State 1
Historic Climax Plant Community

Community 1.1
Historic Climax Plant Community

Figure 4. MLRA 42; SD-2; Gravelly

State Containing Historic Climax Plant Community Mixed-shrub grassland: The historic plant community is believed to have been dominated by grasses, especially bush muhly and black grama, and sometimes dropseeds (Sporobolus spp.). Shrubs, especially creosotebush, are codominants(black grama/shrubs community). Production is generally low (up to 450 lbs/acre) compared to other ecological sites. The biomass of bush muhly and black grama may be equal to that of creosotebush. Few such communities occur in gravelly ecological sites today. Grazing-induced retrogression from this community is characterized by a reduction in the cover of black grama, and may result in an increase in the proportional representation of bush muhly (bush muhly/shrub community). This is paralleled by an increase in bare ground and the cover of fluffgrass (Dasyochloa pulchella). In other cases, bush muhly may either decline alongside black grama or have been a minor component, and threeawns (Aristida spp) may increase (threeawn/shrub community). It is possible that shifts in the dominance of black grama and bush muhly occur in response to climatic variation as well, but this is not known. Additional communities may be observed that differ from the historic climax plant community described in the 1979 range site description due to landscape position or variations in soil texture. Where gravelly sites (as currently defined) occur in the upper portions of fan collars at the bases of desert mountains (i.e. Mt. Summerford, College Ranch, Doña Ana Co.; Wondzell et al. 1996), run-on water and low erosion rates appear to create conditions that are favorable to black grama grassland maintenance and few shrubs occur (black grama community). Further away from the mountain front on the lower fan collar, erosion is greater and the black grama/shrubs community is supported. In areas of gravelly hills or “breaks” along the sides of the Rio Grande Valley, arroyos draining into the valley separate ridges of Gravelly soils (known as ballenas). Soil properties and vegetation vary with position across the ridge and with changing aspect. At one site in Sierra County (Gene Adkins NRCS, Brandon Bestelmeyer, USDA-ARS, and George Chavez, NRCS, personal observations), some ridge tops had less clay and more calcium carbonate than on side slopes. Ridge tops were dominated by creosotebush with a sparse cover of fluffgrass and no other grasses. North-facing slopes supported a mixture of black grama and sideoats grama (Bouteloua curtipendula) as dominants. South-facing slopes often supported large patches of tobosa (Pleuraphis mutica). At a similar site in Socorro County, soil properties did not vary with aspect but vegetation did (see photos). Ridge tops were dominated by creosotebush whereas north-facing slopes were dominated by black grama; south-facing slopes were intermediate. Furthermore, black grama appears to be far less common on gravelly slopes south of Rincon (even on the same soil map unit—Nickel gravelly sandy loam). Thus, the composition of historic plant communities and their resilience to grazing perturbation is highly variable at both small (100 m) and large (100 km) scales, even within restricted areas of SD-2. It may prove useful to split out the gravelly breaks areas from the more level areas as a distinct ecological site. Diagnosis: Cover of black grama and/or bush muhly and other grasses more or less continuous and occurs in shrub interspaces. Shrub density variable, but typical intershrub distance should be several meters to 10s of meters. Depending upon slope and landscape position, rills, gullies, and arroyos may be common. Additional States and Transition Pathways: Transition to shrub-dominated state (1a): Overgrazing is believed to initiate this transition. Gardner (1951) noted that bush muhly was found outside of shrub canopies only in ungrazed sites, indicating that the loss of grasses in interspaces may be caused by grazing. Subsequent competition or loss of intershrub soil fertility, perhaps exacerbated by the allelopathic effects of expanding creosotebush clones, may preclude reestablishment by grasses. Prolonged domination by shrubs may eventually lead to a transition to a shrubland state within which shrub control measures do not result in increased grass cover (transition 3, see below). It is possible that a high cover of stones or gravel may retard erosional soil losses and prolong the window in which grasses may be recovered. Alternatively, climatic changes and/or reduced fire disturbance may drive this transition on certain landscape positions (i.e. where run-on water is not a factor). Generally, the presence or absence of run-on water will cause large variation in the sensitivity of a gravelly site to grazing. A systematic documentation of these relationships would be an important contribution to our understanding of this site. Key indicators of approach to transition: Decreases in grass and litter cover, increases in bare patch size, increases in the frequency and size of rills, gullies, and litter movement Transition to shrubland state (2a): Severe overgrazing causing grass loss with subsequent erosion, gullying, and soil truncation may cause a transition to a shrubland state from which grass does not recover for decades. Severe overgrazing in drought conditions, perhaps followed by heavy summer rains and excessive erosion, may lead a system to bypass the Shrub-dominated state altogether and extinguish most large perennial grasses. In this case, soil loss is often apparent, especially notable in the pedestalling of shrubs. Key indicators of approach to transition: Decreases, sometimes rapid, in grass and litter cover, increases in bare patch size, increases in the frequency and size of rills, gullies, and litter movement, accumulation of gravel and pebbles at the surface, pedestalling.

State 2
Shrub-Dominated

Community 2.1
Shrub-Dominated

Additional States and Transition Pathways: Shrub-dominated state: This state is characterized by a predominance of shrubs (mostly creosotebush and tarbush) with large perennial grasses existing as a subordinate or minor component. Often, shrubs exist as discrete patches with little grass intermixed with areas in which grass is more common and shrub densities are lower. A sparse cover of fluffgrass may occupy the mostly open areas. Localized soil truncation or loss of soil fertility may occur, especially along the sides or arroyos and gullies. In some cases, the erosional-depositional banded vegetation process (e.g. Montana et al. 1990, see Clayey model) occurs on slight (< 1%) slopes where gullying is not apparent. Typically, bush muhly is the dominant grass (Shrubs/bush muhly community), although in some “gravelly hills” situations, black grama persists at low densities (Shrubs/black grama community). This latter situation is often observed on south-facing slopes that are presumably more droughty. On sites with heavier subsurface soils, tobosa may constitute the grass component (Shrubs/tobosa community). Bush muhly is often associated with the bases of shrubs, and may be almost entirely restricted to shrub bases. Nonetheless, this grass constitutes a high percentage of ground cover. Bush muhly establishment appears to be favored under creosotebush, likely due to the interception of wind-born inflorescences and the concentration of nutrients under shrubs (Whitford et al. 1997). Under these conditions, bush muhly may compete with creosotebush and cause creosotebush decadence once the bush muhly volume occupies more than 50% of the aboveground shrub volume (Welsh and Beck 1976). This interaction, in conjunction with the use of effective herbicides such as Tebuthiron, can increase the abundance of bush muhly within this state (Bush muhly/shrubs community). Threeawns may also increase following this treatment. If localized losses of soil fertility or climatic shifts are associated with the transition, however, the conditions promoting shrub establishment at the expense of grasses may persist. Thus, this community would still occupy the shrub-dominated state because intermittent removal of shrubs would be required to maintain grasses within the system. Diagnosis: Cover of black grama and/or bush muhly patchy. Bare expanses of several to 10s of meters are typical. Bush muhly and other grasses may be restricted to the bases of shrubs. Shrub density is moderate, typical intershrub distances may be 2–3 m. Rills, gullies, and arroyos may be common. Evidence of sheet flow in large bare areas present. Pedestalling is apparent. Transition to shrubland state (3): See transition 2a above. Persistent lack of grasses may lead to erosion and soil truncation, and grasses may take decades or more to recover. Transition to mixed-shrub grassland state (1b): Restoration of self-maintaining grass cover may be accomplished through repeated shrub control events. Where seed limitation is a factor, seeding and furrowing may be used to restore grasses, but Gibbens et al. (1993) found this to be unsuccessful on Tencee soils. The use of gully seeders to release seeds when rains flush washes to seed target areas downslope may have promise (Barrow and Havstad, ms). Contour terraces, on the other hand, have not been successful, although they were not maintained (Rango et al. in press). Protection of sites from native herbivores such as jackrabbits may facilitate natural reestablishment of grasses (Havstad et al. 1998).

State 3
Shrubland

Community 3.1
Shrubland

Creosotebush state: In this state, perennial grasses of large stature, including black grama and bush muhly, are largely or entirely absent, with a few individual bush muhly persisting under some shrubs. Typically, creosotebush is the overwhelming dominant. Soil truncation is apparent at this stage and the petrocalcic or calcic horizon may be exposed at the surface. Sheetflow erosion with loss of finer particles may concentrate gravel at the surface to produce a barren desert pavement in shrub interspaces. In some cases, creosotebush is the sole perennial plant. On gravelly soils, Buffington and Herbel (1965) documented the eventual loss of tarbush from the shrub mix to dominance by either pure creosotebush or creosotebush with some mesquite. It is unclear in this study what changes to grass cover accompanied these changes in shrub dominance. Grass reestablishment within this state is virtually impossible. Note that it can be difficult to ascertain when sufficient soil erosion has occurred to preclude rapid grass reestablishment. Sites “written off” prematurely may lead to continued erosion and the option of recovering grasses may be lost. In other cases, shrubs and subshrubs such as mariola and/or zinnia (Zinnia acerosa) and Dyssodia acerosa may be subdominants (Creosotebush/mariola) and may fluctuate in abundance due to climate. Fluffgrass cover may be significant (Creosotebush/fluffgrass community), and where creosotebush has been controlled using herbicides, or where creosotebush cover is limited by shallow soils due to truncation, fluffgrass may be dominant (Fluffgrass/creosotebush community). Whitethorn acacia (Acacia constricta) has invaded and/or expanded within creosotebush shrublands in Las Cruces area over the last 40 years, and might constitute a distinct state. McAuliffe (1994) and Hamerlynck et al. (2000) have suggested that the limited deep soil water recharge on soils with shallow argillic horizons may limit creosotebush growth. Petrocalcic horizons may similarly retard soil water penetration to deep roots (Gile et al. draft ms). Where roots penetrate the petrocalcic, however, water may be funneled to roots (Gile et al. 1998). If soil truncation prohibits grass establishment above the petrocalcic, but creosotebush can exploit water through pipes and cracks over a large area (Gibbens and Lenz 2001), this may explain the success of creosotebush in comparison to grasses on truncated soils. This mechanism may also explain the contrasting roles of restrictive layers between studies in the Sonoran and Chihuahuan deserts if pipes are not present in the argillic layer of McAuliffe’s (1994) study. Diagnosis: Black grama and/or bush muhly typically absent, although bush muhly may occur the bases of a few shrubs. Shrub density may be high with shrub crowns touching. Rills, gullies, and arroyos may be common. Evidence of sheet flow in large bare areas is present. Pedestalling is common, and soil deflation often produces a desert pavement of packed gravel and small stones. Transition to mixed-shrub grassland state (2b): Destruction of gullies and the use of water spreaders may be beneficial. Pitting or other erosion stabilization techniques would probably be needed for the accumulation of organic matter. Seeding would be required. Where physical soil crust/pavement has developed, soil disturbance may promote infiltration. If shallow petrocalcic horizons are exposed, grass recovery would not be possible until soil is added or the horizon was destroyed. Data and information sources and theoretical background: Communities and states are derived largely from information obtained using broad-scale associations recorded by Buffington and Herbel (1965) and Gardner (1951) and by field observations of Brandon Bestelmeyer, USDA-ARS Jornada Experimental Range, Gene Adkins, NRCS Truth or Consequences, Jim Powell, NRCS, retired. Studies by Buffington and Herbel (1965) and Whitford et al. (2001) directly address transitions on gravelly soils in SD-2, and Herbel et al. (1973) and Jerry Barrow and Kris Havstad (unpublished ms) discuss restoration strategies. Three hypotheses for transitions between mixed shrub grassland and shrub-dominated and shrubland states can be identified. Patterns observed by McAuliffe (1994), Gibbens and Lenz (in review) and discussed by Gile et al. (draft ms) support the soil truncation hypothesis. This holds that erosion due to disturbance-induced loss of plant cover, or due to natural, long-term processes, removes soil surface horizons, bringing the calcic or petrocalcic horizon (a characteristic of gravelly site soils) closer to the surface. Because carbonate is relatively impermeable, this may cause runoff to increase and infiltration to decrease. This, in turn, inhibits the establishment of grass, as well as shrubs, and may stress existing shrubs. Despite this stress, shrubs may come to dominate under these conditions by exploiting deeper soil layers through gaps in petrocalcic layers and reproducing via clonal growth. Water may also be funneled and concentrated through gaps (similar to the effect of krotovinas created by burrowing animals; Gile et al. 1997). Increases in creosotebush via clonal growth may require long periods of time without disturbance (McAuliffe 1994). The scenario outlined by Whitford et al. (2001) can be referred to as the allelopathy hypothesis. This explanation proposes that grazing and/or drought creates gaps in the cover of black grama that permit increasing dominance by creosotebush. As creosotebush develops free from competition with grass, it increasingly releases allelopathic chemicals from litter fall that is detrimental to the soil fauna and flora. This, in turn, increases decreases infiltration and nutrient availability, increases erosion, and inhibits grass germination. Alternatively, creosotebush may be a more effective competitor for surface soil water than grasses (the competition hypothesis). Thus, the allelopathy and competition hypotheses can be complementary to the soil truncation hypothesis. Allelopathic/competitive effects of creosotebush may contribute to soil truncation. In both cases, the nutrient concentration hypothesis (Schlesinger et al. 1990; see Sandy model) explains the persistence of shrubs under these conditions. If the mechanism of Whitford et al. operates, then the expansion of creosotebush is the key process responsible for the transition from mixed shrub grassland to a shrub-dominated state. Allelopathic effects may lead to the eventual replacement grasses over time without shrub control. Remediation under this scenario may be difficult, especially if the allelochemicals have persistent effects. On the other hand, if the allelopathy hypothesis is false, then there may be stable coexistence of creosotebush and grasses and grazing management may prevent further degradation even after shrubs have begun to encroach into previously shrub-free settings. In some areas at least, creosotebush has coexisted with grasses for long periods, so some unrecognized factors may limit creosotebush establishment and dominance (perhaps soil instability; McAuliffe 1994). Alternatively, the allelopathy mechanism may not operate in many, or any, situations. The climate change hypothesis may also explain the expansion shrubs into areas of grassland (Neilson 1986; see the Sandy model) and the decline of grasses, especially black grama. The persistence of black grama on certain landscape positions (see below), however, indicates that climate alone is not responsible for the loss of black grama. Reynolds et al. (1999) found that creosotebush is very flexible in the seasonal use of moisture and can adapt its periods of physiological activity to match periods of soil moisture availability. Thus, areas dominated by black grama receiving run-on water may be buffered from the effects of climate change that are important in other landscape positions (e.g. plains). In run-in positions, black grama can continue to successfully impede creosotebush establishment, whereas in other settings, creosotebush has experienced competitive release under the current climate and can capitalize on its physiological flexibility. A reduction in fire frequency may also be associated with grass reduction and shrub expansion, although there is little evidence in support of this mechanism in the gravelly setting.