Ecological dynamics

Overview
The Sandy Ecological Site typically occurs on upland plains, adjacent to or in a mosaic with Deep Sand or Loamy Ecological Sites. The reference plant community of the Sandy site has a grassland aspect characterized by warm- and cool-season grasses, scattered shrubs, and forbs. Blue grama is the dominant grass species accompanied by subdominant western wheatgrass. Fourwing saltbush and winterfat are the dominant shrubs. This site is susceptible to juniper invasion and shrub encroachment. Loss of grass cover and lack of fire may facilitate the transition to the Juniper State. Decreased grass cover due to overgrazing and drought in conjunction with resource competition may cause the transition to the Shrub-dominated State.

Catalog of states and community pathways

Reference State
Reference Plant Community: In the reference plant community, blue grama is the dominant grass species accompanied by subdominant western wheatgrass. Other species that occur in significant numbers include Indian ricegrass, sand dropseed, and spike dropseed. In addition to western wheatgrass and Indian ricegrass, other species such as needle and thread, bottlebrush squirreltail, and New Mexico feathergrass contribute to an important cool-season grass component on this site. Principal shrubs include fourwing saltbush, winterfat, and sand sagebrush. Rocky Mountain beeplant is often the most noticeable forb. Continuous heavy grazing will cause a decrease in cool-season grasses, especially western wheatgrass. The Warm-season Grass Community, dominated by blue grama with subdominant dropseeds, threeawns, and galleta, may result. Western wheatgrass is adapted to fine- to medium-textured soils, and may be naturally less dominant on coarser textured soils (7). Conversely, dropseeds are adapted to coarse- to medium-textured soils and may be naturally more dominant on soils with loamy sand surface textures (7). The Sod-bound Blue Grama Community may occur in response to increased fall/spring moisture following drought (2, 5) or continuous heavy grazing.

Diagnosis: Grass cover is relatively uniform; however, bare ground makes up a large percent of the total ground cover, and grass production during unfavorable years may only average 250 pounds per acre. Shrubs are scattered with canopy cover averaging 5%. Evidence of erosion such as rills and gullies is infrequent.

Additional States:
Shrub-Dominated State: This state is characterized by the predominance of shrubs, especially sand sagebrush, horsebrush, or rabbitbrush. Perennial grasses are subordinate. The grass component is typically a low-vigor, blue grama community with more threeawns, dropseeds, ring muhly, sandhill muhly, and bare ground than in the Reference State.

Diagnosis: Grass cover is patchy, usually dominated by low-vigor blue grama. Shrub cover averages 20% or more. Evidence of wind erosion, such as pedestalling of plants, blowouts, and soil deposition, may be common.

Transition to the Shrub-Dominated State (T1A). Loss of grass cover due to overgrazing or extended drought may facilitate the transition to the Shrub-Dominated State.

Key indicators of approach to transition:
--Loss of cool season grasses
--Decrease in grass and litter cover
--Increases in cover of bare ground
--Increases in shrub seedlings

Restoration Pathway to the Reference State (R2A). Brush control is necessary to reduce the competitive influence of shrubs and reestablish grass dominance. Chemical control or mowing for 2 consecutive years is effective in controlling sand sagebrush. Root plowing and other mechanical control methods that sever the plant below the sprouting zone may reduce horsebrush and rabbitbrush densities. Some positive results have been reported in controlling rabbitbrush with herbicides (1, 8). Follow-up spraying after the initial treatment is necessary to control horsebrush (9). Single treatments may actually increase horsebrush densities. Complete shrub removal should be attempted only after erosion hazard is evaluated. Seeding may be necessary if adequate seed source is not present. Rest from grazing followed by prescribed grazing afterward will help ensure grass establishment.

Juniper-invaded State. This state is characterized by the presence of juniper. Blue grama, dropseeds, galleta, Indian ricegrass, and threeawns are the primary grass species. Western wheatgrass may be present.

Diagnosis: Juniper is present. Grass cover is variable, ranging from relatively uniform to patchy with large, interconnected bare areas.

Transition to Juniper-invaded State (T1B). Loss of grass cover, resource competition, and lack of fire are believed to facilitate juniper invasion. Climatic periods of mild winters and wet summers may produce conditions favorable to juniper establishment, and result in episodic events of juniper expansion (6). Seed dispersal by wildlife and livestock may contribute to the spread of juniper. Birds, rodents, deer and other small mammals may eat the fruits of juniper and aid in spreading juniper seed (3). Sheep and goats may browse juniper and can act as dispersal agents in some areas. Overgrazing and competition for resources in conjunction with drought may favor juniper invasion. During years of limited rainfall, good grass cover may suppress juniper seedling survival by competing directly for soil moisture. Resource competition is more important during juniper seedling establishment when their roots are in the same zone as the grasses (3). Overgrazing may facilitate the establishment of juniper seedlings by providing competition-free areas, but livestock exclusion alone would not prevent juniper establishment. During wet years, competition for available soil moisture is reduced, and juniper seedlings may even establish in good stands of grass (3). Additionally, the natural spatial variability of ground cover may allow woody species to establish on bare areas within good grass stands when adequate moisture is available (4). Where fire was historically important in the development of plant communities on Sandy Ecological Sites by suppressing juniper seedlings, then overgrazing and fire suppression can disrupt natural fire frequencies and may facilitate juniper invasion.

Key indicators of approach to transition
--Decrease or change in composition or distribution of grass cover
--Increase in size and frequency of bare patches
--Increase in amount of juniper seedlings

Restoration Pathway to the Reference State (R3A). Mechanical or chemical brush control can be used to remove juniper and facilitate grass recovery. After brush control, rest from grazing followed by prescribed grazing will assist in grass reestablishment and persistence.

References

1. Cluff, G.J., B.A. Roundy, R.A. Evans, and J.A. Young. 1983. Herbicidal control of greasewood (Sarcobatus vermiculatus) and salt rabbitbrush (Chrysothamnus nauseosus ssp. consimilis). Weed Science. 31: 275-279.

2. Jameson, D.A. 1970. Value of broom snakeweed as a range condition indicator. Journal of Range Management. 23: 302-304.

3. Johnsen, T.N., Jr. 1962. One-seeded juniper invasion of northern Arizona grasslands. Ecological Monographs. 32:187-207.

4. Jurena, P.N. and S. Archer. 2003. Woody plant establishment and spatial heterogeneity in Grasslands. Ecology 84: 907-919.

5. McDaniel, K.C., L.A. Torell, and J.W. Bain. 1993. Overstory-understory relationships for broom snakeweed-blue grama grasslands. Journal of Range Management. 46: 506-511.

6. Miller, R.F., and R.J. Tausch. 2001. The role of fire in pinyon and juniper woodlands: a descriptive analysis. Pages 15–30 in K.E.M. Galley and T.P. Wilson (eds.). Proceedings of the Invasive Species Workshop: the Role of Fire in the Control and Spread of Invasive Species. Fire Conference 2000: the First National Congress on Fire Ecology, Prevention, and Management. Miscellaneous Publication No. 11, Tall Timbers Research Station, Tallahassee, FL.

7. USDA, NRCS. 2002. The PLANTS Database, Version 3.5 (http://plants.usda.gov). National Plant Data Center, Baton Rouge, LA 70874-4490 USA.

8. Whisenant, S.G. 1988. Control of threadleaf rubber rabbitbrush with herbicides. Journal of Range Management. 41: 470-472.

9. William, R.D., D Ball, T.L. Miller, [and others], compilers. 2001. Pacific Northwest weed management handbook. Corvallis, OR: Oregon State University. 408 p.