Physiographic features

This ecological site occurs on terraces
and high floodplain steppes, near springs or seeps, or other areas having a permanent water table close enough to the surface (typically within 3 feet)to influence plant composition and production. These areas are rarely or non-flooded. Rare flooding indicates that flooding is unlikely, but possible under unusual weather conditions (0–5% chance in any year). These are also considered to be “lentic” (standing water) riparian/wetland areas

Table 2. Representative physiographic features

Climatic features

Major Land Resource Area (MLRA) 58AC in Montana is 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 this MLRA, 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.

The majority of the rangeland in MLRA 58AC is within the 11 to 14 inch Mean Annual Precipitation (MAP) 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 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

Soil features

These soils are non-hydric. They are mainly deep to very deep (>40”) with a permanent water table within about three feet of the surface. They are generally in the aquic moisture regime or aquic intergrade. Surface textures will vary- mainly loam, sandy loam, clay loam.

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. This is often a preferred site for grazing by livestock due to the succulent forage, and animals tend to congregate in these areas. In order to maintain the productivity of this site, stocking rates must be managed carefully on adjoining sites with less production to be sure livestock drift onto the Subirrigated Site is not excessive. 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.

Grazing this site when the upper part of the soil is wet can cause compaction. Hummocking (frost heaving) is often a feature of this site. The hummocking can be exacerbated if grazing impact becomes excessive.

Whenever Plant Community 2 (medium and short grasses and sedges) occurs, 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 since a good seed source of the taller grasses should still exist.

Plant Communities 3 and 4 have severely reduced forage production, and contain a high percentage of non-palatable species. Once this site is occupied by these communities, the presence of non-native grasses and undesirable plants will make it 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.

Once established, plants such as Kentucky bluegrass, smooth brome, and Canada thistle are stable and very difficult to remove by grazing alone. The potential for using seeding or mechanical treatment to improve site health can be limited, depending on the depth to the water table.

Wildlife Interpretations:

The Subirrigated ecological site is important for enhancing biodiversity within an otherwise semi-arid environment. Although surface water is rarely present, the high water table allows growth of tall herbaceous and woody plants that provide habitat structure, food and cover for diverse wildlife species. The Historic Climax Plant Community (HCPC) was used by large herds of grazing ungulates, migrating shorebirds and waterfowl, flocks of sage grouse and many songbird species. Uncontrolled livestock grazing has greatly simplified this plant community in many areas. Livestock are attracted to the subirrigated site because of the availability of palatable, succulent forage when upland vegetation is dry. Invasive plants, including Canada thistle, Kentucky bluegrass, redtop and dandelion compete with native vegetation and degrade habitat for many wildlife species. Prescribed grazing strategies can maintain healthy wildlife habitat and promote vegetative productivity on this site. The proximity of uplands, subirrigated sites, riparian habitat and open water creates an exceptionally diverse habitat complex for a wide variety of wildlife.

Plant Community 1: Tall and Medium Grasses and Sedges / Forbs (HCPC):

The mesic environment and abundance of forbs support diverse insect and invertebrate populations ranging from grasshoppers and spiders to dragonflies and pollinating bees. Amphibians, such as Woodhouse’s toad, and reptiles, such as garter snakes, rely on this community for migration and over-wintering habitat. Amphibians can be considered a “keystone species” because of their value as indicators of environmental degradation. The HCPC supports a diverse bird population because of the mix of tall and medium grasses and sedges and abundant forbs. Northern harriers hunt over, and nest in, this community. Shorebirds such as the common snipe and upland sandpiper nest here. LeConte’s sparrow and the savanna sparrow are examples of songbird species using this plant community. Sage grouse broods find abundant insect foods here and adults select succulent forbs. The predominance of grasses and sedges in the HCPC favors grazers and mixed feeders like bison, elk and pronghorn. Thermal and escape cover are limited because of the low shrub coverage. Small mammals, such as the meadow vole, are common and abundant.

Plant Community 2: Medium Grasses and Sedges / Rushes/ Forbs:

Loss of tall grasses and sedges along with a change in the forb component represents a decrease in habitat structural diversity. Insect populations may still be abundant but are less diverse. Amphibian habitat is degraded by a reduction in surface litter and moisture. Breeding bird populations are less diverse as habitat structure is simplified. Common snipes will still use this community for nesting and feeding. Sage grouse broods continue to select this habitat for critical insect foods during their fast growth period follow hatching. Small mammal populations may shift away from dominance by voles to seedeaters like deer mice as ground cover decreases. Forage value for big game declines with the loss of a diverse mix of warm and cool season grasses.

Plant Community 3: Rushes / Medium and Short Grasses and Sedges / Shrubs / Invasive Forbs:

Insect populations decline in abundance and diversity with the loss of succulent forbs and increase in invasive weeds. Amphibians are negatively affected by a further loss of litter cover and an increasingly drier ground surface. Bird species diversity declines as habitat structural complexity is lost. Some species characteristic of drier habitats may increase. For ground-nesting birds, an increase in snowberry and rose cover may partially compensate for the sparser ground cover. Ungulate species will still find palatable forage (i.e., Kentucky bluegrass) but the forage diversity and length of green feed period have declined significantly.

Plant Community 4: Rushes / Non-Native Grasses/ Invasive Forbs:

Insects may be abundant during population highs but species diversity is considerably reduced. Amphibian and reptile habitat is degraded with the drying of the soil surface. Ground-nesting bird habitat is poor following loss of ground cover although an increase in snowberry and rose, if it occurs, partially compensates for duck nesting. Ungulate species find succulent forage but for a shorter period of time compared to higher successional stages. Small mammal species composition further declines.

Hydrological functions

The runoff potential for this site is low. Runoff curve numbers generally range from 61 to 79. The soils associated with this ecological site are generally in Hydrologic Soil Group B. The infiltration rates for these soils will normally be moderate.

A drop in the water table elevation, such as a result of several years of drought conditions will result in a change in the plant community to more drought tolerant species (often non-native).

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): 3

BLM-Soil & Vegetation Inventory Method (SVIM) Data: 5

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

NRCS-Range/Soil Correlation Observations & Soil 232 notes: 10

Contributors

MJR, REL, RSN, POH
RSN

Approval

Kirt Walstad, 6/14/2023