal species, severe disturbances such as periods of well below-average precipitation, severe defoliation, soil erosion, and no fire and no use. The Stony Hills site also can occur on a wide range of slopes and aspects. Steeper slopes, those greater than 20 percent, will tend to have lower total annual production than on slopes less than 20 percent. The plant communities on the shallower slopes may have slightly more cool-season grasses than those on the steeper slopes. Northern and eastern aspects also may produce a slightly higher percentage of cool-season grasses than south-and west-facing slopes.
Kentucky bluegrass occurs on this site and can drive the successional process. Preliminary studies indicate that a threshold may exist when Kentucky bluegrass exceeds 30 percent of the plant community, and native grasses represent less than 40 percent of the plant community composition. Plant communities dominated by Kentucky bluegrass have significantly less cover and diversity of native grasses and forb species. (Toledo, D. et al., 2014).
Livestock and wildlife grazing, fire or the lack of fire, and the introduction of non-native cool-season grasses are the dominant drivers of plant community dynamics.
The following diagram illustrates the common plant community phases that can occur on the site and the transition pathways between communities. These are the most common plant community phases based on current knowledge and experience, and changes will be made as more data is collected. Narratives following the diagram contain more detail pertaining to the ecological processes.
State 1
Reference State
This State represents what is believed to be the natural range of variability that dominated the dynamics of this ecological site prior to European settlement. This site is dominated by warm-season grasses. Cool-season grasses are sub-dominant. In pre-European times the primary disturbances included fire and grazing by large ungulates and small mammals. Favorable growing conditions occurred during the spring, and warm months of June through August. Routine and/or occasional fires, reduced tree cover and contributed to the ecological processes that maintained the reference plant community. Today this State can be found in areas where proper livestock use has occurred and where encroachment of trees, especially ponderosa pine, has been limited and Kentucky bluegrass occurs in minor amounts.
Community 1.1
Bluestem-Sideoats grama-Wheatgrass-Needlegrass/Forbs/Leadplant/Conifers (< 5% Cover)
Figure 8. Stony Hills - 062XY029SD - PC 1.1
Interpretations are based primarily on the Bluestems-Wheatgrass-Sideoats grama-Needlegrass/Forb/Leadplant/Conifer plant community phase. This is also considered to be the Reference or historic plant community. The potential vegetation is about 75 percent grass and grass-like plants, 10 percent forbs, 10 percent shrubs, and 0 to 5 percent trees. Total annual production for a normal growing year is approximately 2,600 lbs./acre. The community is dominated by warm-season grasses including big and little bluestem, sideoats grama, prairie dropseed, and plains muhly. Cool-season grasses and grass-like plants include western and bearded wheatgrass, Columbia and green needlegrass, prairie Junegrass, and threadleaf sedge. Forbs are common and diverse, but prairie coneflower almost always is present, as are leadplant and prairie rose. Conifers may be present, but typically do not exceed five percent of the community. This plant community is productive and resilient to disturbances such as drought and fire. It is a sustainable plant community in regard to soil/site stability, watershed function, and biological integrity.
Community 1.2
Little bluestem-Blue grama-Wheatgrass/Sedge/Forbs/Leadplant/Conifers (2-15% Cover)
This plant community phase is the result of continuous season-long grazing without adequate recovery period, no fire, or drought. The potential vegetation is about 75 percent grass and grass-like plants, 10 percent forbs, 5 percent shrubs, and 2 to 5 percent trees. Total annual production for a normal growing year is approximately 2,000 lbs./acre. The more desirable warm-season grasses are greatly reduced and more grazing-resistant species, such as blue grama and upland sedge, have increased. This community is dominated by warm-season grasses including little bluestem, blue and hairy grama, and plains muhly. Cool-season grasses and grass-like plants include prairie Junegrass, threadleaf sedge, western and bearded wheatgrass, and green needlegrass. Forbs are common and diverse, and prairie coneflower almost always is present. Shrubs include prairie rose, leadplant, and fringed sagewort. This plant community is productive and resilient to disturbances such as drought and fire. It is a sustainable plant community in regard to soil/site stability, watershed function, and biological integrity. Conifers almost always are present, and can increase on this site with the lack of fire.
Community 1.3
Grama-Little bluestem-Sedge-Wheatgrass/Conifers (2-15% Cover)
Figure 9. Stony Hills - 062XY029SD - PC 1.3
This plant community evolved under continuous season-long grazing, heavy continuous grazing, or from overutilization during extended drought periods, and/or no fire. The potential plant community consists of approximately 80 percent grasses and grass-like species, 10 percent forbs, 2 percent shrubs, and 2-5 percent trees. Total annual production for a normal growing year is approximately 1,500 lbs./acre. Dominant grass and grass-like species include blue and hairy grama, threadleaf sedge, and little bluestem. Grasses of secondary importance include western wheatgrass, needlegrasses, sideoats grama, and prairie Junegrass. Forbs commonly found in this plant community include cudweed sagewort, prairie coneflower, spiny phlox, and fringed sagewort. Conifers almost always are present, and can increase dramatically with the lack of fire.
Pathway 1.1A
Community 1.1 to 1.2
Continuous season-long grazing without adequate recovery periods and/or drought will cause an increase in short-grass species and upland sedge. Big bluestem, sideoats grama, and wheatgrasses will decrease, but little bluestem will persist in the plant community. Lack of fire will allow conifers to become established, but in relatively small amounts.
Pathway 1.2A
Community 1.2 to 1.1
Prescribed grazing that provides a recovery period after grazing, along with normal precipitation and periodic fire, will allow this plant community to return to the reference plant community.
Pathway 1.2B
Community 1.2 to 1.3
Continuous season-long grazing without adequate recovery periods, heavy continuous grazing, and/or extended periods of drought will cause an increase in short grasses and upland sedge. Little bluestem and wheatgrasses will persist, but in small amounts. Lack of fire will allow conifers to become established and persist on the site.
Pathway 1.3A
Community 1.3 to 1.2
Prescribed grazing that provides adequate recovery and change in season of use, normal precipitation regime, and periodic fire will over time return this plant community phase to 1.2.
State 2
Native/Invaded State
This State has a mixture of native warm and cool-season grasses and non-native cool-season grasses. It resembles the Reference State, but Kentucky bluegrass has altered the dynamics of site. Kentucky blue grass typically will not exceed 20 percent of the plant community, so the site will not likely be dominated by bluegrass. Once the reference plant communities have transitioned to this State, is unlikely that it will return to the Reference State. Proper grazing management and periodic burning will maintain the productivity of this State. Heavy grazing without adequate recovery, or non-use and no fire, will tend to result in an increase in bluegrass and conifers.
Community 2.1
Little bluestem-Bluegrass-Western wheatgrass/Forbs/Leadplant/Conifers (5-15% Cover)
Figure 10. Stony Hills - 062XY029SD - PC 2.1
This plant community evolved under heavy season-long grazing, invasion of non-native cool-season grasses, and/or non-use or no fire. The potential plant community consists of approximately 75 percent grasses and grass-like species, 15 percent forbs, 5 percent shrubs and 1-5 percent trees. Total annual production for a normal growing year is approximately 1,200 lbs./acre. Dominant grass and grass-like species include little bluestem, Kentucky bluegrass, wheatgrass, and upland sedge. Grasses of secondary importance include needlegrass, blue grama, and prairie Junegrass. Forbs include cudweed sagewort, prairie coneflower, spiny phlox, and fringed sagewort. Conifers almost always are present, and can increase dramatically with the lack of fire.
State 3
Conifer/Invaded State
This State is dominated by conifers or deciduous trees mixed with conifers. Conifer encroachment is common on north- and east-facing slopes, and is very common in the northern LRU (A). This State can be confused with a forest site, except it will not exhibit woodland soil characteristics with an “E” horizon. The Conifer/Invaded State is most likely to occur on the Shirttail series and, to a lesser extent, on the Hilger soils. The midstory and understory may also resemble a forest plant community with shade-tolerant shrubs, grasses, grass-likes, and forbs. Because this plant community transitioned form the Native/Invaded State, Kentucky bluegrass will be well established and may make up at least 20 percent of the herbaceous plant community.
Community 3.1
Ponderosa pine and or Rocky Mountain Juniper > 20% Mature trees
This plant community developed due to pine encroachment, no use and or no fire, and heavy season-long grazing. The potential plant community consists of 60 percent grasses and grass-like species, 15 percent forbs, 5 percent shrubs, and 20 percent trees. Total annual production can be variable, depending upon species composition. The dominant grasses and grass-like species are Kentucky bluegrass, poverty oats, rough-leaf ricegrass, Canada wildrye, slender wheatgrass, and sedge. The dominant forbs include Oregon grape, pussytoes, and western yarrow. Trees can include ponderosa pine, and Rocky Mountain juniper. This site in the northern LRU (A) is likely to include bur oak mixed with the conifer, while the south LRU (C) is more likely to have ponderosa pine and juniper in the plant community. Because this plant community transitioned form the Native/Invaded State, Kentucky bluegrass will be well established and may make up at least 20 percent of the herbaceous plant community.
State 4
Conifer State
This State is dominated by conifers or deciduous trees mixed with conifers. Conifer encroachment is common on north- and east-facing slopes, and is very common in the northern LRU (A). This State can be confused with a forest site, except it will not exhibit woodland soil characteristics with an “E” horizon. The Conifer Encroached State is most likely to occur on the Shirttail series and, to a lesser extent, on the Hilger soils. The midstory and understory may also resemble a forest plant community with shade-tolerant shrubs, grasses, grass-likes, and forbs.
Community 4.1
Ponderosa pine and or Rocky Mountain Juniper > 20% Mature trees
Figure 11. Stony Hills - 062XY029SD - PC 4.1
This plant community developed due to pine encroachment, no use and or no fire, and heavy season-long grazing. The potential plant community consists of 60 percent grasses and grass-like species, 15 percent forbs, 5 percent shrubs, and 20 percent trees. Total annual production can be variable, depending upon species composition. The dominant grasses and grass-like species are poverty oats, rough-leaf ricegrass, Canada wildrye, slender wheatgrass, and sedge. The dominant forbs include Oregon grape, pussytoes, cudweed sagewort and western yarrow. Trees can include ponderosa pine, and Rocky Mountain juniper. This site in the northern LRU (A) is likely to include bur oak mixed with the conifer, while the south LRU (C) is more likely to have ponderosa pine and juniper in the plant community.
Transition 1A
State 1 to 2
Heavy season-long grazing above carrying capacity, and invasion or introduction of non-native cool-season grasses, or no use and no fire can cause a transition to the Native Invaded State (2.0).
Transition 1B
State 1 to 4
Encroachment of conifers and no fire will cause a transition to the Conifer State (4.0). No use or heavy season-long grazing also can facilitate or accelerate this transition. The Conifer State is most likely to occur on the Shirttail series and to a lesser extent on Hilger soils.
Transition 2A
State 2 to 3
Encroachment of conifers, introduction of non-native cool-season grasses and no fire will cause a transition to the Conifer/Invaded State (3.0). No use or heavy season-long grazing also can facilitate or accelerate this transition. The Conifer State is most likely to occur on the Shirttail series and, to a lesser extent, on the Hilger soils.
Transition 3A
State 3 to 2
Fire, prescribed burning, or mechanical brush management followed by long-term prescribed grazing can transition this plant community to the Native/Invaded State (2.0). This process can take an extended period of time, especially if mid-stature cool- and warm-season species make up only a small percentage of the plant community. Kentucky bluegrass may increase and become a dominant species in the plant community.
Restoration pathway 3A
State 4 to 1
Fire, prescribed burning, or mechanical brush management followed by long-term prescribed grazing that provided adequate recovery time and change in season of use can restore this plant community to the Reference State (1.0). This process can take an extended period of time, especially if mid-stature cool- and warm-season species make up only a small percentage of the plant community. If Kentucky bluegrass invades PCP 4.1 the transition will most likely be towards the Native Invaded State (2.0) and not towards the Reference State (1.0).