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 mountain pine beetle infestations 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 on the forest floor.
This site developed under Black Hills climatic conditions with short-term weather variations, light to severe grazing by bison, elk and small mammals, 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, and introduced non-native cool season grasses, are major drivers, along with grazing 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 and Hull-Sieg, 1996). In the absence of fire, encroachment of ponderosa pine is likely to occur on this site. The Loamy ecological site is not as susceptible to encroachment as the Shallow Loamy site but because they typically adjoin, encroachment can occur and untreated, pine will become established. Competition with cool season dominated grasses on the Loamy site hinder establishment of pine seedlings, whereas the gravelly and stony soils on the Shallow Loamy site typically dominated by warm season grasses do not provide that direct competition early in the growing season.
Kentucky bluegrass, smooth brome and timothy are common cool season grass species throughout MLRA 62. Because the Black Hills have higher precipitation amounts and cooler spring and fall periods these non-native cool season grasses can easily become established in a plant community. Improper grazing management, and non-use and/or no fire can cause these species to increase and become the dominant species in a plant community.
Changes will occur in the plant communities due to short-term weather variations, impacts of native and/or exotic plant animal and insect species, grazing management, and severe disturbances, such as periods of well-below average precipitation, severe defoliation, excessive haying or no fire and no use.
The Reference State is difficult to locate in this LRU with the introduction of non-native cool season grassed. Plant community phase 2.1 is most similar to the Reference State but because of the persistence of non-native cools season grasses a restoration pathway to the Reference State is not believed to be achievable.
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
This state represents what is believed to represent the natural range of variability that dominated the dynamics in this ecological site prior to European settlement. The Reference State is unlikely to occur with the introduction of non-native cool season grasses which are common throughout the MLRA.
This site was dominated by cool and warm season grasses. In pre-European times the primary disturbances included fire, insects and grazing by large ungulates and small mammals. Favorable growing conditions occurred during the spring, and warm months of June and July. Routine and/or occasional fires, reduced tree cover and contributed to the ecological processes that maintained the reference plant community.
Community 1.1
Rhizomatous wheatgrass-Needlegrass-Bluestem/Forb/Shrub
Figure 8. Loamy North 062XA010SD PCP-1.1
Interpretations are based primarily on the Rhizomatious wheatgrass-Needlegrass-Bluestem/Forb/Shrub 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, 15 percent forbs, 10 percent shrubs and 0 to 2 percent trees. Total annual production for a normal growing year is approximately 2,500 lbs. /Ac. The community is dominated by cool-season grasses and tall and mid-height warm season grasses. The dominant cool season grasses include western and bearded wheatgrass, porcupine grass, green needlegrass, Richardson’s needlegrass and Columbia needlegrass. Dominant warm season grasses are big bluestem, little bluestem, and prairie dropseed. Other grasses include sideoats grama, slender wheatgrass, and Richardson’s sedge. The dominant shrub is western snowberry, leadplant and rose. This plant community was productive and resilient to disturbances such as drought and fire. It was a sustainable plant community in regards to soil/site stability, watershed function, and biological integrity.
State 2
Native/Introduced
This state represents what is most typically found on this site. The natural range of variability is influences by the presence of non-native cool season grasses, especially Kentucky bluegrass and smooth brome that can dominate the dynamics of this ecological site. Proper grazing management and periodic burning will maintain the productivity of this state. Heavy grazing without adequate recovery, extended periods of drought or non-use and no fire can put this state at risk of crossing a threshold.
Community 2.1
Rhizomatous wheatgrass-Needlegrass-Buestem-Bluegrass/Forbs/Shrubs
This plant community phase closely resembles the PCP 1.1 however non-native cool season grasses have invaded the site and under the current Black Hills climatic conditions will persist in the plant community.
The potential vegetation is about 75 percent grass and grass-like plants, 15 percent forbs, 10 percent shrubs and 0 to 2 percent trees. Total annual production for a normal growing year is approximately 2,400 lbs. /Ac. The community is dominated by cool-season grasses and tall and mid-height warm season grasses. The dominant cool season grasses include western and bearded wheatgrass, Richardson’s and green needlegrass and porcupine grass. Kentucky bluegrass and/or other non-native cool season grasses can make up 2 to 5 percent of the plant community. Dominant warm season grasses are big bluestem, little bluestem, and prairie dropseed. Other grasses include sideoats grama, tall dropseed, slender wheatgrass, and Richardson’s sedge. The dominant shrub is western snowberry, leadplant and rose. This plant community is productive and resilient to disturbances such as drought and fire. It was a sustainable plant community in regards to soil/site stability, watershed function, and biological integrity. Management strategies must include techniques that will not cause Kentucky bluegrass and other non-native cool season grasses to increase significantly as this could put the plant community at risk of transition to the Introduced/Invaded State.
Community 2.2
Rhizomatous wheatgrass-Bluegrass-Smooth brome/Forbs/Shrub
This plant community evolved under continuous season-long grazing, seasonal grazing with no change in season of use or no use and no fire or heavy continuous grazing. This plant community phase is made up of approximately 85 percent grass and grass-like plants, 10 percent forbs, 5 percent shrubs and 0 to 2 percent trees. The community is dominated by cool-season grasses, with most of the warm season grass component being replaced by Kentucky bluegrass and other non-native cool season grasses. The dominant cool season grasses include western and bearded wheatgrass, porcupine grass and Richardson’s needlegrass. Kentucky bluegrass or other non-native cool season grasses can make up 10 to 25 percent of the plant community. This plant community is at-risk of crossing a threshold to the Introduced State.
Pathway 1A
Community 2.1 to 2.2
Non-use and no fire and/or continuous season long grazing or seasonal grazing occurring at the same time every year or heavy continuous grazing will cause an increase in non-native cool season grass species moving this plant community to PCP 2.2.
Pathway 2A
Community 2.2 to 2.1
Prescribed grazing that provides adequate recovery and change in season of use, along with normal precipitation regime and fire may reduce the percent of non-native cool season grasses in PCP 2.2 and restore the tall warm season component typical of PCP 2.1.
State 3
Introduced/Invaded
This state is the result of invasion and dominance of introduced species. This state is characterized by the dominance of Kentucky bluegrass and smooth brome, and an increasing thatch layer that effectively blocks introduction of other plants into the system. Plant litter accumulation tends to favor the more shade tolerant introduced grass species. The nutrient cycle is also impaired, and the result is typically a higher level of nitrogen which also favors the introduced species. Increasing plant litter decreases the amount of sunlight reaching plant crowns thereby shifting competitive advantage to shade tolerant introduced grass species. Studies indicate that soil biological activity is altered, and this shift apparently exploits the soil microclimate and encourages growth of the introduced grass species. Once the threshold is crossed, a change in grazing management alone cannot cause a reduction in the invasive grass dominance. Preliminary studies would tend to indicate this 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).
Once the state is well established, even drastic events such as high intensity fires driven by high fuel loads of litter and thatch will not result in more than a very short term reduction of Kentucky bluegrass. These events may reduce the dominance of Kentucky bluegrass, but due to the large amount of rhizomes in the soil there is no opportunity for the native species to establish and dominate before Kentucky bluegrass rebounds and again dominates the system.
Community 3.1
Kentucky bluegrass-Smooth brome-Timothy/Forb
This plant community is dominated by Kentucky bluegrass and/or other non-native cool season grasses (30 percent or more of the PC). This plant community evolved under no use and no fire or with heavy continuous season-long grazing, or seasonal grazing with no change in season of use or excessive haying.
This plant community is made up of approximately 85 percent grasses and grass-like species, 10 percent forbs, and 5 percent shrubs. Dominant grasses include Kentucky bluegrass, smooth brome, and timothy. Western wheatgrass and some needlegrass may still be found in the plant community. Forbs commonly found in this plant community include cudweed sagewort, goldenrod, scurfpea, and western ragweed. Production will be significantly reduced when compared to the interpretive plant community. The period when palatability is high is relatively short, as Kentucky bluegrass matures rapidly. Energy capture is also reduced. Runoff is high and biological activity in the soil is likely reduced significantly in this phase.
State 4
Conifer
This state is greatly influenced by conifers, primarily ponderosa pine but rocky mountain juniper can also be present. Ponderosa pine canopy was found to significantly reduce precipitation reaching the forest floor by an average of 30 percent due to interception in area of intermediate and dense canopy (Wrage, 1994). This state will develop when conifers encroach onto the site from adjacent forest sites or ecological sites that have been invaded. Encroachment and the establishment of conifers on this site is primarily the result of no fire and grazing management that reduced the competitive nature of the native herbaceous plant community. Once conifers become established on this site, non-native cool season grasses will increase especially in the shaded areas.
Community 4.1
Native/Introduced grasses/15-30% cover of Ponderosa pine
This plant community phase is the result of no use and no fire or on site that have had heavy continuous grazing resulting in increased bare ground where pine seedlings can become established. The potential vegetation is approximately 70 percent grasses and grass-like plants, 5 percent forbs, 5 -10 percent shrubs and up to 30 percent conifers. The majority of grasses will be cool season species with Kentucky bluegrass increasing because of its shad tolerance.
State 5
Early Seral
This state is the result of very heavy, concentrated disturbance such as cropping, concentrated rodent activity, or concentrated livestock areas. This state can develop as a result of invasion by highly competitive weed species such as Canada thistle, hound’s tongue, leafy spurge, or knapweeds. Extended periods of drought accompanied by heavy grazing can also push an at risk plant community phase to this state. In most cases, this phase is dominated by annual and/or pioneer perennial species. Bare ground is also typically much higher than on any other plant community phase.
Community 5.1
Pioneer Perennial/Introduced Species/Bare ground
This plant community developed under continuous heavy grazing or heavy disturbances such as heavy use areas, abandoned cropland, and livestock feeding areas. The potential plant community is made up of 60-80 percent grasses and grass-like species, 15 to 35 percent forbs, 2 to 5 percent shrubs, 0-10. Percent bare ground can be very high in this plant community. The dominant grass is often Kentucky bluegrass. Other grasses may include annual bromegrass (field brome and cheatgrass), sedge, sand dropseed, and western wheatgrass. The dominant forbs include fetid marigold, sweetclover, western ragweed, cudweed sagewort, and other invader-like species. The dominant shrubs include fringed sagewort, broom snakeweed. A wide variety of other early seral plant species can occupy this site in varying amounts. This plant community is susceptible to invasion of Canada thistle and other non-native species because of the relatively high percent of bare ground.
Transition 1A
State 1 to 2
Invasion and/or encroachment of non-native grasses such as Kentucky bluegrass, smooth brome and timothy, and disruption of natural disturbance regimes, typically as a result of fire suppression following settlement led this state over a threshold to the Native/Introduced State 2.0.
Transition 2A
State 2 to 3
Introduction or invasion of non-native cool season grass species. Non-use and no fire. Excessive haying of native grass species and or heavy continuous grazing. This transition is most likely going to occur from PCP 2.2
Transition 6A
State 2 to 4
Encroachment and establishment of conifer trees into this state due to no-use and no fire can cause a transition to the Conifer State.
Transition 2B
State 2 to 5
Severe and frequent defoliation which can be exacerbated by drought and/or additional livestock or wildlife use. Heavy use areas and livestock feeding areas can also cause this transition.
Restoration pathway 3A
State 3 to 2
Early season prescribed burning followed by long term prescribed grazing to promote establishment of native species. Chemical and/or mechanical treatment followed by seeding of native species may accelerate the reestablishment of structural functional groups similar to PCP 2.1 however the resulting plant community may not achieve management goals. This restoration pathway can take many years and in the end may not be successful.
Transition 6A
State 3 to 4
Encroachment and establishment of conifer trees into this state due to no-use and no fire can cause a transition to the Conifer State.
Transition 3A
State 3 to 5
Severe and frequent defoliation which can be exacerbated by drought and/or additional livestock or wildlife use will push this plan community to the Early Seral State.
Restoration pathway 4A
State 4 to 2
Reintroduction of fire or prescribed burning and/or, mechanical brush management to remove encroachment, followed by long term prescribed grazing to promote reestablishment of native species may accelerated the reestablishment of structural/functional groups similar to PCP 2.1 however the resulting plant community may not achieve management goals.
Transition 4A
State 4 to 3
Wild fire or prescribed burning or brush management to remove conifer encroachment followed by prescribed grazing management will transition back to the Introduced State.
Transition 4B
State 4 to 5
Wild fire, prescribed burn or brush management will treat conifer encroachment on this site but without proper grazing management this state will transition back to the Early Seral State.
Restoration pathway 5A
State 5 to 2
Removal of severe grazing disturbance (frequency and intensity), normal precipitation and fire regime, and time. Chemical and/or mechanical treatment followed by seeding of native species may accelerate the reestablishment of structural/functional groups similar to PCP 2.1 however the resulting plant community may not achieve management goals. This restoration pathway can take many years and in the end may not be successful.
Transition 5A
State 5 to 3
Removal of severe grazing disturbance (frequency and intensity), normal precipitation regime. Chemical and/or mechanical treatment followed by seeding of native and introduced species.
Transition 6A
State 5 to 4
Encroachment and establishment of conifer trees into this state due to no-use and no fire can cause a transition to the Conifer State.
