State 1
Meadow Grassland

Community 1.1
Meadow Grassland

Meadow Grassland Plant Community: The historic plant community is dominated by giant sacaton and alkali sacaton and harbors several other grass species including vine mesquite (Panicum obtusum) and tobosa (Pleuraphis mutica). Sedges and rushes are also common. Inland saltgrass and alkali muhly (Muhlenbergia asperifolia) often occur in flooded patches, perhaps were salts accumulate due to poor drainage or due to disturbance. Little is known about the environmental variables along which grass species segregate. Reductions in the sacaton species, particularly giant sacaton, may occur with grazing pressure or perhaps due to reduction in the water table. These communities used to occur alongside meadow communities on low salinity soils in the Rio Grande valley, but the non-saline communities have been largely eliminated by agriculture (Fosberg 1940). Diagnosis: Giant sacaton and alkali sacaton are co-dominant. Sedges and rushes present. Additional States: Transition to alkali sacaton-saltgrass state (1a): Increasing salinity due to changes in flooding regime (i.e. decreased frequency or volume) and/or to changes in the permeability of the soil due to soil-surface exposure and compaction, initiated by grazing or disturbance, may lead to the loss of giant sacaton. Without repeated flooding and drainage of salts into the ground water, sites with shallow water tables accumulate salts due to capillary rise (Hendrickx et al. 1999). Key indicators of approach to transition: Increases in bare ground cover, salinity, reduced frequency and volume of flooding. Transition to cottonwood state (2a): This transition used to occur independently of human influence and is considered a natural component of dynamics. Overbank flooding with sediment agradation may kill grasses and provide a substrate for germination and growth of cottonwood, as well as exotics. Transition to mixed exotics state (3a): May be due to disturbance, via mechanical means or by overgrazing, where propagules of exotics are available. Transition to saltcedar state (4a): Reduced flooding frequency with associated increases in salinity, or perhaps due to reduced water tables. This probably also requires disturbance to vegetation and the presence of propagules of exotics. The lowering of the water table and increasing salinity may reduce the resistance of the meadow communities to saltcedar invasion (Tesky 1992, Di Tomaso 1998). Disturbances (e.g. grazing) may facilitate saltcedar establishment by reducing competition (Di Tomaso 1998, Stromberg 1998). Periodic flooding disturbances, however, may inhibit the establishment of saltcedar. Key indicators of approach to transition: Increases in bare ground cover, increased salinity, reduced frequency and volume of flooding, lowering of water table.

State 2
Saltgrass Grassland

Community 2.1
Saltgrass Grassland

Saltgrass grassland: This state is characterized by a dense mat of saltgrass that may be interspersed with patches of alkali muhly. The dense root mass of saltgrass precludes establishment by other species and diversity is generally low in this state. Saltgrass is believed to tolerate a higher level of salinity than the sacaton species and is able to expand in compacted, clay soils by virtue of its rhizome morphology (Hansen et al. 1976). Saltgrass is also very flood tolerant (Uchytil 1990). Patches in this state are often associated with saltcedar-dominated patches. Diagnosis: Saltgrass is extremely dominant, almost completely covering the ground surface. Exposed soil often reveals encrusted salt. Transition to mixed exotics or saltcedar state (10a): See 4a above. Disturbance to saltgrass may be especially important to allow invasion, considering the dominance exerted by saltgrass. Campbell and Dick Peddie (1964) note that saltcedar does not grow well in saltgrass-dominated communities when the water table is within 3 feet (1 m). Observations in the Rio Grande valley by Darrell Reasner (NRCS Socorro), however, dispute this claim. He feels that tamarisk establishment is not limited in saltgrass. Transition to meadow grassland state (6): Unknown. May require the disruption of saltgrass root systems, but this would make the community susceptible to invasion by exotics (transition 10a or 11a). The return of flooding events and/or irrigation (see Hendrickx et al. 1999) to remove salt concentrations would probably be needed if salt limits sacaton establishment. Seeding may also be necessary.

State 3
Saltcedar

Community 3.1
Saltcedar

Saltcedar: Saltcedar may come to dominate in systems where water tables are lowered and where disturbance has occurred. Dense stands of saltcedar occur in areas where the water table is from 5-20 feet (1.5-6 m; Tesky 1992). Saltgrass may persist in depressions and areas between saltcedar patches, but this grass tends to be absent within saltcedar patches. Saltcedar has been described as an opportunistic species that is not a strong competitor (Everitt 1998). Nonetheless, two feedback mechanisms may promote extreme dominance by this species. First, salts drawn from below the soil surface are excreted through glands in stems and leaves and concentrate on the soil surface, producing a saline crust that inhibits germination and survival of other species. Second, saltcedar is tolerant of fire. Litter accumulation in dense stands leads to fire frequencies of 16-20 years in some systems (Tesky 1992). In principle, this might help to exclude less fire tolerant species but the increased salinity and competitive interactions with established, dense saltcedar stands probably play a primary role in maintaining saltcedar dominance. Within the saltcedar state, natural (flooding) and management-induced disturbances (mowing) may promote a grassland aspect (Taylor and McDaniel 1998). Fire disturbances may also temporarily alter species composition of tamarisk communities. Diagnosis: Saltcedar is dominant, sometimes forming impenetrable stands. Grass cover (particularly saltgrass and alkali muhly) may occur in areas where saltcedar densities are lower. Transition to meadow grassland state (4b): Unknown. A combination of herbicide use, burning, and mechanical control can be used to clear saltcedar (Taylor and McDaniel 1998). The use of subsurface herbicide techniques may be useful (Hollingsworth et al. 1979). The reestablishment of flooding regimes and high water tables would probably be needed to restore and sustain a meadow grassland. Irrigation using water flow diversions may achieve this (Taylor and McDaniel 1998). Transition to saltgrass state (10b): Unknown. In principle, this transition can be brought about by the removal of saltcedar but without restoring the conditions required for the establishment of meadow grasslands (e.g. changed hydrology). Transition to cottonwood state (11b): Techniques applied by Taylor and McDaniel (1998) have been successful in establishing this state in the Bosque del Apache National Wildlife Refuge. This includes the removal of saltcedar via herbicides, fire, and mechanical control alongside irrigation and plantings of cottonwoods and black willow (Salix nigra).

State 4
Cottonwood

Community 4.1
Cottonwood

Cottonwood: This state is characterized by the development of a cottonwood canopy alongside other woody species such as black willow, coyote willow (Salix exigua), and tornillo and an understory of herbs. Grasses are usually a minor component. Transition to meadow grassland state (2b): Removal of cottonwood canopy, or perhaps aging and mortality of cottonwoods over longer periods. Transition to saltcedar state (11a): Removal of cottonwoods and subsequent colonization by saltcedar.

State 5
Mixed Exotics

Community 5.1
Mixed Exotics

Mixed exotics: This state may exhibit dominance by Russian olive and/or other invasives, such as Russian knapweed, in disturbed settings. Little is known about the conditions under which different species invade. Because exotic species may coexist with native meadow grassland species, we speculate that the changed hydrology and salinization that accompanies the transition to a saltcedar state does not necessarily occur in this state. Where soil drying is a factor, upland species such as saltbush (Atriplex canescens) may colonize. Transition to meadow grassland state (3b): Variable recovery of grasses may be achieved by mowing and/or the use of herbicides to remove exotic shrubs if hydrology is not the limiting factor (Muldavin et al. 1999). Transition to saltcedar state (12): Disturbance and subsequent colonization by saltcedar. Data and information sources and theoretical background: Communities and states are derived largely from observations of Darrel Reasner, NRCS Socorro. Despite a proliferating literature on the ecology of cottonwoods and saltcedar, there is little work on the ecology of grasses in the riparian context. The mechanisms underlying the transitions rely largely on the idea that giant and alkali sacaton require conditions of relatively high soil moisture and low soil salinity relative to species defining the other states. The reduced frequency and volume of flooding may lead to accumulations of salt where the water table is high, leading to a turnover to more salt tolerant taxa. Grazing may also contribute to this turnover. Continued dominance of saltgrass, once it has established, may be enforced by its competitive preemption of space. For the invasion of opportunistic exotic taxa, disturbance is assumed to play an important role with or without changes in salinity by reducing competitive interactions with native species. On the other hand, a reduction in flooding disturbances has been invoked to explain the increase in saltcedar (Tesky 1992). It is possible that disturbance type is important. Alternatively, the role of flooding disturbance in inhibiting saltcedar establishment may apply to river banks and not to the more distant salt meadow sites. Similarly, the role of water table depth is unclear. Salinity seems to be promoted by shallow water tables but tamarisk is observed to dominate where water tables are relatively deep. Everitt (1998) notes that channel narrowing and subsequent sediment deposition over the last century due to depleted flow has led to a rise in dry-season water tables beneath the Rio Grande flood plain, and thus and increase in wetland conditions. The roles of salinity, water table depth, and flooding frequency on the vegetation dynamics of this ecological site have yet to be explored in detail.

State 6
Alkali Sacaton-Saltgrass

Community 6.1
Alkali Sacaton-Saltgrass

Alkali sacaton-saltgrass: This site differs from the meadow grassland largely by the absence of giant sacaton, a large grass that is an important structural element of the historic community. It is unknown which other elements may differ within this state. Soil salinity is presumably higher within the state. Saltgrass assumes greater importance. Diagnosis: Giant sacaton is absent over extensive areas, and alkali sacaton is dominant. Saltgrass may dominate many patches. Transition to saltgrass state (5): The transition is caused by increasing salinity with soil compaction, and may also require mechanical disturbance or heavy grazing. If sacaton species are eliminated and the relatively unpalatable saltgrass expands in area, the dense roots of saltgrass effectively preempt space and inhibit germination by other species. Saltgrass reproduction by seed is poor under highly saline conditions, so vegetative reproduction through rhizomes is usually responsible for expansion (Hansen et al. 1976, Cluff and Roundy 1988). Key indicators of approach to transition: Increases in bare ground cover and loss of sacaton plants, increases in salinity, increasing soil compaction. Transition to saltcedar state (7): See 3a above. Transition to mixed exotics state (9): See 4a above.