State 1
Historic Climax Plant Community

Community 1.1
Historic Climax Plant Community

Grassland: The historic plant community is believed to be diverse grassland community dominated by black grama, with bush muhly, Arizona cottontop, cane bluestem, tobosa, and threeawns as subordinates. Sideoats grama (Bouteloua curtipendula) and plains bristlegrass (Setaria leucopila) are also common. Winterfat (Krascheninnikovia lanata) and succulents such as ocotillo (Fouqueria splendens) and sotol (Dasylirion spp.) or yuccas (Yucca spp.) may be common, although in Sierra County they are not (Gene Adkins, NRCS Truth or Consequences). Creosotebush, mesquite, and tarbush densities are low and these shrubs are minor components. Grazing-induced retrogression from this community is characterized by a reduction in the cover of black grama (Bush muhly/black grama community). Secondarily, bush muhly, cane bluestem, Arizona cottontop, sideoats grama and winterfat will decrease. This is accompanied by an increase in the proportional representation tobosa, threeawns, burrograss (Scleropogon brevifolius), fluffgrass (Dasyochloa pulchella), snakeweed (Gutierrezia sarothrae), and shrubs (tobosa/bush muhly community). Diagnosis: Black grama and bush muhly grass cover dominant, tobosa may also be dominant. Several grass species represented. Shrubs are sparse and there are few signs of erosion. Additional States and Transition Pathways: Transition to tobosa-bush muhly/shrub state (1a): The cause of this transition is presumed to be due to grazing or other soil disturbance, possibly in combination with climate change, that allows shrubs to attain and maintain dominance. Key indicators of approach to transition: Increases in bare ground and bare ground patch size, decreases in black grama and bush muhly cover, possibly increased germination of creosotebush and tarbush. Changing climate may also drive these changes. Transition to grassland state (1b): Shrub control measures (e.g. tebuthiron application) have achieved desired results on gravelly loam soils and may be used to recover a productive grassland. The transition is probably contingent on grazing deferment and high summer rainfall to facilitate grass establishment. Transition to eroded shrubland state (2): With persistent lack of grass and litter cover and high rates of erosion, soil surface conditions may be altered such that grass reestablishment is rare across the site. This is likely to be due to the loss of nutrients and water-retaining organic matter with the loss of the A horizon due to wind and water erosion. Key indicators of approach to transition: Increases in bare ground cover and patch size, loss of grass cover, increased evidence of erosion (rills, gullies, deepening of gullies, pedestalling), including loss of the A horizon. Transition to tobosa-bush muhly/shrub state (3): Unknown, but presumably requires shrub removal and treatment of the soil surface (pitting) to increase infiltration, and seeding. Data and information sources and theoretical background: Communities and states are derived largely from Jim Powell, NRCS, retired and observations by Brandon Bestelmeyer, USDA-ARS Jornada Experimental Range. Mechanisms driving transitions are assumed to be similar to those described for the Gravelly ecological site. Specifically soil truncation due to erosion and the competitive or allelopathic effects of creosotebush on other plants and soil microbes are believed to cause transitions. Because erosion to a petrocalcic horizon is not a factor on these soils, as it is in gravelly soils, erosion to an argillic (Bt) soil horizon may limit grass survival and establishment. The loss of organic matter needed to retain water at the rooting depth of grasses may be a key mechanism.

State 2
Tobosa-Bush muhly/Shrub State

Community 2.1
Tobosa-Bush muhly/Shrub State

Tobosa-bush muhly/shrub: In this state, creosotebush or tarbush is either dominant or co-dominant and tobosa, bush muhly, or burrograss are dominant grasses. Mesquite (Prosopis glandulosa) may also be present. Black grama has usually been reduced to a minor component if it is present at all and overall grass cover is lower than in the grassland state. It is unknown whether different communities within this state are spatially segregated in response to minor soil texture variation or if they occur on the same sites over time in response to grazing or climate variation. It is likely that grazing-induced retrogression within this state leads to a reduction of tobosa and bush muhly and increasing representation of burrograss (creosotebush-burrograss). Shrub control within this state may increase cover of perennial grasses only temporarily (e.g. bush muhly/tobosa community), especially if shrub propagules reestablish (e.g. due to summer drought with good spring/ winter rains) or shrub kill is incomplete. Diagnosis: Creosotebush/tarbush density is moderate and perennial grasses are patchy. Bare patches may be common and large (> 2m). Evidence of erosion, including gullies and rills, may be apparent in bare areas but not necessarily.

State 3
Eroded Shrubland State

Community 3.1
Eroded Shrubland State

Eroded shrubland state: Creosotebush or tarbush is dominant and grasses, including tobosa, burrograss, or fluffgrass occur only in small patches. As in the tobosa-bush muhly/shrub state, soil texture variation may alter the degradation trajectory. On Eba soils, tarbush may dominate whereas on Hap soils, creosotebush may dominate. Permeability of water may be low across the site where the Bt horizon has been exposed and soil stability and fertility are low. Note that in the field, this state can be expressed as a mosaic with the tobosa-bush muhly state. Diagnosis: Creosotebush/tarbush density is moderate to high. Grasses occur in relatively small patches separated by large amounts of interconnected bare ground. Evidence of erosion, especially pedestalling of grasses and shrubs and terracettes, are apparent throughout the site.