Ecological dynamics

MLRA-42, SD-1: Sandy

Overview

The sandy site occurs largely on eolian or fluvial deposits of various ages derived from river sediments. Landscape positions include relict basin floor and lower to middle piedmont slopes. Sandy soils are usually old enough to have developed calcic or argillic horizons, and so are often higher on the landscape and farther from Rio Grande than deep sand soils. Sandy sites may intergrade with both loamy sites and gravelly sand sites.

The historic community type is dominated by black grama (Bouteloua eriopoda) and depending on texture variations and rainfall patterns may feature subdominance by dropseeds (Sporobolus cryptandrus and flexuosus), Indian ricegrass (Achnatherum hymenoides), or galleta (Pleuraphis jamesii). Heavy grazing leads to reductions of palatable grasses (especially black grama and ricegrass) and possibly the persistent loss of black grama. On soils that have not developed a stage 3 or greater calcic horizon (i.e., the calcic horizon is not white with many nodules), sand sage (Artemisia filifolia) may be present historically and fill in areas where grasses have been lost. On soils with well-developed calcic horizons, sand sage tends to be absent. Mesquite may be present on these soils but persistent grass loss and dune formation has not been observed. Herbicides may be used to increase the grass component in sand sage dominated areas, but it is unclear whether sand sage will inevitably reestablish.

Catalog of states and community pathways

State Containing Historic Plant Community

Black grama Grassland:

Black grama and dropseeds are likely to have dominated soils included within this site. The abundance of subordinate species, however, probably varied among soils. Indian ricegrass and scattered sand sage may have been present on soils with a deeper loamy sand mantle over the calcic horizon or with a poorly developed (stage 2 or less) calcic horizon within 1 m. Loamier soils featured substantial amounts of galleta. Scattered mesquite (Prosopis glandulosa) may have been present, especially on the soils with argillic and shallow calcic horizons. Periodic fires may have limited mesquite abundance, but its effect on sand sage is unknown. There is evidence that fire may have favored sand sage6. Continuous heavy grazing, especially during drought, reduces the dominance of black grama and in some cases Indian ricegrass, leaving dropseeds, threeawns and snakeweed (Xanthocephalum spp.) which may increase in overall cover. Because Indian ricegrass is a cool-season plant, its abundance may also respond to the seasonality of rainfall, being favored in wet springs3. On loamier soils in concave positions that collect surface water runoff, burrograss and galleta may become dominant under continuous grazing. There is evidence that periodic fires may have a characteristic of this state (see photos).

Diagnosis: Grass cover is uniform with bare patches typically > 50 cm in width. Black grama is dominant and stabilizes much of the soil surface such that there is little evidence of wind erosion. Sand sage and/or mesquite are present, but not abundant.

Additional States:
Bunchgrass-snakeweed: Black grama abundance is persistently depressed or eliminated, leaving snakeweed and bunchgrasses including dropseeds and Indian ricegrass as dominants. This reinforces relatively high erosion rates and persistent soil degradation. Overall grass cover is substantially reduced, but shrubs are not dominant. It is not clear if this state actually exists. All bunchgrass-dominated communities we observed had a black grama component. Black grama recovery (from trace to 8%) with cessation of livestock grazing may occur over 20 years (1976-1996) in the extreme southern part of SD-14,5 on Turney loamy sand soil. Thus, the lack of black grama reestablishment observed in warmer areas (SD-22) may not apply to SD-1 as long as some plants remain. Bunchgrass-snakeweed communities would then be considered as occurring within the black grama grassland state, although black grama recovery may take decades.

Diagnosis: Black grama is very rare or absent. Bunchgrasses and/or snakeweed are dominant. Shrubs may be present but are not dominant. Bare ground patches > 1m are common and bare ground cover is interconnected. Evidence of wind and water erosion includes blowouts and litter dams.

Transition to Bunchgrass-Snakeweed (1a) Loss of black grama grass cover due to continuous heavy grazing through drought periods causes the transition. Once most plants are eliminated, recovery of black grama cover via stolons would require periods of high rainfall, and be very slow even with grazing rest. Establishment by seed appears to be limited. Soil degradation during periods of low grass cover (specifically, loss of fungal associates and soil water holding capacity) may also limit black grama reestablishment.

Key indicators of approach to transition:
? Overutilization, decadence, and mortality of black grama.
? Absence of stolons
? Increasing size of bare ground patches

Transition back to Black Grama Grassland (1b) The recolonization of black grama may be facilitated by prescribed grazing and seeding in SD-1, although establishment rates are low.

Transition to Sand Sage (4b) Bunchgrass-dominated communities may be produced by the use of herbicides in the sand sage state. The seed pool of sand sage may be large. Reestablishment of sand sage during periods of high winter-spring rainfall may occur, especially if measures are not taken to ensure high grass abundance or the reestablishment of black grama.

Sand Sage Dominated: Sand sage is dominant and black grama is a subdominant. Cover of sand sage may exceeds 25%. Sand sage competes with grasses for space and soil water, thereby limiting grass recovery. Basal bare ground cover is greater than in the black grama grassland, but canopy cover is similar. Thus, soil degradation does not appear to be significant. Remaining black grama may decline with continuous grazing, leaving dropseeds, threeawns, and Indian ricegrass as primary grasses depending on rainfall patterns. This state has not been recorded on soils with white, stage 3 calcic horizons within 100 cm (e.g., Madurez, Turney) but has been recorded on soils with stage 2 calcic development to this depth (Pajarito, Wink).

Diagnosis: Sand sagebrush is dominant or co-dominant with black grama and bunchgrasses. Bare ground patches are usually < 1m. Where sand sagebrush is dense, there is little evidence of erosion.

Transition to Sand Sage Dominated State (2a) Sand sagebrush may expand due to reductions in black grama cover and increased resource availability. Climatic variation (increased winter/spring precipitation) may also play a role. Reductions in fuel loads and fire frequency may contribute to persistent sand sage dominance, but sand sage is only top-killed by fire and has resprouted vigorously in other studies6.

Key indicators of approach to transition:
? Increase in size and frequency of bare patches.
? Increased amounts of sand sage seedlings.

Transition back to Black Grama Grassland (2b) Herbicides can be used to reduce sand sage dominance and allow the reestablishment of grasses. Prescribed grazing may sustain grasses as dominants under suitable climatic conditions.

Sand Sage: Sand sage is dense and dominant, but black grama has been lost and is difficult to recover. Scattered bunchgrasses may remain or be absent and reappear in wet years. Because of the overall high canopy cover of sand sage, soil erosion by wind is low. The low cover of grass is due to the high dominance and resource monopolization of sand sage.

Transition to Sand Sage (3) Continuous heavy grazing within the sand-sage dominated state will remove remaining grass patches and these patches may be replaced by sand sage during periods of high winter/spring rainfall.

Key indicators of approach to transition:
? Overutilization, decadence and mortality of remaining grasses

Transition to Bunchgrass Snakeweed (4a) Similar to 2b except black grama cannot be recovered without seeding and perhaps repeated herbicide applications. Poor grazing management and high winter spring rain may lead to recovery of sand sage dominance (4b).