Ecological dynamics

Vegetation Dynamics:
Very Shallow produces about 75 to 200 pounds per acre of biomass annually.

The Very Shallow ecological site in MLRA 7 has at least four different variations on the low shrub-short grass theme for the Reference Community. Sandberg bluegrass is co-dominant in every variation:
1. Scabland sagebrush – Sandberg bluegrass
2. Scabland sagebrush / thymeleaf buckwheat / rock buckwheat – Sandberg bluegrass
3. Thymeleaf buckwheat – Sandberg bluegrass
4. Narrowleaf goldenweed (Stenotus s.) – Sandberg bluegrass

In the spring this site has a rich diversity of native annual and perennial forbs on most sites. Very Shallow ecological site supports edible species that have been an important food source for the Native Americans for many generations. Bitterroot and biscuitroot are the main species harvested for food.

Sandberg bluegrass is a shallow rooted, perennial bunchgrass, perfectly suited to Very Shallow sites. It has short leaves and a green to purplish panicle seed head. On most sites Sandberg is an understory grass, but on the Very Shallow ecological site it is the dominant grass. It begins growth in the fall then grows rapidly in the spring and sets seed before moisture is gone. Sandberg bluegrass is resistant to drought, grazing, trampling and fire.

In most years many bunchgrasses remain dormant in the fall. Sandberg bluegrass, however, greens up every year. And so, from late fall through winter and into early spring these Very Shallow ecological sites provide important green forage for deer, elk and upland birds. But these sites also dry up sooner than adjoining sites due to the very shallow soil depth. Grasses are dry from May through September.

Scabland sagebrush, locally known as stiff sagebrush, is low mounding and strongly scented with the characteristic sage odor. It is low and spreading with a conspicuously woody base. The base is often heaved from the soil by frost action. The trunk is very irregular, spreading above the base in a dense cluster of short, rigid, and rather brittle branches up to sixteen inches in length. Scabland sagebrush leaves are forked into three deep lobes like fingers. Unlike other sagebrush species, the leaves of scabland sagebrush are deciduous, and by fall, all the leaves have dropped. The ground under each plant will have a pile of dead leaves.

Fire: The vegetative cover is too low to carry fires, so these sites rarely burn
Grazing: In many pastures the Very Shallow ecological sites are not attractive to grazing animals and so are rarely if ever grazed.
Based on inherent protection from both fire and grazing, most Very Shallow ecological sites are stable and in excellent condition.

For more grazing management information refer to Range Technical Notes found in Section I Reference Lists of NRCS Field Office Technical Guide for Washington State.

State and Transition Diagram: Very Shallow

This state and transition model (STM), explains the general ecological dynamics for the Very Shallow ecological site. The STM illustrates the common plant communities that can occur on the site. Boxes around each state represent the ecological threshold, which if crossed, is not reversible without human intervention. Arrows within a state represent the pathway between plant communities, while the arrows between states represent the transition or recovery between the states. Plant species composition is represented as a percentage of total annual production (pounds). The composition of pristine sites can vary somewhat due to variations in site conditions.


Reference State for Very Shallow Ecological Site

State 1 Narrative:
The Reference State represents non-invaded communities composed of native species. Invasive annual grasses are not present. Very Shallow ecological sites rarely burn, and in most cases, receives minimal grazing. This ecological site is the most stable ecological site on the landscape.

Reference Community 1.1 is the classic Very Shallow, dominated by Sandberg bluegrass with one or more low shrub species. The low shrub component may be scabland sagebrush and one or more Eriogonum low shrub species. Community 1.1 is mostly very stable. remaining in State 1 regardless of climate or management. Similarity Index scores are typically higher on Very Shallow than other ecological sites on the landscape.

Community 1.2 represents the degraded phase which is quite rare in MLRA 7. The species are native, but Sandberg bluegrass has a diminished presence and forbs are more prominent. Community 1.2 still has enough Sandberg bluegrass present, to shift back to reference community 1.1, given the right conditions,

Reference State Community Phases:
1.1 Reference Sandberg bluegrass – low shrub
1.2 Forb – low shrub Native forbs – low shrub

Dominant Reference State Species:
Sandberg bluegrass and scabland sagebrush and other low shrub Eriogonum species

At-risk Communities:
• All communities in the reference state are at risk of moving to State 2. The seed source of cheatgrass is nearby and blowing onto most sites annually
• Community 1.1 has a high Sandberg bluegrass cover and is thus, at low risk of moving to State 2, Forb-Annual Grass
• Community 1.2 has low amount of Sandberg bluegrass cover and a high amount of forb cover, and is at considerable risk of moving to State 2
 
Reference Community 1.1 for Very Shallow:
Plant species composition is represented as a percentage of total annual production (pounds). The composition of pristine sites can vary somewhat due to variations in site conditions.

Similarity Index Similarity Index
Dominant Low Shrub/Half Shrub
25% 50 lbs.

ARRI2 stiff sagebrush
ERTH4 thymeleaf buckwheat
ERSP7 rock buckwheat

Other Shrubs / Half Shrubs
2% 4 lbs.

SADOI purple sage
PUTR2 antelope bitterbrush
ERDO Douglas buckwheat
ERHE2 Wyeth buckwheat
ERIOG buckwheat

Dominant Short Grass

POSE Sandberg bluegrass 55% 110 lbs.

Mid-Grasses 3% 6 lbs.

ELEL5 bottlebrush squirreltail
ACTH7 Thurber needlegrass
PSSP6 bluebunch wheatgrass

Annual Grass

VUOC sixweeks fescue 1% 2 lbs.

Most Common Forbs 10% 20 lbs.

NEST5 narrowleaf goldenweed
PHHO spiny phlox
PHLOX phlox
L1PU11 granite gilia
LOMAT lomatium / biscuitroot
BAHO Hooker balsamroot
VITR3 sagebrush violet
ERIGE2 fleabane

Other Forbs 5% 10 lbs.

ERNI2 snow buckwheat
ASPU9 woollypod locoweed
PENI pediocactus
PENST penstemon
NOTR2 weevil microseris
EPILO willow herb
ALLIU wild onion
LERE7 bitterroot

Below Normal Above
Estimated Production (pounds / acre) 75 150 200

 
1.1a Result: Shift from Reference Community (low shrub – short grass) to Community 1.2 (forb – low shrub). Sandberg bluegrass has been much reduced but remains in the community. The native forb component has increased. There may be a few invasive forbs.

Primary Trigger: heavy spring grazing pressure (heavy to severe grazing intensity) to Sandberg bluegrass. The grazing pressure can come from elk, cattle or feral horses.

Ecological process: consistent spring defoliation pressure to Sandberg bluegrass causes poor vigor, shrinking crowns and mortality. Grass roots begin to die, and this opens the soil for native forbs to increase via seedlings. The hoof action by large ungulates can disturb the soil surface enough to make them vulnerable to annual grass and forb invasion.

Indicators: decreasing Sandberg bluegrass cover and increasing cover of native forbs.


1.2a Result: Shift from forb – low shrub community back to the Reference Community.
Sandberg bluegrass reestablishes dominance over the native annual forb component as it exerts competitive advantage for resources and space. So, Sandberg bluegrass displaces the forbs to become co-dominant with the low shrub component.

Primary Trigger: Defoliation pressures are removed, allowing Sandberg bluegrass to recover and reestablish dominance over the forb component.

Ecological process: With reduced grazing pressure Sandberg bluegrass experiences increased plant vigor and root production, expanding its size and competitive abilities through seedlings and tillering. Soils stabilize with the removal of the hoof action and increased volume of roots.

Indicators: decreased forb cover and increased cover of Sandberg bluegrass.


State 2: Forbs &/or Annual Grass

State 2 Narrative:
This state represents the ecological changes that occur when there is a shift from dominance by perennial native grasses to forbs or annual grass dominance in the herbaceous layer. The shrub components generally remain in the overstory.

Most Very Shallow sites never cross the threshold into State 2 as they are not attractive to grazing animals and rarely burn (limited forage values and surface rocks). The exception being chronic heavy grazing in the spring from migrating elk, feral horses or livestock. As the cover of Sandberg bluegrass significantly declines the site becomes open to invasion by invasive annuals, however.

Invasive annual grasses, which are common & frequently dominant on adjacent Loamy ecological sites, do not often compete as well on Very Shallow sites. However, the cheatgrass seed blows onto Very Shallow sites annually and can become a minor component. In a year with heavy snowfall and early spring rain, such as 2017, the site had far more moisture that the plant community could utilize. This is the perfect opportunity for cheatgrass seed, which is capable of rapid germination and growth to establish in significant amounts across the site. In following years when moisture is normal or below normal, native species will utilize most of the available moisture and cheatgrass seed will not germinate or make viable plants. Therefore, in most cases, these micro-bursts of cheatgrass tend to be episodic and mostly a temporary condition on Very Shallow sites.

A reduction to Sandberg bluegrass cover allows annual grasses the opportunity to colonize and invade on a more permanent basis. Heavy grazing use disrupts the soil surface and the moss-lichen layer via animal hooves, which in turn, causes loss of both soil structure and biological crust. When this happens site resistance to erosional forces are greatly diminished as well.

State 2 may exhibit either a significant decrease in pedestaling due to the lack of bunchgrass cover and heavy use trampling by ungulates, or there will be a significant increase in pedestaling due to increased erosion from water flows around the remaining bunchgrasses.


Community Phases for State 2:

Community Phase 2.1: dominated by native forbs and invasive annual grasses. Forbs which increase in the altered conditions and are competitive with invasive grasses, can include lomatium, fleabane, willow herb, yarrow and onion. Typical invasive grasses may include annual bromes, medusahead and sixweeks fescue.


Transitions
T1 Result: Shift from Reference Community Phase 1.1 to Sate 2 Community Phase 2.1, resulting in the shift in functional groups to forbs and non-native annual grass dominance.

Primary Trigger: Extensive spring grazing with heavy use to Sandberg bluegrass. The grazing pressure can come from elk, cattle or feral horses.

Secondary Trippers: a micro-burst of cheatgrass could put Community 1.2 at risk. The trampling of very shallow soils, displacing and disturbing the surface soil structure by grazing animals could also trigger transition to State 2. A micro-burst of annual grasses could allow even pristine sites to be invaded. Community 1.2 is the community most at risk and is also the pathway for crossing the threshold from State 1 to State 2.

Ecological process: consistent spring defoliation pressure to Sandberg bluegrass causes poor vigor, shrinking crowns and plant mortality. Most or all Sandberg bluegrass plants are lost from the community, and this allows native forbs to increase and invasive annuals (forbs and grasses) to colonize and expand. This facilitates plant community changes from Community 1.2 to Community 2.1.

Indicators: Declining vigor and cover of Sandberg bluegrass, declining soil biotic crust and, increasing gaps between perennial bunchgrasses.


No Recovery
State 2 is considered non-reversible. Restoration of Sandberg bluegrass, the low shrub component, native forbs and the soil biotic crust would be extremely difficult, labor intensive and costly. Seedlings and plugged plants need soil moisture and time to germinate and become established. In most years, seeds and plugs may not have a chance as site conditions on the Very Shallow ecological site can change quickly and the non-native species are much more adaptable under these conditions. Drying winds and bright sun can turn a snowy or muddy site into a hard crust before plants are established. Timing of all recovery efforts would have an extremely narrow window of opportunity on these altered sites.