Desert shallow sandy loam (CORA)

The following state and transition model was developed based upon a priori knowledge of the ecological site (e.g., past experience and published literature) ecological principles and logical hypotheses. To the greatest degree possible, empirical data was used to validate concepts in the model. A confirmation of the existence of states using cluster analyses can be viewed at this link. Analyses aimed at modeling transitions can be viewed at this link.

Fig. 1. State-and-transition diagram for desert shallow sandy loam. Solid boxes represent ecosystem states. Dashed boxes indicate phases within states (red signifies a phase that is at-risk of transition to another state). Arrows indicate transitions. In some cases, phases within the reference state are not connected to any others by arrows; this is our method of representing spatial variants of the reference state that are dictated by abiotic factors (click to enlarge image)

S1. REFERENCE SHRUBLANDS. Multiple distinct vegetative communities can be observed. They appear to largely be dictated by abiotic factors rather than disturbance and successional processes. Soil depth and proportion of the surface covered by rocks seem to dictate dominant vegetation, and biological crust cover (as rock increases, the amount of available habitat for crusts decreases). Most of the reference communities contain Coleogyne ramosissima. Sites with low to moderate surface rock, and shallow depth (indicated by exposures of bedrock) tend to favor C. ramosissima shrublands.

S1P1. ROCKY SHRUBLANDS. This phase is characterized by surfaces dominated by small rocks. The vegetative community is quite distinct, being dominated by Chrysothamnus viscidiflorus. Elymus elymoides and Atriplex canescens are the most common palatable species. Biological crusts are unimportant, as there is little available habitat. Invasion by Bromus tectorum is uncommon, and of minor severity.

S1P2. BLACKBRUSH SHRUBLANDS – CRUSTED. This phase is characterized by low surface rock cover, and shallow soils indicated by bedrock exposures. The vegetation is naturally dominated by C. ramosissima and Ephedra spp. Biological crusts are common but cover is generally low. Invasion by Bromus tectorum is uncommon, and of minor severity.

Transitions from this phase are modeled at this link.

S1P3. BLACKBRUSH SHRUBLANDS. This phase is identical to S1P5, except that biological crust cover may be compromised by surface disturbances. Total plant cover may be reduced. Invasion by Bromus tectorum is uncommon, and of minor severity.

Transitions to this phase are modeled at this link.

S1P4. GRASSY SHRUBLANDS. This phase was not directly observed in available data, but is inferred based upon a parallel phase in semidesert shallow sandy loam sites. It is presumed to be the precursor of S2, though this cannot be tested directly since S2 sites occur only (with one exception) in currently grazed sites. Based on the native palatable species in S2, this phase might contain Aristida purpurea and Pleuraphis jamesii. Biological crusts are probably common but not abundant.

Transitions from this phase are discussed at this link.

S2. ANNUALIZED. Based upon physical attributes (relatively low exposed bedrock and surface rock) and some floristic similarities, this state is likely to arise via grazing disturbance to S1P1. It is dominated by native unpalatable shrubs such as Gutierrezia sarothrae. Also, Bromus tectorum may be a major community component, even codominating. Due to the potentially high contribution of B. tectorum to total cover, inter- and intra-annual variation in total cover is possible. Biological crusts are typically eliminated or occur in low abundance.

Transitions to this state are discussed at this link.

S3. SEVERELY ERODED. This state is largely theoretical. When a site is naturally lacking in surface rocks, its soil erodibility can be enhanced by loss of biological crusts (this occurs previously in the transition from SIP3 to SIP4). Erosivity, the ability of erosive forces to move sediment, is largely modified by properties of the plant community. When both erodibility and erosivity are high, erosion is certain to occur. If grazing intensity or drought mortality (or other disturbance such as ORVs, seismic explorer rigs, etc.) is so great that the erosivity-dampening properties of the vegetative community are degraded, a positive feedback may be initiated whereby erosion prevents vegetation recovery.

OTHER TRANSITIONS. This ecological site is closely aligned with Semidesert shallow sandy loam (JUOS-CORA), which can be viewed simply as a wetter version of Desert shallow sandy loam. Recent global-change type droughts in the Colorado Plateau region suggest that drought mortality can occur quickly in pulses. Pinus edulis, an important species of Semidesert shallow sandy loam (JUOS-CORA) is particularly susceptible. We can envision that a prolonged drying trend or an extreme drought could transition the states and phases od Semidesert shallow sandy loam (JUOS-CORA) to corresponding states and phases here in the Desert shallow sandy loam (CORA) ecological site. A state-and-transition model can illustrate possible transitions between these two ecological sites.

Fig. 2. A state-and-transition model illustrating the states and phases of both Semidesert shallow sandy loam and Desert shallow sandy loam. Transitions in blue indicate transitions precipitated by droughts linked to climate change (click to enlarge image)