Ecological dynamics
The vegetation of the shallow loamy ecological site is part of the Black Hills ecosystem consisting of a mixture of forests and grasslands resulting from the interaction of a varied topography, geology, climate and natural disturbance. Frequent fires, periodic droughts, and episodic mountain pine beetle infestations are also thought to have contributed to maintain this park-like forest structure of large prairie areas scattered throughout the Black Hills. Ponderosa pine is a fire adapted species as it has evolved to coexist and flourish with fire; particularly frequent, low-intensity fires that consume mostly small seedlings, prune lower branches from large trees, and reduce woody fuels debris on the forest floor. The result is a mosaic of conditions ranging from open grassland areas to groups of young seedlings to clumps and groups of large older trees. When large crown fires did occur, they probably did not completely consume all trees within the landscape, but left large healthy trees as sources of seeds to eventually recolonize the burnt areas (Shepperd and Battaglia, 2002). Between 1388 and 1900, fire intervals in the Black Hills ranged from 16 to 20 years (Brown and Hull-Sieg, 1996).
With the advent of fire suppression over the past 120 years, forests density has increased and grassland areas decreased across the whole landscape. Historical records indicated the presence of large areas covered by tall grasses on the outer edges and slopes of the limestone plateau but did not mention ponderosa pine encroachment. McIntosh (1949) conducted botanical field work in the Black hills during the summers of 1924 to 1930 and noted that pines were invading the grassland. Gardner and Thompson (1972) also mentioned that pine encroachment was most evident on areas dominated by warm season grasses. Today, ponderosa pine encroachment is still quite evident on the shallow loamy ecological site where trees seem to germinate and survive well on the rocky and gravelly soils in the absence of fire.
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 historic native vegetation on this site consisted of mixed prairie grasses occurring on ridges and uplands. The vegetation was predominately warm season grasses mixed with some cool-season grasses portraying a mixed grass prairie characteristic. Little bluestem is the dominant species. Big bluestem and sideoats grama are in association with little bluestem. Subdominant cool season grasses include needlegrasses and rhizomatous wheatgrasses. In the understory, are blue grama, hairy grama, and threadleaf sedge. Kentucky bluegrass is present but typically does not dominate the plant community. Forbs make up a significant proportion in the mixture. Common forbs include cudweed sagewort, scurfpeas, coneflowers, gayfeather, asters along with hood phlox and pussytoes in the understory. Shrubs include fringed sagewort, broom snakeweed, green sagewort, prairie rose, and a few yucca plants scattered on the landscape. A few ponderosa pine and juniper are often scattered across the site. Rocks, and rock fragments are very noticeable and bare ground can be present but not common.
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 the natural range of variability that dominates the dynamics in this ecological site. This site is dominated by warm-season grasses, with cool-season grasses being subdominant. In pre-European times the primary disturbances included fire and grazing by large ungulates and small mammals. Favorable growing conditions during the spring as well as the warm months of June and July along with routine or occasional fires, reduces tree cover and contributes to the ecological processes that maintain the reference plant community.
Today a similar state can be found in areas where proper livestock use has occurred and where the encroachment of trees, especially ponderosa pine, has been limited.
Community 1.1
Bluestem-Sideoats grama-Rhizomatous wheatgrass/Ponderosa pine (< 2%)
Figure 8. SwLy North - PCP 1.1 (Spring)
Figure 9. SwLy North - PCP 1.1 (Summer)
Interpretations are based primarily on the Bluestem-Sideoats grama-Rhizomatous wheatgrass plant community phase. This is also considered to be the reference or historic community. The potential vegetation is about 80 percent grass, 10 percent forbs, and 10 percent trees and shrubs. Average annual production for this plant community phase on a median year is 1,700 lbs/Ac. The community is dominated by tall and mid-height warm-season grasses with cool-season grasses being subdominant. The dominant grasses include little bluestem, big bluestem, and sideoats grama. Western wheatgrass and needlegrasses also comprise a significant amount of the plant community.
Ponderosa pine can be scattered throughout the site but will not exceed 2 percent canopy cover. Other grasses include prairie dropseed, tall dropseed, blue grama, sedge and slender wheatgrass. This plant community is productive and resilient to disturbances such as drought and fire. This is a sustainable plant community in regards to soil/site stability, watershed function, and biological integrity.
Community 1.2
Rhizomatous wheatgrass-Needlegrass-Little bluestem-Blue grama/Ponderosa pine (< 2%)
This plant community is a result of heavy continuous grazing, seasonal grazing with no change in season of use between grazing years, or continuous season-long grazing by livestock. The potential plant community is made up of approximately 70 percent grasses, 15 percent forbs, and 15 percent shrubs and trees. Dominant grasses include western wheatgrass, beardless wheatgrass, needleandthread, green or Columbia needlegrass and little bluestem. Short warm-season grasses such as blue grama and hairy grama present in the understory.
Sideoats grama, tall dropseed, and prairie dropseed may also be present. Invasive, non-native grasses such as Kentucky bluegrass, may contribute to biomass production but do not dominate this ecological site. Forbs contribute substantially to the biomass production in this plant community. Big bluestem has decreased dramatically and may be nearly absent from the site. Ponderosa pine can be scattered throughout the site but will not exceed 2 percent canopy cover. The herbaceous species within this plant community are well-adapted to grazing.
Pathway 1.1A
Community 1.1 to 1.2
Continuous season-long grazing or heavy continuous grazing with stocking rates above the carrying capacity for the entire growing season or seasonal grazing during the middle of the growing season every year will cause a decrease in big bluestem, little bluestem and sideoats grama. Rhizomatous wheatgrass and needlegrass will increase initially but will decrease in the plant community if grazing management is not changed.
Pathway 1.2A
Community 1.2 to 1.1
Long-term prescribed grazing that provides adequate recovery and change in season of use, along with normal precipitation regime and periodic fire can restore the tall warm-season component of this plant community.
State 2
Shortgrass State
This state is dominated by short-grass species and sedges and is the result of continuous season-long grazing or heavy continuous grazing. Desirable species have been over utilized and removed or greatly reduced in the plant community. The site is has increased runoff and depending on percent bare ground the site can be susceptible to erosion. This state can be very resistant to change.
Community 2.1
Blue grama-Sedge/Fringed sagewort/Ponderosa pine (< 8%)
This plant community develops under continuous, season-long grazing or heavy continuous grazing. The transition may be accelerated by over-utilization during extended drought periods. This plant community is composed of approximately 65 percent grasses, 15 percent forbs, and 20 percent shrubs and conifers. Dominant grasses and grass-like plants include, blue grama, hairy grama, thread-leaf sedge, plains muhly, sideoats grama, and prairie junegrass. Other grasses include rhizomatous wheatgrass, needlegrass’s, little bluestem and sand dropseed. Cheatgrass, field brome, smooth brome, and Kentucky bluegrass may also invade the site. Forbs commonly found in this plant community include prairie coneflower, dotted gay feather, and common mares-tail. Shrubs commonly found on this site include fringed sagewort, broom snakeweed, soapweed yucca, and prairie rose. When compared to the Reference State, short statured grasses have increased significantly and become more dominant on the site. Shrubs become more common on the site as herbaceous species become reduced in vigor and cover. It is also common for Ponderosa pine to increase in this plant community.
State 3
Conifer/Herbaceous State
This state is dominated by conifer and is a result of heavy continuous grazing and lack of frequent fire or no use and no fire resulting in encroachment of conifers. Areas of intermediate and dense ponderosa pine canopy was found to reduce precipitation reaching the forest floor by an average of 30 percent (Wrage, 1994). Native warm-season grasses such as bluestems, sideoats grama, blue grama and cool-season grasses such as wheatgrasses and needlegrasses declined as overstory canopy cover increases. Shade tolerant grasses, such as poverty oatgrass, bluegrasses, and rough-leaved ricegrass increase. Forbs such as cudweed, sagewort and shrubs such as wood rose increased under intermediate canopy closure (< 25 percent). In the absence of fire, this plant community phase will be resistant to change. Ponderosa pine canopy can continue to increase over time, reducing herbaceous production and increasing bare ground. This state will have lower water infiltration rates, increased runoff, and potential for soil erosion. The resulting plant community is less productive for grazing animals than the other states.
Community 3.1
Ponderosa pine/Bluegrass-Sedge-Wheatgrass/Fringed sagewort Plant Community
Figure 10. SwLy North - PCP 3.1 Ponderosa Pine
This plant community is characterized by the dominance of conifers as a result of heavy continuous season-long grazing, and no fire, or no use and no fire. Ponderosa pine make up approximately 30 percent of the plant community, and shrubs approximately 15 percent. Grasses and forbs make up approximately 55 percent of the plant community. Dominant grass and grass-like plants include Kentucky bluegrass, slender wheatgrass, needleandthread, rough-leaved rice grass, and upland sedge. Dominant forbs include cudweed sagewort, coneflower, and dotted gayfeather. Prevalent shrubs include fringed sagewort, wild rose, poison ivy, and skunkbush sumac. Most of these shrubs rarely exceed one foot in height. Ponderosa pine is the dominant tree species; and Rocky Mountain juniper is sometimes present as a subordinate tree, but does not dominate.
Transition 1A
State 1 to 2
Continuous season-long grazing or heavy continuous grazing will cause a transition from the Reference State to a plant community dominated by shortgrass species in the Shortgrass State. Ponderosa pine may increase on this site but typically not greater than 8 percent canopy cover.
Transition 1B
State 1 to 3
Heavy continuous season-long grazing with stocking rates well above the carrying capacity for the entire growing season combined with the absence of fire to control conifer seedling establishment, or no use, no fire and conifer encroachment will lead toward a conifer dominated state, State 3. More shade tolerant grasses will become dominant in this state.
Restoration pathway 2A
State 2 to 1
Long-term prescribed grazing which provides growing season grazing deferment along with stocking rates not exceeding carrying capacities and periodic fire or prescribed burning will restore this plant community to the Reference State. The Shortgrass State can be resistant to change and grazing deferments and favorable growing conditions will in time help reestablish the plant community however management goals may not be achieved.
Transition 2A
State 2 to 3
Continuous season-long grazing or heavy continuous grazing with stocking rates above the carrying capacity for the entire growing season combined with the absence of fire to control shrub and conifer seedlings’ establishment, or no use, no fire and encroachment will lead toward a conifer dominated state, State 3.
Restoration pathway 3A
State 3 to 1
Fire or prescribed burning or mechanical brush management, plus long-term prescribed grazing may move the Conifer State back to the Reference State depending upon the plant community and climatic conditions. This could be a long-term process and the results may not
be achieved or meet management goals. Seeding may be successful following wildfire; however, it is recommended to use native species that replicate the structural functional groups in PCP 1.2.
Transition 3A
State 3 to 2
Depending upon the existing herbaceous plant species in the understory, prescribed fire and/or mechanical brush management to remove conifers and long-term prescribed grazing may transition the Conifer State to the Shortgrass State. Grazing deferments and favorable growing conditions will in time help to reestablish the plant community; however, management goals may not be achieved.