State 1
Historic Climax Plant Community

Community 1.1
Historic Climax Plant Community

State-transition model: MLRA 36, WP-3: Bottomland DRAFT—12 June 2001 STATUS—First abbreviated draft Overview The historic plant community type (HCPC) of this site is dominated by giant sacaton (Sporobolus wrightii) or alkali sacaton (Sporobolus airoides). Vine mesquite (Panicum obtusum) and sideoats grama (Bouteloua curtipendula) may also be common. Reduced plant cover with increased erosion and subsequent channelization causes transitions to two alternate states. In cases where the slope is slight (<1%), small channels tend to form and the subsequent lowered soil moisture average leads to a transition to a tobosa (Pleuraphis mutica)-dominated state. In cases where slopes are more steep (>1%) or where soils or increased erosion withi the tonosa-dominated state permit the formation of deeper channels, extensive soil drying can leave a sparse grassland with sacaton individuals concentrated along channel margins. Eventually, these conditions result in the establishment of upland grass species, including blue grama (Bouteloua gracilis). No quantitative information exists concerning the causes of transitions among grassland types or to shrublands or annuals. No systematic studies exist regarding the effects of range management on grassland-shrubland/annual community transitions in the lowland ecological site group. Information sources and theoretical background: Communities, states and transitions are derived from the observations of Gene Atkins and Elizabeth ??. Communities are usually defined by the primary and secondary dominant plant species, but sometimes emphasize dominant species of differing life-forms. 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). Catalog of states and community pathways 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 (Pleuraphis mutica), galleta (Pleuraphis jamesii). Western wheatgrass (Elymus smithi) may also occur at high elevations within this subresource area. 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. Each of these has relatively high palatability when compared to tobosa and during the growing season. Thus, reduction of populations of bottomland grass species due to overgrazing and erosion is likely in the absence of proper management. 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. Diagnosis: Basal cover of giant sacaton and/or alkali sacaton ranges from x to x. Mean bare patch size does not exceed x cm. Tobosa grassland: This grassland occurs as gullying increases and soil moisture levels decline during flood periods. Channels, however, are small and stable within this state. Tobosa and mat muhly (Muhlenbergia richardsonis) dominate overall, but giant sacaton may occur in wetter patches. Diagnosis: Basal cover of giant sacaton and/or alkali sacaton ranges from x to x. Tobosa ranges from x to x. Mean bare patch size ranges from x-x. Small gullies are present and physical soil crusts and shrink-swell cracking is visible in bare patches. Drained grassland: Most of the area in this state may have been reduced to either bare ground or annuals. Patches of sacaton may occur along the margins of deep gullies that characterize this site in its early stages. Over time, upland grasses such as blue grama will increase, and with mat muhly, dominate the site. Over extended periods (e.g. 50-100 years) agradation of soil into channels will lead to a return of flooding cycles, and bottomland grasslands may reestablish. Thus, this site may exhibit a cyclic sequence of states over the long term. Diagnosis: Bare ground or annual cover exceeds x. 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, often up to x cm deep. Physical crusting and shrink-swell cracking of the bare soil surface is extensive. Upland grasses may dominate the site. Catalog of transitions GT: Burning, mowing for hay in conjunction with heavy grazing may reduce grass cover, and subsequent gullying and diversion of water may produce changes in the soil moisture regime that facilitates a shift in dominance to tobosa. Key indicators of approach to threshold: 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. Reversing practices: Grazing giant sacaton during dry summers or fall may expose crowns to freezing temperatures and cause grass mortality (Cox 1988). Frequent utilization during the spring when grasses are palatable and growing, however, may weaken them. Giant sacaton, for example, is relatively slow to recover from fire, taking from 2-3 years. Thus, the appropriate timing of grazing and fire is critical.?? GD: Causes are similar to GT under conditions of steeper slope or easily erodable soils. TD: If the conditions permitting rapid erosion are sustained and channels deepen, this transition may occur on even shallow slopes. Key indicators of approach to threshold: Increases in bare ground cover, increases in the size of bare ground patches, decreases in the cover of tobosa, deepening of gullies. This site is strongly dominated by giant sacaton. Alkali sacaton, tobosa, vine-mesquite, scattered shrubs such as fourwing saltbush, Apacheplume, and occasionally yucca on the more medium textured soils, also occur. Forbs and shrubs occur as minor components of the plant community when the site is at its potential. *Western wheatgrass may be found at certain higher elevations and in the northern portions of the site’s range of occurrence.