Ecological dynamics

MLRA-42, SD-1: Bottomland

Overview

The loamy site occurs largely on relatively level, lower portions of piedmont slopes. Soils classified as loamy vary continuously in texture and soil moisture regime such that different potential communities occupy different soil series and different landscape positions within soil series. Loamy soils intergrade with sandy soils and clayey soils.

This site is associated with Draw ecological sites, and often occurs at the downslope ends of draws. The historic plant community type of this site is dominated by either giant sacaton (Sporobolus wrightii) or alkali sacaton (Sporobolus airoides), the distinction depending upon unknown factors (perhaps salinity). Vine mesquite (Panicum obtusum) and sideoats grama (Bouteloua curtipendula) may also be common. Reduced cover and hummocking of these grasses characterize initial stages of degradation due to overgrazing, and perhaps due to reductions in soil moisture availability with changes in hydrology. Transitions to first tobosa (Pleuraphis mutica) and then to burrograss (Scleropogon brevifolius)-dominated states may occur in response to the redistribution of run-in water via overgrazing and subsequent erosion and gullying or changes in hydrology. **On soils where blue grama is common, it may also be driven to a burrograss state. Although soil-sealing may occur in some loamy soils (especially in the burrograss state), shrub invasion is not usually observed.

Catalog of states and community pathways

State Containing Historic Plant Community

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.

Additional States:

Galleta-dominated: Black grama abundance is persistently depressed or eliminated, leaving galleta, dropseeds, and burrograss as dominants. Blue grama may also be present. Burrograss tends to increase with grazing pressure. Overall grass cover may be somewhat reduced, but galleta and other grass growth may compensate black grama’s absence. This state is not frequently observed.

Diagnosis: Black grama is very rare or absent. Galleta, dropseeds, or burrograss are dominant. Bare ground patches up to 1 m across may be observed, with patchy evidence of soil sealing on some soils.

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.

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.

Burrograss-Dominated: Burrograss is dominant and interspaces between plants are sealed. Erosion may have stripped away surface soils. Galleta, dropseeds, and other perennial grasses are rare or absent. Burrograss seeds are well adapted to establishment in soils that have developed physical crusts due to raindrop impact1,3. These plants also reproduce by stolons. Long-term monitoring within the Sevilleta NWR indicates that the burrograss-dominated state may be quite stable. Over 20 years in ungrazed conditions on the Turney loam map unit, burrograss canopy cover fluctuated between 15-28% in one transect and 5-14% in another, with very little or no recruitment of other species 5, 6. Burrograss cover may decline to very low levels with drought and continuous grazing and take decades to recover. Burrograss is relatively tolerant of drought3.

Diagnosis: Burrograss is dominant, other grasses are usually rare, and soil sealing in exposed soil is extensive.

Transition to Burrograss state (2a, 3) Intense, continuous heavy grazing, possibly with drought, drives blue grama, galleta and other grasses to low overall cover. Exposed soils (loam and sandy clay loam surface) seal due to raindrop impact or surface horizons are eroded away during this low-cover period. Persisting burrograss recovers over time and other species do not.

Key indicators of approach to transition:
? Overutilization, decadence, and mortality of perennial grasses
? Increase in size and frequency of bare patches
? Evidence of soil physical crusts, lack of cryptobiotic crust.

Transition back to Grama Grassland (2b) Seeding with grazing deferment and soil-surface disturbance would be necessary. If erosion has stripped away part or all of the A horizon, then soil addition would be necessary.

Contributors. Data and ideas were provided by Darrel Reasner, Gary Garrison, George Chavez, Elizabeth Wright, Will Hooper, and David Trujillo.