State 1
Historic Climax Plant Community

Community 1.1
Historic Climax Plant Community

Figure 4. SD-2; Draw

Tobosa-dominated grassland state: The historic plant community is believed to be tobosa-dominated and may include alkali sacaton or vine mesquite in microsites (depressions) where water collects for periods longer than a few days. Blue grama may have been dominant or co-dominant with tobosa in many draws (Phil Smith, BLM Las Cruces, personal communication) and blue grama tends to increase in abundance with grazing rest. Sideoats grama (Bouteloua curtipendula) and cane bluestem (Bothriochloa barbinodis) are also present. Production ranges from 2000 lbs/acre in favorable years to 600 lbs/acre in unfavorable years. At its most pristine, the draw site is characterized by a high diversity of grasses. Retrogression within this state results in the reduction or loss of blue grama, vine mesquite, cane bluestem, and other grasses due to grazing and/or a reduction of soil moisture availability, and a homogeneous stand of tobosa remains. These grasses may become locally extinct, but could be reintroduced through natural or management means. Mesquite and other shrubs or trees (e.g. desert willow Chilopsis linearis, littleleaf sumac Rhus microphylla, fourwing saltbush Atriplex canescens) may be present and persist in low densities, especially at draw margins. Tobosa is reputedly fire tolerant. Given the large fuel loads typical of draw sites, fire may have been an important feature of the site. Diagnosis: Cover of grasses high, nearly continuous. Alkali sacaton and vine mesquite present. Mesquite are small or sparse if present, and bare patches are small (< 50 cm) and infrequent. Litter cover dominates most plant interspaces. This site appears as a grassy swale dominated by mid-grasses, rhizomatous and stoloniferous short-grasses, and some tall-grasses. It may, in some instances, have a component of shrubs and trees (such as desert willow) that are both dominant in stature and natural to the site, where occasional deeper soil wetting occurs. Substantial amounts of woody species beyond an aspect sort of dominance usually represent an abnormal increase or an invasion of these plants. Both numbers and kinds of forbs are quite variable and are dependent upon moisture and temperature conditions at any given time. Mesquite may be considered to have become naturalized to the site in minor amounts. Transition to burrograss state (1a): This can occur in response to changes in water flow or soil permeability catalyzed by overgrazing. Removal of tobosa may increase the rate of water flow down the draw and result in gullying. Channelization of subsequent flood waters into the gully diverts run-in water that favors tobosa and produces conditions more suitable for burrograss (see Paulsen and Ares 1962, Gibbens and Beck 1988). The physical blockage of water flow by dams may have a similar effect. Alternatively, exposure of soil to raindrop impact may promote soil crusting and erosion and reduce infiltration. A temporal decline in rainfall amounts and water flow in some draws could lead to similar changes without much grazing pressure (Campbell 1931). Key indicators of approach to transition: Decreases in grass diversity and tobosa cover, increases in burrograss cover, increases in bare ground cover, decreases in litter cover, decreases in flooding frequency and duration. The presence of blockage to surface flow would be an important indicator. Transition to mesquite-tobosa woodland state (2a): Overgrazing of tobosa, erosion, and resultant channelization or the impedance of surface water flow may facilitate the encroachment of honey mesquite or other shrubs into draws. The mechanism underlying this response may be a reduction in soil moisture that favors mesquite establishment. Alternatively, increasing mesquite establishment in degraded draws may be due to diminished competition from tobosa or lowered frequency of fire-generated disturbances due to grazing. Fire may remove (or topkill) small (< 3.5 yr old, < 0.5 inch stem diameter) mesquite (Wright et al. 1976). The maintenance of a shrubfree grassland using fire cannot be achieved once herbaceous cover is less than 600 lbs/acre (Wright and Bailey 1980). Thus, once grass biomass has permanently declined below this value (e.g. when burrograss dominates), the control of mesquite with fire is not possible. Key indicators of approach to transition: Decreases in grass cover, increases in bare ground cover, decreases in litter cover, decreases in flooding frequency and duration. If mesquite plant density is increasing, then the transition may have occurred. It is possible that mesquite seedlings normally increase until the next fire disturbance, but there are no data to support or refute this. Transition to annual-dominated state (6a): This can occur if continued extreme overutilization, mechanical disturbance due to offroad vehicle use, and subsequent soil loss and/or physical degradation decreases water infiltration and seedling germination. Under such conditions, only an ephemeral, annual-dominated community may persist. Key indicators of approach to transition: Decrease in tobosa or burrograss cover, increased decadence of grasses, increases in bare ground cover and mean bare ground patch size, evidence of soil physical crusting and shrink-swell cracking, and erosion including rills and pedestalling.

State 2
Mesquite-Tobosa-Woodland

Community 2.1
Mesquite-Tobosa-Woodland

Additional States: Mesquite-tobosa woodland state: The amount and duration of soil moisture is hypothesized to be below a value that permits the persistence and growth of mesquite or, in some cases, other shrubs such as little-leaf sumac. Alternatively, fire frequency may be too low to suppress woody growth once the fuel provided by tobosa is lowered below a certain level (Wright et al. 1976). Mesquite tends to increase under these conditions. Tobosa may be locally extirpated due to grazing pressure but may recolonize with shrub removal and restored hydrologic inputs, and usually exists somewhere in the site. Diagnosis: Tobosa cover discontinous and patchy. Vine mesquite and alkali sacaton are usually absent. Large (> 1 m tall) mesquite are present, bare ground is associated with shrubs and patches may be large (> 2m). Transition to mesquite woodland state (3): Overgrazing or disturbance reduces the remaining tobosa. Soil degradation, gullying, and nutrient loss then accelerates the continued reduction of tobosa. Key indicators of approach to transition: Decreases in tobosa cover, increases in bare ground cover, decreases in litter cover, decreases in flooding frequency and duration, terracetes, rills, gullies, soil sealing. Transition to tobosa-dominated grassland state (2b): Mechanical removal of mesquite with restored hydrology, if necessary, and subsequent recovery of tobosa cover and fire frequencies. Seeding should not be necessary.

State 3
Burrograss Grassland

Community 3.1
Burrograss Grassland

Burrograss grassland: This state is dominated by burrograss. Bare ground dominates the remaining ground cover. The availability of soil moisture is hypothesized to be lower in this state than in the tobosa-dominated state (c.f. Gile and Grossman 1997, Herbel and Gibbens 1989). This state has been found on the downslope side of earthen water tanks and does not appear to be common. Transition to mesquite woodland state (4a): This can occur when mesquite is introduced after burrograss dominance, if the environmental conditions are suitable for mesquite germination. Fire cannot be used to control mesquite in burrograss grasslands. Key indicators of approach to transition: Unknown, perhaps only presence of mesquite seed vectors. Transition to tobosa-dominated grassland state (1b): Mechanical destruction of gullies or dams, and the use of water spreaders to promote a more even distribution of water could create conditions favorable to tobosa establishment and dominance (c.f. Rango et al., in press). Transition to annual-dominated state (7): As for 6a above.

State 4
Mesquite Woodland

Community 4.1
Mesquite Woodland

Mesquite woodland: Grasses are sparse, especially tobosa. Burrograss may dominate some patches, but bare ground and mesquite dominate the aspect of the site. Soil compaction may be important and permeability may be low. Diagnosis: Tobosa is absent or reduced to a few plants or patches. Burrograss may dominate some patches. Large (> 1 m tall) mesquite are abundant, and bare ground patches are interconnected or is nearly continuous. Transition to burrograss grassland state (4b): Removal of mesquite using mechanical means and herbicide. This may result in increases in burrograss cover if mesquite competes with the grass. Mesquite may recolonize. Transition to tobosa-dominated grassland state (5): Removal of mesquite using mechanical means and herbicide with restoration of soil permeability (pitting?) and surface hydrologic inputs. Seeding is probably necessary.

State 5
Annual-Dominated

Community 5.1
Annual-Dominated

Annual-dominated: The extreme drying and formation of physical soil crusts with extreme reductions in grass cover inhibits perennial plant establishment. Under these conditions, and in the absence of established shrubs, annual plants dominate (e.g cocklebur; Xanthum strumarium). Transition to tobosa-dominated grassland state (6b): Restoration of soil permeability (pitting?) and surface hydrologic inputs. Seeding is probably necessary. Information sources and theoretical background: Communities and states are derived largely from observations by Brandon Bestelmeyer and Jim Powell. Communities are usually defined by the primary and secondary dominant plant species, but sometimes emphasize dominant species of differing life-forms. Transitions are derived from expert opinion and are founded upon two hypotheses (same as in the Bottomland site). The channelization hypothesis holds that the loss of herbaceous vegetation cover increases erosion and channelization, and that channelization reduces soil moisture availability to grasses across broad areas. Changes in soil moisture availability, in turn, lead directly to changes in the composition of dominant plants (Gile and Grossman 1997). The fire hypothesis holds that vegetation change is limited only by limitations in the dispersal and growth of dominant shrub species. Once shrub propagules are present, vegetation change is inevitable without periodic disturbances such as fire (Brown and Archer 1989). Finally, the competition hypothesis holds that tobosa grassland maintenance depends upon the competitive exclusion of shrub seedlings due to limitations in light or nutrients (c.f. Van Auken and Bush 1990). There may be a threshold grass density below which the probability of shrub establishment increases rapidly, leading to a transition to the shrubland type.