State 1
Historic Climax Plant Community

Community 1.1
Historic Climax Plant Community

Figure 4. MLRA 42; SD-2; Loamy

State Containing Historic Climax Plant Community Black grama-tobosa grasslands: This state is now rare throughout SD-2. On Berino fine sandy loam soils west of Las Cruces (the Corralitos Ranch), a tobosa-black grama community occurs in which tobosa is the overwhelming dominant and black grama is restricted to small patches. In other settings (e.g. the Jornada Experimental Range), black grama appears to have colonized where sand has blown onto silt loam soils. Dominance by black grama, however, has not been observed on soils classified to this site. Other than tobosa and black grama, alkali sacaton is a relatively consistent feature of this site. Threeawns or dropseeds (Sporobolus spp.) may achieve dominance or subdominance depending upon variation in soil texture and natural disturbances. On more gravelly soils, bush muhly (Muhlenbergia porteri) cover may be significant. In any case, the historic community was probably extremely diverse due to the intermediate nature of this site along the gradient of soil texture. Grazing-induced retrogression would be expected to reduce this diversity and to severely reduce the dominance of black grama. Subordinate species of lower palatability and higher tolerance to disturbance, including tobosa and threeawns, would be expected to attain dominance. As long as sufficient black grama cover persisted and climatic conditions favored black grama reproduction, then recovery would be possible. Diagnosis: Ranging from near co-dominance between black grama and other grasses to the representation of black grama in isolated patches. Mesquite, tarbush, and creosotebush are absent. Signs of erosion, including gullies, rills, and litter movement are infrequent. Transition to burrograss-tobosa-threeawn grassland (1a): Grazing and/or drought may limit the growth of black grama (see also transition 1, Sandy model). Disturbance to individual black grama plants due to trampling or other physical disturbance may be especially important. Key indicators of approach to transition: Loss of grass species diversity, reduction in black grama cover and increased decadence of black grama plants, decreases in overall grass cover coincident with increases in the relative cover of burrograss, tobosa and/or threeawns, increases in bare patch size. If climate is not favorable for black grama reestablishment, the transition may be inevitable. Transition to shrub-invaded state (2a): Mesquite invasion may be due to introduction by cattle or humans (see Sandy model), although tarbush and creosotebush are wind-dispersed. Surfacesoil disturbance, reduced grass cover, and the presence of large bare patches may facilitate the germination of seeds of all shrubs. This mechanism assumes that shrub invasion is coincident with grazing-induced loss of grass (black grama) cover, as is the case on the Jornada Experimental Range. The reduction of periodic fire in conditions where black-grama/tobosa grasslands have sufficient grass production (e.g. >600 lbs/acre; Wright et al. 1976) may also have facilitated shrub invasion. These factors may be independent of management (Herbel and Gibbens 1996) if climate has played a large role. Key indicators of approach to transition: Same as for 1a if mesquite is limited by competition with grasses, in addition to the presence of maturing shrubs. If fire is important, then a persistent reduction in production and litter cover coincident with reduced fire frequencies may portend shrub invasion.

State 2
Burrograss-Tobosa-Threeawn Grassland

Community 2.1
Burrograss-Tobosa-Threeawn Grassland

Additional States: Burrograss-tobosa-threeawn grassland: There are many communities within this state due to the sensitivity of species to slight variations in soil conditions, or perhaps due simply to chance colonization. The state is characterized by the reduced ground cover of grasses overall and low cover of black grama in particular. On eroded sandy ridges (e.g Algerita, sandy loam, eroded phase), threeawns, yucca (Yucca elata), and fluffgrass (Dasyochloa pulchella) may dominate. In some cases, alkali sacaton may dominate; it is unclear why (but perhaps related to salinity). Local variation in grass densities may occur depending on microtopography or soils. On siltier and/or more calcareous soils, burrograss may be dominant (perhaps without disturbance). On heavier soils with higher run-in water such as Algerita sandy clay loam, tobosa may dominate. In some cases, annual grass and forb species may be especially abundant. On slight (< 1°) slopes, erosional-depositional vegetation banding (Mauchamp et al. 1990) may be apparent, with bare strips being a prominent feature (e.g. Algerita sandy loam; Gile and Grossman 1997). This pattern may be initiated by disturbance to vegetation and is maintained through positive feedbacks. The extent of the bare area may not increase over time, although the bare areas may shift upslope. Diagnosis: Black grama low to absent. Other perennial grasses including tobosa, burrograss, and threeawns ranges are dominant. Cover of perennial grasses and litter is discontinuous, bare areas are interconnected. Signs of erosion may be present. Transition to shrub-invaded state (3a): See transition 2a above. Communities dominated by burrograss are unlikely to have sufficient fuel to carry fire. Thus, the presence of shrub propagules at a site may be all that is required for this transition. Transition to black grama-tobosa grassland (1b): If climate is responsible for black grama decline, then this transition may be impossible to manipulate. May require soil amendments to increase fertility for black grama (i.e. restoration of mycorrhizal associates). Very little is known about the constraints to the presence and dominance of individual grass species.

State 3
Shrub-Invaded Grassland

Community 3.1
Shrub-Invaded Grassland

Shrub-invaded grassland: This state usually features a substantial amount of shrub cover, although in some cases, a savanna-like aspect with abundant grass (e.g. tobosa/mesquite community) can persist for long periods. Different shrubs have a tendency to invade depending on variations in soil texture—mesquite invades more often on more coarse variants (e.g. sandy loams over sandy clay loam or clay loam B horizons; tobosa or threeawn/mesquite communities) with low carbonate and tarbush and creosotebush are dominant shrubs where surface layers are finer (sandy clay loams, fine sandy loams) and soils with higher carbonate (such as the carbonate-rich Reagan soils). Tarbush may do especially well on silt-loam soils, although this has not been quantified. Creosotebush frequency increases as gravel content increases (e.g. > 5%; David Trujillo, NRCS, personal communication). Mesquite exhibits a more inverted-conical growth form on loamy soils relative to sandier soils, perhaps due to the lack of sand accumulation around shrub bases. Grass cover is often (but not always) substantially lower than in the grassland state and is patchy. It is possible that accelerating practices (shrub control) are needed to maintain a shrub-invaded state. While in the shrub-invaded state, tobosa and bush muhly have been reestablished as dominants in response to herbicide application on creosotebush and tarbush on Dona Ana sandy loam soils in the Jornada Experimental Range (Herbel et al. 1983). Diagnosis: Shrubs, including mesquite, tarbush, and/or creosotebush are present. The cover of perennial grasses, including tobosa, burrograss, and threeawns, ranges from interconnected to patchy but large (> 2 m) bare patches are usually common. Black grama may be present but is rare (so far as we have observed). Transition to shrub-dominated state (4): Continued grazing or drought may lead to the loss of remaining grasses, sheet erosion or gullying, and soil truncation and soil drying. This prevents further establishment by grasses or shrubs in most areas. Soil surface physical crusting may also be important. Some shrubs (e.g. creosotebush) may expand via clonal growth. Furthermore, competition by shrubs for water at grass rooting depths (see Gibbens and Lenz 2001) may lower grass survival and reproduction. Shrub encroachment alone may produce these changes without grazing such that this transition occurs if shrub removal is not applied (see Sandy model). Key indicators of approach to transition: Increases in shrub cover and/or density, loss of grass cover, biotic soil crusts, increased bare ground and bare ground patch sizes, evidence of increasing erosion including litter movement, rills, gullies, and pedestalling of grasses and shrubs. Presence of physical soil crusts. Transition to shrub-dominated state (2b). Shrub removal with increases in grass cover and fuel loads for fire may be required to reestablish the competitive dominance of grass or to facilitate periodic disturbances to shrubs. It is more likely, however, that the conditions promoting shrub invasion will remain in place such that fire and shrub control must be continually applied, perhaps at frequencies and under conditions not conducive to grass recovery. In any case, the utility of fire to maintain grassland cover in SD-2 is unknown and is currently being investigated (Drewa and Havstad 2001).

State 4
Shrub-Dominated

Community 4.1
Shrub-Dominated

Shrub-dominated grassland: This state is characterized by soil surface degradation and truncation (Gile and Grossman 1997) and little grass cover. Shrubs may be very dense. In some cases, only small patches of tobosa, burrograss and/or threeawns may exist, and fluffgrass may be an important component of cover. Buffington and Herbel (1965) have documented a great deal of turnover in the composition of dominant shrubs within the Dona Ana-Reagan map unit of the Jornada Experimental Range. Generally, the trend has been a shift from tarbush to a mix of creosotebush and mesquite. They speculate that this is due to the relatively greater motility of tarbush seeds compared to mesquite or creosotebush. Grass cover may vary depending upon the species of invading shrubs. Grass cover is reputed to be higher where tarbush has invaded in comparison to other shrubs, but this may simply represent the sequence of shrub invasion paralleling the effects of erosional soil degradation and grass loss, rather than any direct effect. In tarbush shrublands that currently receive little grazing pressure, microbiotic crusts may be very common. This may represent the first steps towards long-term recovery. Diagnosis: Density of shrubs, including mesquite, tarbush, and/or creosotebush ranges is high. The cover of perennial grasses including tobosa, burrograss, fluffgrass, and threeawns is low and restricted to small patches or scattered plants. Bare ground is dominant and interconnected or continuous. Soil truncation is apparent and pedestalling and other signs of erosion are ubiquitous. Either biotic or physical crusts, usually both, are common. Transition to burrograss-tobosa-threeawn grassland state (5). Shrub removal in combination with soil amendments, or pitting and seeding, may catalyze progress towards recovery of a grass component. If grass establishment fails, and increased production of shrubs and annuals may be observed. Such approaches have not met with success on loamy soils in SD-2, perhaps due to rainfall limitations after seeding. Where erosion has exposed saline or carbonate-rich soils, chemical treatments (or flushing) to reduce these compounds would also be required. Data and information sources and theoretical background: Some information is available from a number of Jornada Experimental Range long-term monitoring plots (known as the permanent quadrats) that occur on the Dona-Ana-Reagan map unit (Dona Ana County soil survey). These provide data on the dominants occurring in 1m2 plots from 1915/16 or 1926/27 to 2001. Unfortunately, there are clear discrepancies in the boundaries and composition of the soil map unit in the Dona Ana County soil survey and the vegetation maps documented in 1998 by Robert P. Gibbens (unpublished). The Desert Project soil survey overlaps part of the area mapped by Gibbens and this association is relied upon heavily in this presentation. The extension of the Desert Project mapping effort to the north will be invaluable. Information on dynamics is provided by Buffington and Herbel (1965). Other observations by Jim Powell, NRCS retired, and Brandon Bestelmeyer, USDA-ARS Jornada Experimental Range are reported. As described in detail in the Clayey site Overview: Data and information sources and theoretical background, the soil truncation hypothesis holds that erosion exposes a hardened argillic B horizon, or calcium carbonate-rich horizon, that resists infiltration and may inhibit establishment. Hypotheses regarding the limitations placed upon black grama and the invasion of mesquite are discussed in the Sandy overview section.