Ecological dynamics

The reference plant community on the Sandy ecological site developed under the prevailing climate over time along with the soils in their topographic location. Grazing and/or browsing by local and nomadic wildlife influenced the plant community as well. The Tallgrass/Midgrass Community (1.1) is composed of tall and midgrasses, associated forbs, and scattered woody shrubs. Dominant tallgrasses are sand bluestem (Andropogon hallii) and little bluestem (Schizachyrium scoparium). Midgrasses present included sideoats grama (Bouteloua curtipendula), sand lovegrass (Eragrostis trichodes), dropseeds (Sporobolus spp.), sand paspalum (Paspalum strumarium), fall witchgrass (Digitaria cognata), hairy grama (Bouteloua hirsuta), plains bristlegrass (Setaria leucopila), hooded windmillgrass (Chloris cucullata) and perennial three-awns (Aristida spp.). Some cool-season grasses occur in small amounts such as needle & thread (Stipa comata). Typically associated forbs include dotted gayfeather (Liatris punctata), annual wild buckwheat (Eriogonum annuum), queen’s delight (Stillingia sylvatica), prairie clover (Dalea purpurea), catclaw sensitivebriar (Mimosa aculeaticarpa var. biuncifera), gaura (Gaura spp.), and numerous annual forbs. Primary shrub species are sand shinoak (Quercus havardii) and sand sagebrush (Artemesia filifolia), with lesser amounts of skunkbush sumac (Rhus aromatica), sand plum (Prunus angustifolia), and southwestern rabbitbrush (Chrysothamnus pulchellus). Nutrient cycling, the water cycle, watershed protection and biological processes are functioning at their peak.

Fire plays an important role in the function of most plains sites, especially the tallgrass communities. Tallgrasses such as sand bluestem and little bluestem are dependent on fire to stimulate them and remove old growth that would accumulate on the soil surface. Fire also suppresses shrubs. Overall, fire helps to keep a balance between the vegetation. Opening up canopies and stimulating forb growth improved wildlife habitat. The deep-rooted species that grow on the site are not easily damaged by fire. Shrubs usually re-sprout, but are suppressed for a time allowing grasses to dominate. In the absence of periodic fire woody plants will slowly increase and with grazing pressure can begin to dominate the site. Practices such as brush management may be necessary to keep the community in balance if periodic fires no longer occur.

Periodic grazing and trampling by migrating herds of bison and resident herds of pronghorn antelope probably occurred during drought periods. However, long rest periods followed once the large herds of bison and antelope moved out of the area. Thus the resilient grassland could re-establish itself and maintain the reference community. Major forces influencing transition from the reference community include continued overgrazing by livestock and the decrease in frequency and intensity of fires. As livestock and wildlife numbers increase and grazing use exceeds a plants ability to sustain defoliation, the more palatable and generally more productive species decline in stature, productivity and density.

The productive capacity of this site is moderately high if good management is applied. The tallgrasses are sensitive to overgrazing and will decrease under continuous heavy grazing. Moderate grazing and periodic rest, especially during the mid to late growing season, are necessary to sustain them. Overgrazing causes retrogression. The tallgrasses give way to an increase in midgrasses such as sideoats grama, dropseed species and perennial three-awns. The better quality perennial forbs are replaced with less palatable species such as gaura, western ragweed, and camphorweed.

Annual forbs such as annual wild buckwheat increase. Sand shinoak, sand sagebrush, and skunkbush sumac can increase to >20 percent of the total plant community. The decrease in density and stature of the tallgrass vegetation and increase in density of the woody vegetation brings about a new plant community, a Midgrass/Shrub Community (2.1).

In the Midgrass/Shrub Community (2.1), the transition back to the reference is possible with proper grazing management, brush and pest management. Prescribed burning could be used if the conditions allow. The production of vegetation has shifted from mostly herbaceous vegetation to more woody, although the herbaceous vegetation biomass is still the largest amount. Nutrient cycling, the water cycle, watershed protection and biological functions have changed little.

If long-term abusive grazing continues, a threshold will be crossed to a Shrub Dominant Community (3.1). In this degraded state, typical vegetation will be annuals and western ragweed along with an increase in perennial three-awns, red lovegrass, gummy lovegrass, and sandbur. A significant increase in the canopy of sand sagebrush and sand shinoak can be expected for this plant community. The loss of herbaceous cover and increased bare ground encourages accelerated erosion. Nutrient cycling, the water cycle, watershed protection and biological functions have been severely reduced. The plant community is so degraded that it cannot reverse retrogression without extensive energy and management inputs. Restoration will require prescribed grazing with rest periods during the growing season, re-seeding bare areas with adapted native grass species, and chemical and/or mechanical brush management and some form of pest management. With the reduced amounts of grass fuel, prescribed burning is usually not an option in this phase.

The sandy site is not particularly resistant to heavy grazing but exhibits amazing resilience. If even a small seed source of the tallgrasses remains and some old root crowns are still viable, then recovery is possible with minimal re-seeding. Recovery can occur fairly rapid if the competitive plants are controlled and proper grazing management is applied. Full recovery and maintenance of the reference community requires continued proper grazing management as well as occasional brush and pest management.

NOTE: Rangeland Health Reference Worksheets have been posted for this site on the Texas NRCS website (www.tx.nrcs.usda.gov) in Section II of the eFOTG under (F) Ecological Site Descriptions.

STATE AND TRANSITIONAL PATHWAYS: (DIAGRAM)

Narrative:
The following diagram suggests some pathways that the vegetation on this site might take. There may be other states not shown on the model. This information is intended to show what might happen in a given set of circumstances. Local professional guidance should always be sought before pursuing a treatment scenario.

Changes in the structure and composition of the plant community may be due to management and/or natural occurrences. At some point thresholds are crossed as indicated by the lined box on the State and Transition Diagram. This suggests that once changes have progressed to a certain point, the plant community has been altered to the extent that a return to the former state is not possible unless some form of energy is applied. These changes take place on all ecological sites. Some sites support communities that are more resistant to change than others. Also, some sites are more resilient and can heal or restore themselves more easily. Usually, changes in management practices alone, such as grazing techniques, will not be sufficient to restore former plant communities. An example of energy input might be the implementation of chemical brush management to decrease the amount of woody/cacti shrubs and increase the amount of grasses and forbs. This shift in community balance could not be brought about with grazing alone. The amount of energy required to bring about a change in plant community balance may vary a great deal depending on the present state and the desired result.

As it relates to this site, the plant community balance is more fragile than some of the other sites. This balance is necessary for the community to function properly. Tallgrasses are not as resistant to grazing as short and midgrass species. The soil is more fragile since it is sandy and can result in plant and soil disturbance from hoof action. If cover is very poor, wind erosion will occur.