State 1
Historic Climax Plant Community

Community 1.1
Historic Climax Plant Community

Figure 4. SD-2; Bottomland

Bottomland grasslands: The historic plant community is dominated by giant sacaton and alkali sacaton either alone or in mixture, and harbors several other grass species including vine mesquite, tobosa, burrograss, and sideoats grama. It is not known what conditions or circumstances lead to the differing abundances of the sacaton species among bottomlands, or if shifts in their relative abundances occur over time. Alkali sacaton may dominate on more saline soils. Each of these species has relatively high palatability when compared to tobosa and burrograss during the growing season. Thus, reduction of populations of bottomland grass species due to overgrazing and erosion is a risk. The giant sacaton grasslands have been reduced to 5% of their original extent (Cox 1988), thus there is great interest in preserving the remaining stands. Grazing giant sacaton during dry summers or fall may expose crowns to freezing temperatures and cause grass mortality (Cox 1988). Burning of bottomland grasslands may do more harm than good. Giant sacaton, for example, is relatively slow to recover from fire, taking from 2-3 years. Suitable burning strategies for this site are unknown, but post-fire protection of grasslands from grazing can aid the recovery of grasses.

Diagnosis: Giant sacaton and/or alkali sacaton dominates (often more than 50% basal cover) and cover is uniform. Open patches are few and less than 2 m in length, most ground is covered with litter. Mesquite is generally absent.

Transition to tobosa-dominated state (1a): Transitions from bottomland communities to tobosa-dominated communities can occur in response to diversion in run-in water flow catalyzed by overgrazing or blockage of surface
flow. Removal of grasses may increase the rate of water flow in parts of the bottomland and result in gullying. Channelization of subsequent flood waters into the gully diverts run-in water and reduces the amount of time that areas of the bottomland are submerged. Once the duration of submersion is reduced below an unknown value, tobosa establishment and persistence may be favored at the expense of bottomland grasses, especially in the
presence of grazing. It is not known what conditions promote either the transition to shrub-invaded grassland versus a mesquite-free, tobosa-burrograss grassland.

Key indicators of approach to transition: Increases in bare ground cover, tobosa, and burrograss cover, increases in the size of bare ground patches, decreases in the cover and reproduction of giant and alkali sacaton, appearance of water flow patterns, rills, gullies, and debris dams associated with open spaces, reduced frequency and duration of flooding.

Transition to shrub-invaded state (2a): Overgrazing or poorly-timed grazing, with consequent reduction in grass cover and subsequent gullying and diversion of water may produce changes in the soil moisture regime or fire regime that may facilitate the establishment of mesquite and other shrubs. It is unknown whether increased shrub establishment requires only open space or requires changes in soil moisture or fire frequency. The historic role of fire in bottomlands is unknown, and the absence of fire and reduced soil moisture may act together. Tarbush may also invade.

Key indicators of approach to transition: Increases in bare ground cover, increases in the size of bare ground patches, decreases in the cover and reproduction of giant and alkali sacaton, appearance of water flow patterns, rills,
and debris dams associated with open spaces, reduced frequency and duration of flooding. Increases in tobosa and burrograss may also accompany this transition. The presence of mesquite seedlings and plants may indicate that a transition is underway.

State 2
Shrub-Invaded Grasslands

Community 2.1
Shrub-Invaded Grasslands

Additional States:
Shrub-invaded grasslands: Honey mesquite and/or other shrubs such as little-leaf sumac are common. Shrub increase is generally associated with declines in the sacaton species and increases in tobosa, burrograss, and bare ground. Plant distributions are often very patchy, with clumps of mesquite occupying drier parts and grasses or perhaps annuals dominating others. The expansion of mesquite may cause further reductions in grass cover, but if sufficient grass remains to control erosion, this situation may be stable.

Diagnosis: Medium to large (>50 cm tall) mesquite present. Giant sacaton and/or alkali sacaton cover and litter cover is discontinuous, tobosa, burrograss, or bare ground may dominate many large (>2 m) patches.

Transition to mesquite woodland (3): Transitions to a mesquite woodland state may occur in response to continued gully development and lowering of soil moisture, alongside soil surface degradation, either before (transition 4a) or after (3) mesquite have established. This may be caused by continued overgrazing and reduction of grass cover.

Key indicators of approach to transition: Increases in bare ground cover and the size of bare ground patches, decreases in grass cover, presence of deep gullies.

Transition to bottomland grassland (2b): Mechanical/herbicide removal with restoration of hydrology and grass cover would be needed to restore bottomland grasslands.

State 3
Tobosa-Burrograss Grassland

Community 3.1
Tobosa-Burrograss Grassland

Tobosa-burrograss grassland: This grassland is believed to occur as gullying increases (or flows are blocked by dams) and soil moisture levels during flood periods decline. Tobosa and/or burrograss dominate overall, but giant sacaton may occur along gully margins or in wetter patches. Mesquite is absent or rare, perhaps due to dispersal limitation.

Diagnosis: Giant sacaton and/or alkali sacaton restricted to wetter patches. Tobosa and burrograss dominant in large, many large (> 2m) bare patches. Large gullies are present and physical soil crusts and shrink-swell cracking is visible in bare patches.

Transition to mesquite woodland (4a): See transition 3 above.

Transition to annual-dominated state (5a): The causes, indicators, and reversing practices for this transition are believed to be similar to those for transitions 3 and 4a. In this case, however, the presence of shrink-swell clays causes root destruction of perennials and only annual species occur. Persistent disturbance and soil degradation may produce a similar effect.

Transition to bottomland grassland state (1b): Gully destruction and water-spreading to redirect flood waters across the area in conjunction with transplants or seeding of bottomland grasses (especially with vine mesquite and alkali sacaton) may possibly reverse this transition.

State 4
Mesquite Woodland

Community 4.1
Mesquite Woodland

Mesquite woodland: Communities in this state are largely bare ground but may contain patches of tobosa, burrograss, and perhaps sacaton species. Mesquite cover may be thick.

Diagnosis: Large (> 1 m tall) mesquite are numerous. Giant sacaton and/or alkali sacaton either absent or restricted to a few patches. Tobosa and burrograss also restricted to patches. Bare ground extensive and well connected. Large gullies are present and physical soil crusts and shrink-swell cracking is visible in bare patches.

Transition to tobosa grassland state (4b): Shrub removal with gully destruction and water-spreading to redirect flood waters across the area, in conjunction with transplants or seeding of tobosa and other grasses, may improve infiltration conditions and facilitate subsequent reintroduction of bottomland grasses. Alternatively, one may attempt to restore bottomland grasses immediately. Neither has been attempted so far as we are aware.

State 5
Annual-Dominated

Community 5.1
Annual-Dominated

Annual-dominated: Most of the area in this state has been reduced to either bare ground or annuals. Patches of giant sacaton, burrograss and tobosa may occur. Deep gullies are a prominent feature of this site.

Diagnosis: Bare ground or annual cover is nearly continuous. Giant sacaton and other grasses occur only in isolated patches, sometimes on the fringes of the annual-dominated area, or not at all. Gullies are present or other features obstruct surface flow of water. Physical crusting and shrink-swell cracking of the bare soil surface is extensive.

Transition to tobosa grassland state (5b): Same as for 4b without need for shrub removal.

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 Draw 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). For bottomlands, the historic role of fire is far from clear. Finally, the competition hypothesis holds that
sacaton 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.