Semi-desert stony loam (ATCO)

Background
Semidesert stony loam (shadscale) ecological sites, are situated on stony, often cobbley alluvial and colluvial soils. They are deep and well drained, and typically on mild to moderate slopes. They are at least slightly calcareous. A large proportion of the soil surface is covered by rock, making the soil well-armored against wind erosion, but these soils may be susceptible to water erosion. The current draft ecological site description does not provide a state-and-transition model, but does mention some ecosystem dynamics in response to grazing and fire. In the description, grazing decreasers included Hesperostipa comata, Achnatherum hymenoides, Artemisia bigelovii and Ephedra torreyana (all palatable except for Ephedra). Grazing increasers include Artemisia tridentata, Gutierrezia sarothrae, Pleuraphis jamesii, and Atriplex confertifolia. Although fire dynamics were mentioned, our a priori state and transition model did not include fire-related transitions because these communities are characterized by low cover (usually Helianthus, Juniperus, Pinus, or Bromus tectorum.

With these assertions in mind we developed the following state-and-transition model:

Fig. 1. State-and-transition diagram for Semi-desert stony loam (shadscale). Solid boxes refer to ecosystem states, and dashed boxes represent phases within those states (red being at-risk of state transition). Click image to enlarge.

















State 1. REFERENCE SHRUBLANDS.
State one consists of sparse shrublands over growing shallow soils with a large amount of surface rock. Atriplex confertifolia is a common species, which may co-occur with Atriplex canescens, Artemisia spp., Ephedra torreyana. There may be a grass component such as Hesperostipa comata, Achnatherum hymenoides, Pleuraphis jamesii or Elymus elymoides. Rock cover is 35% on average, ranging from 9 - 75%. Biological soil crusts may be present but habitat is constrained by rock cover. Plant cover averages 18%, with most sites having less cover, and some extremely productive sites with cover up to 45%. Basal gaps average 1.25 m. Shrubs are usually about twice as abundant as grasses. A large proportion of the soil that is armored by rock, but soil aggregate stability of exposed soil surface may be low, averaging 2-3 using the Herrick soil stability kit. Like many ecological sites surface soil stability is controlled largely by biological crust cover, which explain around 60% of the variation in aggregate stability.

Fig. 2. Photograph of Semidesert stony loam (ATCO) from Witwicki et al. 2009. Click to enlarge image.
















SIPhase1: PALATABLE SHRUBLANDS
Total plant cover is about 15%, and a typical shrub to grass ratio is ~ 5:1. These sites are more likely to contain cool season bunchgrasses such as Achnatherum hymenoides and Hesperostipa comata, the palatable shrubs Artemisia bigelovii and Atriplex canescens, and the unpalatable Ephedra torreyana. Biological crust cover is usually sparse.

S2Phase2: LESS PALATABLE SHRUBLANDS
Total plant cover is about 8 %, and a typical shrub to grass ratio is ~ 3.3:1. These sites are most likely to contain higher relative abundance of Atriplex confertifolia, along with unpalatable shrubs Guttierezia sarothrae. The warm season perrennial grass Pleuraphis jamesii may increase in importance. Biological soil crust cover is usually sparse. Juniperus osteosperma may occasionally be an important species.

State 2. GRAZING IMPACTED GRASSY SHRUBLAND
Total plant cover is about 28%, though smaller statured species are dominant, suggesting that standing biomass may be lower than the previous state. A typical shrub to grass ratio is ~ 1. This state is dominated by the grazing tolerant grass, Pleuraphis jamesii, and the palatable shrubs are greatly reduced. Guttierrezia sarothrae is a common sub-dominant and Bromus tectorum may have invaded.

State 3. ANNUALIZED
Information on total plant cover is lacking, Palatable shrubs have largely been replaced by unpalatable ones such as Coleogyne ramosissima and G. sarothrae. Grasses typically account for more cover than shrubs by a ratio of ~3.5:1. This is driven by annual grasses which account for ~70% of the grass cover; the perrennial grass to shrub ratio is close to 1. Bromus tectorum dominates and as a result, vegetative cover fluctuates within and among years. The exposed soil surface is erodible, especially in highly erosive thunderstorms.


Proceed to this link to read a summary of a data-derived validation of the phases of state 1, and this link to read a summary of a data-derived effort at modeling transitions.