Ecological dynamics
This site developed through time under the influence of climate, geological materials, fire, plants and animals. In comparison to normal upland range sites, environmental characteristics of this site limit herbage production and subsequent fuel accumulation. Although the role of natural fire is probably less significant in the development of this site, fires may have occurred on a natural interval of 10-12 years (Frost 1998). Research consistently shows that precipitation is the principal factor altering productivity (Heitschmidt et al. 2005). The same authors concluded that grazing reduces herbage standing crop, whereas its effects on aboveground net primary production varies with timing of grazing and precipitation events, along with the functional and structural composition of the plant community.
The resultant historic climax plant community (HCPC) is the basis for plant community interpretations. The HCPC has been determined by evaluating rangeland relic areas, and other areas protected from excessive disturbance.
The HCPC is comprised of a mixture of tall and medium height cool season grasses, forbs and shrubs. About 85% of the annual production is from grasses and sedges, most of which is produced during the cool season. Forbs and shrubs contribute 5 and 10%, respectively, to total annual production. Total vegetative production averages 600 lbs/ac in normal years, 350 lbs/ac in “unfavorable” years, and 900 lbs/ac in “favorable” years.
This site is moderately resilient to disturbance because plant growth is limited by soil characteristics. Departures from the HCPC generally result from management actions, drought, and/or a change in the natural fire regime. The site is considered fragile in the sense that vegetative vigor and composition will rapidly decline with continued adverse impacts. With favorable precipitation and/or prescribed grazing treatments the plant community can return to the HCPC. However, succession may be slow. Trends in plant community dynamics, states, transitional pathways, and thresholds have been evaluated and determined through experience and research.
Successional pathways of Dense Clay 10-14” p.z. ecological sites cannot be satisfactorily described using traditional theories of plant succession leading to a single climax community (Briske et al. 2005). As the HCPC regresses to an early seral state, it is theorized that a threshold is crossed somewhere within the mid-seral state. Plant communities occurring below this threshold are in a steady state. Succession back to the HCPC usually does not occur within a reasonable length of time, and/or without a large input of energy.
Three plant communities and the successional pathways that commonly occur within the Reference State (State 1) are shown in the following diagram. In addition, the transitions from Plant Community A (State 1) and from Plant Community B (State 1) to Plant Communities C (State 2) and D (State 3), are also illustrated. Ecological processes are discussed below in the plant community descriptions.
Community 1.1
Historic Climax Plant Community (HCPC)
Mid and tall cool-season perennial grasses, short cool-season perennial grasses, 5 percent native forbs, 10 percent native shrubs, 35 percent bare ground, total annual production about 600 lbs/ac
Mid and tall cool-season perennial grasses, short cool-season perennial grasses, 5 percent native forbs, 10 percent native shrubs, 35 percent bare ground, total annual production about 600 lbs/ac
Western/thickspike wheatgrasses and green needlegrass are common cool season mid grasses on this ecological site. They account for about 80% of total plant production in the HCPC. Needleandthread, another cool season mid-grass is common and tends to replace the green needlegrass when it is stressed by lack of moisture, grazing pressure, etc. About 5% of the total production is comprised of a mix of cool season short grasses and grasslike plants. These species include sandberg bluegrass, prairie junegrass, needleleaf sedge and threadleaf sedge.
American vetch, a cool season nitrogen-fixing legume, is one of the most important members of the forb community. Milk vetches and prairie thermopsis are additional members of the legume family which may occur in the HCPC. Other common forbs include onion, hoods phlox, scarlet globemallow, wooly plantain, and biscuitroot. The latter group contains a mix of warm and cool season species whose relative occurrence on the site is largely influenced by the timing and amount of precipitation. Forbs contribute about 5% of the total annual production.
Nuttall saltbush and winterfat are the two most important browse species occurring on the site. While Nuttall saltbush expresses itself during the cool season, winterfat is a warm season plant. Shrubs such as big sagebrush, greasewood, silver sagebrush, pricklypear cactus and fringed sagebrush may also be found in the HCPC. Shrubs normally make up about 10% of the total annual production.
Broom snakeweed, annual bromes, and annual forbs are not a part of the HCPC. Their presence indicates possible ecological deterioration, or downward trend. Trend is difficult to interpret because large areas of bare ground between plants are fairly common.
Total annual production averages 600 lbs/ac during normal years. However, production declines as the site regresses from the HCPC to lower successional communities. Regression may result from grazing management strategies that do not allow adequate recovery periods between grazing events, drought, and/or the disruption of the normal fire sequence.
Following regression from the HCPC, soil structure appears to influence species composition. Surface layers that are vesicular, with large angular peds (blocky clods, etc) appear to limit or restrict the establishment and growth of short grasses (especially blue grama) (White and Lewis 1969). Consequently, the percentage of bare ground increases as the production of green needlegrass and western wheatgrass decline on these soils.
In contrast, production of shallow-rooted and/or predominantly lateral-rooted species such as needle and thread, blue grama, sandberg bluegrass, prairie junegrass, hairy goldenaster, and hoods phlox increase when surface soils are more granular, with smaller peds, etc. The large cracks that form between the peds of the heavy soils are theorized to restrict shallow root growth and/or shear the lateral roots that have thin cortices, resulting in a reduction of the short grasses.
Regardless of soil shrink/swell realities, winterfat and Nuttall saltbush may also be replaced by broom snakeweed, fringed sagewort, etc on the site. Cheatgrass and Japanese brome may invade the site. As the result of these vegetative changes, there is less litter to protect the soil and less infiltration. Hydrologic cycles are impaired when plant communities are unable to effectively use precipitation.
Plant basal cover averages 15%. Litter varies from 40-50%. Consequently, bare ground may range from 35-55%. Thus, infiltration is not optimized and runoff and erosion are not minimized on the Dense Clay 10-14” p.z. ecological site. Runoff and soil erosion normally increase as the HCPC regresses to earlier seral states.
(Insert HCPC Plant Community photo)
The major plant species composition and production by dry weight are shown for the HCPC in the following table. Total annual production has been derived from several sources, and has been adjusted to represent a typical annual moisture cycle.
Table 6. Annual production by plant type
| Plant type |
Low
(kg/hectare) |
Representative value
(kg/hectare) |
High
(kg/hectare) |
| Grass/Grasslike |
336 |
572 |
857 |
| Shrub/Vine |
39 |
67 |
101 |
| Forb |
17 |
34 |
50 |
| Total |
392 |
673 |
1008 |
Table 7. Ground cover
| Tree foliar cover |
0%
|
| Shrub/vine/liana foliar cover |
0%
|
| Grass/grasslike foliar cover |
0%
|
| Forb foliar cover |
0%
|
| Non-vascular plants |
0-1%
|
| Biological crusts |
0-1%
|
| Litter |
40-50%
|
| Surface fragments >0.25" and <=3" |
0-1%
|
| Surface fragments >3" |
0-1%
|
| Bedrock |
0%
|
| Water |
0%
|
| Bare ground |
30-40%
|
Table 8. Soil surface cover
| Tree basal cover |
0%
|
| Shrub/vine/liana basal cover |
1-5%
|
| Grass/grasslike basal cover |
9-12%
|
| Forb basal cover |
1-2%
|
| Non-vascular plants |
0%
|
| Biological crusts |
0%
|
| Litter |
0%
|
| Surface fragments >0.25" and <=3" |
0%
|
| Surface fragments >3" |
0%
|
| Bedrock |
0%
|
| Water |
0%
|
| Bare ground |
0%
|
Table 9. Canopy structure (% cover)
| Height Above Ground (m) |
Tree |
Shrub/Vine |
Grass/
Grasslike |
Forb |
| <0.15 |
– |
0-1% |
0-20% |
0-25% |
| >0.15 <= 0.3 |
– |
0-40% |
0-50% |
0-50% |
| >0.3 <= 0.6 |
– |
0-50% |
0-25% |
0-25% |
| >0.6 <= 1.4 |
– |
0-10% |
0-5% |
– |
| >1.4 <= 4 |
– |
– |
– |
– |
| >4 <= 12 |
– |
– |
– |
– |
| >12 <= 24 |
– |
– |
– |
– |
| >24 <= 37 |
– |
– |
– |
– |
| >37 |
– |
– |
– |
– |
Community 1.2
Plant Community A - Pre-threshold Vertic
Reduced production, mid and short cool-season grasses, more than 40 percent bare ground.
Reduced production, mid and short cool-season grasses, more than 40 percent bare ground.
Range inventories conducted by NRCS on the Fort Peck and Fort Belknap Reservations indicate similarity indices of 45-64% are indicative of Community A. Non-prescribed grazing and drought reduce plant height and plant litter. Bare ground increases as the production of HCPC species decline. Surface runoff and soil temperature increases, and infiltration decreases. Production of shallow-rooted short grasses and sedges is limited by the large cracks that form between the peds as the soils dry. Pricklypear cactus is conspicuous in this community.
In contrast to the HCPC, total annual production averages about 475 lbs/ac. However, western and thickspike wheatgrasses and green needlegrass still contribute 70% of the annual production. However, they are less vigorous and individual plant growth is reduced from what it is in the HCPC. Production of the short grasses and lower-successional forbs increases slightly to what it was in the HCPC. Although total shrub production did not change, production of silver sagebrush tends to replace production of winterfat and Nuttall saltbush.
Plant community A is called a “pre-threshold community”. It is critical that this community be recognized and strategies implemented to prevent further regression. Although this community can improve to the HCPC through successional processes, further disturbance will result in regression to a lower state (State 2). Once Community A regresses to a lower state, normal successional processes are restricted.
Community 1.3
Plant Community B - Pre-threshold Non-Vertic
Reduced production, mid and short cool- and warm-season grasses, increase of lower successional forbs, more than 40 percent bare ground.
Reduced production, mid and short cool- and warm-season grasses, increase of lower successional forbs, more than 40 percent bare ground.
NRCS range inventories indicate that Community B is characterized by similarity indices of 45-64%. The Community is dominated by a mix of medium and short grasses. Blue grama, threadleaf sedge, needle and thread and sandberg bluegrass increased in the community by replacing some of the mid grasses. However, western and thickspike wheatgrass and green needlegrass continued to contribute about 50% of the total annual production (average of 375 lbs/ac.).
In comparison to the HCPC, sandberg bluegrass, blue grama, and other short grasses have increased. In addition, sand dropseed and tumblegrass may appear in the community. Warm season forbs increase and replace American vetch and other high-successional forbs. The warm season half-shrub, fringed sagewort, may also increase in this community. Pricklypear cactus and broom snakeweed are conspicuous.
In comparison to the HCPC, litter varies from 25-35%. Bare ground increases to 50-60%. Thus, rills, flow patterns and movement of litter deposits are visible.
Plant community B is called a “pre-threshold community”. It is critical that this community be recognized and strategies implemented to prevent further regression. Although this community can improve to the HCPC through successional processes, further disturbance will result in regression to a lower state (State 3). Once Community B regresses to a lower state, normal successional processes are restricted.
Pathway 1.1A
Community 1.1 to 1.2
Soils are vertisols, or have vertic properties. The soil surface structure is vesicular, irregular, coarse crumb. Granules in blocky clods, large peds, bordered by large cracks when dry.
Non-prescribed grazing, no fire, drought (3 to 6 years)
Non-prescribed grazing, drought and/or a cessation of the natural fire regime will cause regression from HCPC to Community A. The pathway to Community A occurs on soils with vesicular, irregular, coarse granules, and blocky clods. Large cracks form between the peds (aggregates) as these soils dry. The regression to either Plant Community A may occur within a couple of years.
Pathway 1.1B
Community 1.1 to 1.3
Soils are not vertisols, or do not have vertic properties. Soil surface structure is not characterized by large peds, block clods and large cracks when dry.
Non-prescribed grazing, no fire, and fire.
Non-prescribed grazing, drought and/or a cessation of the natural fire regime will cause regression from HCPC to Community B. The pathway to Plant Community B occurs on soils with much smaller soil peds. Large cracks between peds are not a normal occurrence on these soils. The regression to either Plant Community B may occur within a couple of years.
Pathway 1.2A
Community 1.2 to 1.1
The Dense Clay 10-14” p.z. site is resilient within the Reference State. Normal growing conditions, the implementation of prescribed grazing, or the re-introduction of the natural fire regime will move Plant Community A to the HCPC. This succession can occur within a couple of years.
Pathway 1.3A
Community 1.3 to 1.1
The Dense Clay 10-14” p.z. site is resilient within the Reference State. Normal growing conditions, the implementation of prescribed grazing, or the re-introduction of the natural fire regime will move Plant Community B to the HCPC. This succession can occur within a couple of years.
State 2
Degraded Vertic State
Community 2.1
Plant Community C
Short cool-season perennial grasses, mid-height cool-season grasses, pricklypear cactus, more than 50 percent bare ground, total annual production about 325 lbs/ac.
Short cool-season perennial grasses, mid-height cool-season grasses, pricklypear cactus, more than 50 percent bare ground, total annual production about 325 lbs/ac.
Regression of Plant Community A crosses a threshold and results in Plant Community C. This community is a steady state, which is resistant to change. It is characterized by a significant reduction in production of medium-height, cool season grasses. The amount of bare ground increases significantly. The wheatgrasses contributed about 50% of total annual growth. Individual wheatgrass plants produce few seed heads and can be low in vigor. Similarity indices during the NRCS inventories on Fort Peck and Fort Belknap (2001-2004) were less than 35% for this Community. Pricklypear cactus, broom snakeweed and annual bromes are often common in this community. Total annual production averaged 325 lbs/ac, a 20% reduction from Community A.
Litter cover averages about 15%. Water flow patterns are numerous and there is moderate active pedestalling. Bare ground is moderately to much higher than expected. Compared to the HCPC, production of tall and medium height grasses has decreased, and bare ground has increased. The ecological processes of energy flow, hydrologic cycle, and nutrient cycle are disrupted.
Resilience management. *Successional Pathways Between Communities C and D:
Differences in soil structure are largely responsible for the species composition of these two plant communities. Therefore, successional pathways between these communities are unlikely.
State 3
Degraded Non-Vertic State
Community 3.1
Plant Community D
Short warm- and cool-season perennial grasses, pricklypear cactus, increase of lower successional forbs, about 50 percent bare soil, total annual production about 300 lbs/ac.
Short warm- and cool-season perennial grasses, pricklypear cactus, increase of lower successional forbs, about 50 percent bare soil, total annual production about 300 lbs/ac.
Regression of Plant Community B ends in Plant Community D. This community is a steady state, which is resistant to change. It is characterized by a mix of warm and cool season short grasses. Blue grama, needleandthread and sandberg bluegrass are the most common plants. Western and thickspike wheatgrasses contribute about 15% to the total annual production. Production of low-successional forbs decreased relative to Community B. Total annual production usually varies from 250-300 lbs/ac during favorable years.
The NRCS inventories indicate a few winterfat and Nuttall saltbush plants persist in this community. However, dry weight production of these high-successional shrubs decreased relative to the dry weight production of broom snakeweed, pricklypear cactus and fringed sagewort.
Resilience management. *Successional Pathways Between Communities C and D:
Differences in soil structure are largely responsible for the species composition of these two plant communities. Therefore, successional pathways between these communities are unlikely.
Transition T1A
State 1 to 2
Non-prescribed grazing, no fire, extended drought (greater than 7 years)
Plant Communities A and B are much less resistant to disturbance than the HCPC. Lower production, lower vegetative cover, less litter, and increased bare ground increases susceptibility to disturbance, stress etc. Extended drought and non-prescribed grazing are the most common causes of retrogression to State 2.
Transition T1B
State 1 to 3
Non-prescribed grazing, no fire, extended drought (greater than 7 years)
Plant Communities A and B are much less resistant to disturbance than the HCPC. Lower production, lower vegetative cover, less litter, and increased bare ground increases susceptibility to disturbance, stress etc. Extended drought and non-prescribed grazing are the most common causes of retrogression to State 3.
Restoration pathway R2A
State 2 to 1
Prescribed grazing, extended period of favorable precipitation
Plant community C is resistant to significant succession. The adverse soil conditions and a theorized inadequate seed bank of high successional species greatly restrict potential for succession to State 1. Although succession usually does not occur within a reasonable length of time, anecdotal evidence indicates succession may occur with the combination of prescribed grazing, the resumption of a normal fire regime, and an extended period of favorable precipitation. Favorable environmental factors may favor succession of Plant Community C to Plant Community A. This possibility is depicted by a dashed arrow in the state and transition diagram.
In comparison to “normal” ecological sites (Silty 10-14” p.z., Clayey 10-14” p.z. and Sandy 10-14” p.z.) having soils > 20 inches in depth, the average annual above ground production on this Dense Clay 10-14” p.z. ecological site is 50-60% less. Mechanical treatments and range seeding are not recommended on this site.
Restoration pathway R3A
State 3 to 1
Prescribed grazing, fire, extended period of favorable precipitation
Plant community D is resistant to significant succession. The adverse soil conditions and a theorized inadequate seed bank of high successional species greatly restrict potential for succession to State 1. Although succession usually does not occur within a reasonable length of time, anecdotal evidence indicates succession may occur with the combination of prescribed grazing, the resumption of a normal fire regime, and an extended period of favorable precipitation. Favorable environmental factors may favor succession of Plant Community D to Plant Community B. This possibility is depicted by a dashed arrow in the state and transition diagram.
In comparison to “normal” ecological sites (Silty 10-14” p.z., Clayey 10-14” p.z. and Sandy 10-14” p.z.) having soils > 20 inches in depth, the average annual above ground production on this Dense Clay 10-14” p.z. ecological site is 50-60% less. Mechanical treatments and range seeding are not recommended on this site.
