Ecological dynamics

The Sand Hills ecological site is unique in many respects. The site developed in wind-worked alluvial or eolian deposits. The lack of soil development and recent climatic conditions played a major role in plant community development. Typically, these are developmentally very young soils. Areas of the Sand Hills site now supporting a plant community were bare dunes as recently as 50 to 60 years ago. Other sites appear to be more mature and support a more diverse and seemingly stable plant community. Different stages of plant community development can be observed over the range of the site. The reference plant community for this site is difficult to describe because of the different ages of sediments, and the stage of development of the plant community. The age of soil deposition and stability appear to be major factors in this process.

The Tallgrass/Shrubs Community (1.1) is a mixture of tallgrasses (50-60 percent), shrubs (35-45 percent) and perennial forbs (10-15 percent). The amount of annual forbs found is dependent on the moisture situation in a given year. The dominant tallgrasses are sand bluestem (Andropogon hallii), giant sandreed (Calamovilfa gigantea), little bluestem (Schizachrium scoparium) and the taller dropseed species (Sporobolus spp). Lesser amounts of switchgrass (Panicum virgatum) and Indiangrass (Sorghastrum nutans) are found under more favorable moisture conditions. Usually Indiangrass occurs where finer textures occur in the subsoil. A good variety of forbs exist but the amount varied greatly from year to year. The more commonly found forb species are queen’s delight (Stillingia sylvatica), gaura (Gaura spp.), and annual wild buckwheat (Eriogonum annuum). The major shrubs are sand shinoak (Quercus harvardii), sand sagebrush (Artemisia filifolia), and skunkbush sumac (Rhus aromatica). Lesser amounts of sand plum (Prunus angustifolia), and southwestern rabbit brush (Chrysothamnus pulchellus) are usually present in the Texas portion of the MLRA. The woody shrubs are more stable from year to year and can better withstand the climatic extremes; therefore it is probable that shrubs were a major component of the historic plant community. In dryer years the grasses and forbs decline somewhat and in wet years forb growth can be profuse. With abundant early spring moisture, annual wild buckwheat can be the dominant herbaceous plant present. With good summer moisture, the taller grasses become more visible and produce a larger portion of total biomass. The environment in the Sand Hills is harsh and those species that are deep rooted, soil stabilizing and drought tolerant have the advantage. The shrubs and the taller grasses, along with certain tap-rooted forbs best fit that description.

The plant community on this site is influenced by moisture and aspect. North and east aspects will support a slightly different plant community than the south and west aspects. Production levels can be slightly higher on the north and east aspects.

Fire no doubt plays a part in the sites’ ecology, as is true of nearly all plains sites. Since cover is sparser than adjacent sites and bare areas exist throughout the site, fuel continuity may not have always been sufficient for a complete burn. There are historical accounts of fires having burned up to the Sand Hills and dying out. There were undoubtedly times when the site did burn and present day observations bear this out. These results are varied. If the soil is bare in early spring and moisture is deficient then some post-burn erosion is likely. The resulting blowing sand can cut off plant bases and seedlings. However, if post-burn rainfall is adequate, erosion can be minimal, and recovery can take place. Frequently, the site requires three or four growing seasons to reach equilibrium. Biomass production is reduced for at least three years. A moderate shift toward grass vegetation with shrub suppression lasting several years will usually result.

Abusive grazing with domestic livestock for prolonged periods will usually see the most palatable plants pressured severely. As retrogression proceeds, this site will move towards the Shrub/Midgrass Community (2.1). Tallgrasses decline and midgrass species increase. The shrub component increases dramatically to >60 percent. Heavy disturbance by hoof action to the soil surface can cause an increase in annual species. There will be a marked increase in western ragweed, dropseed species, perennial three-awns (Aristida spp.), and a wide variety of lower successional species. Sand shinoak, sand sagebrush and skunkbush sumac will increase dramatically. Some sites may have an increase in yucca (Yucca glauca) plants. Bare areas may become more numerous and wind erosion will increase.

Shrubs are only slightly utilized during the grazing/browsing process, and little pressure is placed upon them. The balance between grass, forbs, and shrubs is rather delicate; and over utilization of the more palatable species favors shrubs. Diversity will be less than under reference conditions. In this phase, ecological processes have changed somewhat, but the pathway back toward the reference community can be initiated through prescribed grazing and selective brush management. Prescribed burns require the utmost caution. Generally there is not enough fine fuel to carry an effective fire. Other tools can be used with less risk. In appropriate conditions careful use of herbicides can suppress shrub growth without destabilizing the site. Care should be taken to avoid treating the tops of dunes and areas of poor cover.

If abusive grazing continues, along with periodic droughts, retrogression will move towards a Shrub Dominant Community (3.1). In extreme cases, shrubs will dominate with mainly annual forbs and grasses being present. Sand shinoak, sand sagebrush, and skunkbush sumac increase. Some sand hills sites may see a major increase in yucca plants. Numerous bare areas will be scattered throughout the site. The rolling mini-dune areas may begin to show signs of wind erosion from the lack of deep-rooted grass cover. The plant community can become so degraded that it cannot recover without extensive energy and management inputs. At this point, a major threshold has been crossed. Restoration of the Shrub Dominant Community (3.1) requires re-seeding, prescribed grazing (3 - 5 consecutive years deferment during the growing season), and extensive brush and pest management.

The shrub dominant community is generally stable. However, plant diversity is important to ecological processes, and in the management of the site for wildlife. The state of the plant community certainly influences the quality of habitat for quail, pronghorn and mule deer frequenting the site. All of these species prefer a mixture of grass, forbs and shrubs. A variety of shrubs, tallgrasses, and forbs will be more beneficial for wildlife habitat and livestock grazing, and will aid in the function of the ecological processes such as nutrient cycling and the hydrological cycle.

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 diagram. This information is intended to show what might happen in a given set of circumstances; it does not mean that this would happen the same way in every instance. 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.

The plant community balance of this site is more fragile than some of the short grass sites. This balance is necessary for the community to function properly. Tall grasses are not as resistant to grazing as short and mid grass 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.

STATE AND TRANSITIONAL PATHWAYS: (DIAGRAM)