Sandy, Desert Shrub

Description

The historic plant community is dominated by black grama (Bouteloua eriopoda) and dropseeds (Sporobolus flexuosus, S. cryptandrus, and S. contractus) may be secondary dominants. Bush muhly (Muhlenbergia porteri) and threeawns (Aristida spp.) are other common grasses. Soaptree yucca (Yucca elata), longleaf ephedra (Ephedra trifurca), sand sage (Artemisia filifolia), and snakeweed (Gutierrezia sarothrae) are common shrubs. Through its high basal cover, high litter cover, and consequent low rates of erosion and high infiltration rates, black grama can regenerate through tillering and possibly by seed in some years.

Overutilization of black grama and increasing rates of soil erosion and deposition when bare ground cover is high may result in an increasing relative abundance of dropseeds, threeawns, or snakeweed. It is also possible that in coarser soils, such as loamy sands, dropseeds dominate naturally. Two seasons without summer rains will also lead to black grama decline, but grasses such as dropseeds and threeawns are thought to be more sensitive to drought than black grama (Herbel et al. 1972). Snakeweed or dropseeds may become dominant within this state but black grama can gradually recover as long as some plants persist. Gibbens and Beck (1987) and Bestelmeyer et al. (2013) provide evidence for recovery of black grama within communities dominated by dropseeds or other plants. Campbell and Bombarger (1934) indicate that black grama can recover within snakeweed-dominated grassland.

Characteristics and indicators

Black grama is highly dominant and cover is largely continuous. There is evidence of black grama reproduction by stolon and possibly by seed. There are few large bare gaps (>1 m). Litter cover is abundant. Soil stability values range from 4-6 and poorly-developed biological crusts (Pedoderm class PDB) may be common in wet periods. Mesquite (Prosopis glandulosa) is sparse or absent (< 5% foliar cover). Black grama foliar cover levels approaching 5%, especially with loss of plants in large patches, should be cause for concern.

Resilience management

Black grama is the key plant of this site class due to its dominance in recent history, its high forage value, and its consequent sensitivity to grazing. Shifts away from black grama dominance are due to overgrazing and/or multi-year periods of summer or spring drought, or due to an accelerated rate of honey mesquite (Prosopis glandulosa) seed flow into black grama grassland and the establishment and growth of these shrubs. Continuous heavy grazing leads to declines in the proportional representation of black grama declines because it is preferred by cattle over other species, particularly outside the growing season (Paulsen and Ares 1962). Once black grama is locally extirpated, it tends not to recover because reproduction is primarily vegetative via stolons (Canfield 1939, Neilson 1986). Competition for water with neighboring shrubs, particularly in drought, can also limit grass growth, survival, and reestablishment (Snyder and Tartowski 2006).

Consequently, resilience of the black grama grassland state is focused on 1) grazing management to avoid substantial reductions of black grama and 2) control of mesquite shrubs when mesquite recruitment is occurring. Stocking rates and grazing periods that leave stems (stolons) long enough for new plants (ramets) to take root and adequate cover to protect soils through the spring windy season is critical. This management apparently allowed black grama populations to survive extreme drought episodes in the 1930s and 1950s that led to B. eriopoda losses elsewhere (Bestelmeyer et al., 2013). Timely control of mesquite using aerial or spot treatments of a foliar herbicide blend (see http://aces.nmsu.edu/pubs/_b/B822/welcome.html). Herbicide treatment should be considered when there is evidence that mesquite recruitment is occurring. Individual, large mesquite plants, however, may be desirable wildlife habitat elements.

Submodel

Description

Black grama continues to be the dominant perennial grass, but mesquite cover is sufficiently high to compete for space and resources with grasses. Mesquite plants may be very small and difficult to detect. Although fire may kill small (< 1.5 yr old; Wright et al. 1976) mesquite, it is unlikely that fire frequencies have been sufficient to remove mesquite from a grassland once a source of mesquite propagules has been connected to a grassland. Valentine (1936) indicates that beyond a height of 1-2 feet, mesquite begins to exclude grasses from around plant bases.

Characteristics and indicators

Black grama is the dominant perennial grass and mesquite are present and usually conspicuous, exceeding 5% foliar cover when leafed out. Black grama cover may be substantial but areas around shrubs devoid of grass. Typically, large patches devoid of black grama are common. Black grama foliar cover levels approaching 5% should be cause for concern.

Resilience management

Management of black grama is similar to State 1, but there should be additional concern for the effects of competition with shrubs, especially mesquite (P. glandulosa). Bestelmeyer et al. (2013) showed that, under heavy grazing pressure, the presence of shrubs (estimated at 7% foliar cover) increased the loss of black grama. Thus, managers might consider more conservative stocking rates in the presence of shrubs than indicated by grass production alone.

Pierce et al. (2018) found that removal of shrubs, on a gradient from 2-18% foliar cover of mesquite, resulted in a doubling of black grama production and reproduction. Brush control using herbicides (e.g. 2,4,5-T) resulting in at least a 30% mesquite kill can result in increases in grasses (Herbel et al. 1983). Newer herbicides, such as remedy/reclaim or sendero with electrostatic application may be most effective at killing shrubs. The use of fire may also be possible in more productive environments with continuous grass cover and wetter climates (average >10" per year). Wright et al. (1976) observed that honey mesquite less than 2-3 years old were killed by fire, apparently because the bud zone meristem was still exposed. Older mesquite tolerate fire or other disturbances by resprouting from the bud zone if above ground parts are destroyed or damaged. Fire, however, may damage or kill black grama plants and reduce reproduction in more arid parts of Chihuahuan Desert (9” annual precipitation), especially if fires are followed by drought (Drewa et al. 2006).

Submodel

Description

Bunchgrasses, including dropseeds (Sporobolus airoides, S. cryptandrus, and S. contractus) and threeawns (Aristida purpurea, A. ternipes) are the dominant perennials grasses and black grama is rare (<5%).

Characteristics and indicators

Black grama foliar cover is < 5% and lower than that of bunchgrasses, but total plant canopy cover may range from 60-75% where perennial grasses remain healthy. Black grama exists as isolated patches, often featuring evidence of soil aggradation within these patches and soil erosion surrounding the patches where bunchgrasses occur (soil redistribution class is 3a). The % of a line transect in gaps >1 m may range from 37-56%. Poorly developed biological crusts (pedoderm PDB) are rare, but average surface soil stability values range from 3-5, subsurface (2.5 cm) values are 1-2. Mesquite cover is variable, but usually > 5% and must be < 15%.

Submodel

Description

Mesquite are dominant and intershrub areas are typically eroded with a sparse vegetation cover of annual plants, perennial forbs, and subshrubs. On soils that are moderately deep or deep to restrictive soil horizons, soil accumulates on mesquite to form coppice dunes (or nabkas; Langford 2000) over time, which attain heights over 2 m on deeper soils. Saltbush (Atriplex canescens) may be common in mesquite coppice dunes. Perennial grasses, restricted to bunchgrasses, may establish and attain high cover in shrub interspaces during sequences of wet years (Peters et al., 2014). In other periods, grasses cannot establish due to soil water limitations, the instability of the substrate, and rodent and lagomorph herbivory on grass seedlings. In some cases, soil truncation exposes clay- and carbonate-rich subsoil that is very hard and does not permit plant establishment.

Characteristics and indicators

Mesquite is dominant and may form coppice dunes from 0.5-3 m high. Bunchgrasses are usually absent or restricted to coppice dunes. In extremely wet years, bush muhly, threeawns, and dropseeds may colonize interdunes . There is usually abundant evidence of wind erosion and deposition including extreme pedestalling, plant burial, highly sorted sand, ripples, and an exposed B horizon in interdunes.

Resilience management

Grazing rest during wet seasons to promote a transition to a Grassy shrubland state.

Description

Lehmann lovegrass monocultures occur, often as large patches occurring within areas otherwise dominated by bunchgrass/mesquite states.

Characteristics and indicators

Lehmann lovegrass is the dominant perennial grass.

Resilience management

Fire may promote Lehmann lovegrass dominance, but fire is rare in typic aridic, thermic soil climates. Little is known about management practices to promote or limit its expansion.

Description

Formerly a community phase within the shrubland state, recent evidence suggests the grassy shrubland is a persistent state. Abundant cover of bunchgrasses, including bush muhly, dropseeds, and threeawns in the interspaces of mesquite shrubs or coppice dunes. Grass patches may accumulate soil and litter, creating environments that favor grass establishment and persistence. Black grama is not known to colonize shrubland states.

Characteristics and indicators

Mesquite is dominant, and bunchgrass cover may be patchy or co-dominant with mesquite (up to 30% foliare cover) nearly continuous in mesquite interspaces. Soil accumulation may be observed.

Resilience management

Care should be taken to manage grazing use to promote bunchgrass survival and recruitment during dry years. Severe drought may lead to a transition back to the shrubland state irrespective of grazing management.