Physiographic features

This ecological site occurs on nearly level to strongly sloping sedimentary plains, terraces, and fans. The slopes range from 0 to 15 percent, but are mainly less than 8 percent. This site occurs on all exposures and aspect is not significant. This site is very similar to a normal Silty site, with some significant variations in the plant community composition.

Table 2. Representative physiographic features

Climatic features

MLRAs 58A and 60B are considered to have a continental climate characterized by cold winters, hot summers, low humidity, light rainfall, and much sunshine. Extremes in temperature are typical. The climate is the result of this MLRA’s location in the geographic center of North America. There are few natural barriers on the northern Great Plains and the winds move freely across the plains and account for rapid changes in temperature. Seasonal precipitation is often limiting for plant growth. Annual fluctuations in species composition and total production are typical depending on the amount and timing of rainfall.
Temperatures can be very extreme in this part of Montana. Summer daytime temperatures are typically quite warm, generally averaging in the mid to upper 80º’s F for July and August. Summertime temperatures will typically reach in the 100º’s F at some point during the summer, and can reach 90º F any month between May and September. Conversely, winter temperatures can be cold, averaging in the mid teens to mid 20º’s F for December and January. There will typically be several days of below zero temperatures each winter. It is not uncommon for temperatures to reach 30–40º F below zero, or even colder, most any winter.

Spring can be windy throughout these MLRA’s, with winds averaging over 10 mph about 15 percent of the time. Speeds of 50 mph or stronger can occasionally occur as a weather system crosses this part of Montana.

MLRAs 58AE and 60BE have been divided into two distinct precipitation zones for the purpose of developing ecological site descriptions: 10–14” Mean Annual Precipitation (MAP) and 15–19” MAP.


10–14 inch zone:

The majority of the rangeland in these areas falls within the 11 to 13 inch range. During an average year, 70 to 75 percent of the annual precipitation falls between April and September, which are the primary growing season months.

Snowfall is not heavy in the area, averaging 28 total inches in the 10-14 inch MAP (Yellowstone Valley). Heavy snowfall occurs infrequently, usually late in the winter or early spring. Snow cover is typically 1 to 3 inches.

The frost free (32º F.) season averages about 105 to 145 days each year in the uplands, to nearly 170 days along the Yellowstone River Valley.

For local climate station information, refer to http://www.wcc.nrcs.usda.gov/cgibin/state.pl?state=mt.

Table 3. Representative climatic features

Influencing water features

There are no influencing water features for this site.

Soil features

These soils are more than 20 inches deep and are mainly loams or silt loams and occasionally silty clay loams. They are strongly alkaline in the subsoil, usually within 6 to 12 inches of the surface. They tend to be slightly to moderately saline and very strongly alkaline in the upper 2 to 2.5 feet. In their "natural" condition, these soils will appear very similar to a "normal" silty ecological site. When tilled, however, they become very salty at the surface, severely limiting plant production and the potential for reseeding.

Table 4. Representative soil features

Animal community

Livestock Grazing Interpretations:

Managed livestock grazing is suitable on this site as it has the potential to produce an abundance of high quality forage. Management objectives should include maintenance or improvement of the plant community. Shorter grazing periods and adequate re-growth
after grazing are recommended for plant maintenance and recovery. Heavy stocking and season long use of this site can be detrimental and will alter the plant community composition and production over time.

Whenever Plant Community 2 occurs (medium and short grasses), grazing management strategies need to be implemented to avoid further deterioration. This community is still stable, productive, and healthy provided it receives proper management. This community will respond fairly quickly to improved grazing management including increased growing season rest of key forage plants. Grazing management alone can usually move this community back to one more similar to potential if a good seed source of the taller grasses still exists.

Once this site is occupied by Plant Community 3 or 4, it will be more difficult to restore it to a community that resembles the potential with grazing management alone. Additional growing season rest is often necessary for re-establishment of the desired species and to restore the stability and health of the site. Additional rest or accelerated practices are often necessary for re-establishment of the desired species and to restore the stability and health of the site.

Plant Communities 4 and 5 have extremely reduced forage production (<300 pounds per acre), and are comprised of mostly of non-preferred species. As salts are brought up to the surface the soils become extremely limited for plant growth. These communities are very difficult to restore to a more productive stage.

Wildlife Interpretations:

The following is a description of habitat values for the different plant communities that may occupy the site:

Plant Community 1: Tall and Medium Grasses/ Forbs/ Shrubs (HCPC):
The predominance of grasses plus a diversity of shrubs and half-shrubs in this community favors grazers and mixed feeders such as bison, pronghorn and elk. When this plant community is adjacent to large blocks of sagebrush-grassland, it can provide quality sage grouse lek sites and brood habitat. Complex plant structural diversity and litter cover provide habitat for a wide array of small mammals (both seed eaters, i.e., deer mice, and herbivores, (i.e. sagebrush vole) and neotropical migratory birds. Diverse prey populations are available for raptors such as ferruginous hawks and prairie falcons. The diversity of grass stature and life forms, along with scattered shrubs provides habitat for many bird species including the upland sandpiper, sharp-tailed grouse, loggerhead shrike, grasshopper and vesper sparrow, lark bunting, chestnut-collared longspur and western meadowlark. This community is especially favorable for ground-nesting birds because of the abundant residual plant material and litter available for nesting, escape and thermal cover.

Plant Community 2: Medium and Short Grasses/ Medium Shrubs:
An increase in sagebrush cover may benefit sagebrush-grassland obligates such as Brewer’s sparrow and sage grouse. Thermal and hiding cover for big game improves with an increase in shrub cover. Loss of tall preferred grasses, residual grass cover and litter generally reduces habitat value for ground-nesting birds. Some species, such as lark buntings, will use this community as sagebrush and/or greasewood cover increases somewhat. Small mammal species diversity declines as the herbaceous plant composition is simplified. The western harvest mouse may take advantage of an increase in greasewood.

Plant Community 3: Short Grasses/ Medium Shrubs / Half-shrubs:
The shift to short grasses and annuals plus a loss of ground cover and residual grass greatly reduces habitat value for ground-nesting birds. Small mammal species composition is simplified and dominated by seed-eaters, particularly deer mice. Nutritive value for ungulates suffers from the loss of desirable forbs, shrubs and half-shrubs and palatable grasses. The increase in greasewood may provide thermal cover for big game. The general loss of plant structural diversity significantly reduces wildlife habitat values.

Plant Community 4: Short Grasses/ Cacti/ Annuals:
This community has very limited habitat value for wildlife in general following the significant loss of plant species diversity and ground cover. Mountain plovers and horned larks may nest on the sparsely covered surface. Sage grouse may use this community for lek sites when it is adjacent to sagebrush-grassland.

Plant Community 5: Annual Grasses and Forbs: This community is found on “go back” land following abandonment of cropping. Wildlife habitat values are very limited because of a lack of plant species and structural diversity and ground cover.

Hydrological functions

The runoff potential for this site is moderate, depending on slope and ground cover/health. Runoff curve numbers generally range from 66 to 84. The soils associated with this ecological site are generally in Hydrologic Soil Group B. The infiltration rates for these soils will normally be moderately slow.

Good hydrologic conditions exist on rangelands if plant cover (grass, litter, and brush canopy) is greater than 70%. Fair conditions exist when cover is between 30 and 70%, and poor conditions exist when cover is less than 30%. Sites in high similarity to HCPC (Plant Communities 1 and 2) generally have enough plant cover and litter to optimize infiltration, minimize runoff and erosion, and have a good hydrologic condition. The
deep root systems of the potential vegetation help maintain or increase infiltration rates and reduce runoff.

Sites in low similarity (Plant Communities 3, 4, and 5) are generally considered to be in poor hydrologic condition as the majority of plant cover is from more shallow-rooted species and shrubs.

Erosion is minor for sites in high similarity. Rills and gullies should not be present. Water flow patterns, if present, will be barely observable. Plant pedestals are essentially non-existent. Plant litter remains in place and is not moved by erosion. Soil surfaces should not be compacted or crusted. Plant cover and litter helps retain soil moisture for use by the plants. Maintaining a healthy stand of perennial vegetation will optimize the amount of precipitation that is received. (Reference: Engineering Field Manual, Chapter 2 and Montana Supplement 4).

Recreational uses

This site provides some recreational opportunities for hiking, horseback riding, big game and upland bird hunting. The forbs have flowers that appeal to photographers. This site provides valuable open space and visual aesthetics. Caution should be used during wet weather periods.

Other information

The following is an example of how to calculate the recommended stocking rate. This example does not use production estimates from this specific ecological site. You will need to adjust the annual production values and run the calculations using total annual production values from the ecological sites encountered on each individual ranch/pasture. Before making specific recommendations, an on-site evaluation must be made.


Example of total annual production amounts by type of year:
Favorable years = 2200 lbs/acre
Normal years = 1480 lbs/acre
Unfavorable years = 1200 lbs/acre

It is recommended that on slopes of 30% or less, stocking rate should be derived from the total annual production pounds minus 500 pounds for residual dry matter and 25% harvest efficiency. On slopes over 30%, stocking rate is derived from total annual production pounds minus 800 pounds for residual dry matter and 25% harvest efficiency. Refer to the NRCS National Range and Pasture Handbook for a list of Animal Unit Equivalents.

Sample Calculations using Favorable Year production amounts:

< 30% slopes: AUM/AC = [(2200-500)(0.25)]/915 lbs/month for one AU = 0.46 AUM/AC
AC/AUM = (1.0 AU)/(0.46AUM/AC) = 2.2 AC/AUM

> 30% slopes: AUM/AC = [(2200-800)(0.25)]/915 lbs/month for one AU = 0.38 AUM/AC
AC/AUM = (1.0 AU)/(0.38 AUM/AC) = 2.6 AC/AUM

NOTE: 915 lbs/month for one Animal Unit is used as the baseline for maintenance requirements. This equates to 30 lbs/day of air-dry forage (1200 lb cow at 2.5% of body weight).

Inventory data references

NRCS-Production & Composition Record for Native Grazing Lands (Range-417): 2

BLM-Soil & Vegetation Inventory Method (SVIM) Data: 2

NRCS-Range Condition Record (ECS-2): 5

NRCS-Range/Soil Correlation Observations & Soil 232 notes: 15

Contributors

Bob Leinard
JVF, REL, RSN, MJR, SKW, SVF, POH