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 the 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. For the purposes of a site description, it is assumed that the more diverse plant community is more indicative of the reference plant community that developed over time and was observed by European settlers.

This site is a tallgrass prairie dotted with sparse stands of shinoak and sand sagebrush. The duned areas produce a greater variety of tall bunchgrasses than the flatter areas of the site with a strong midgrass component scattered throughout the entire site. The Tallgrass/Midgrass/Shrubs Community (1.1) is a mixture of tallgrasses (50 percent), midgrasses (25 percent), shortgrasses (5 percent), shrubs (15 percent) and perennial forbs (5 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 dropseed (Sporobolus giganteus), Havard’s panicgrass (Panicum havardii), little bluestem (Schizachrium scoparium) and traces of giant sandreed (Calamovilfa gigantea). Midgrass species include sand dropseed (Sporobolus cryptandrus), spike dropseed (sporobolus contractus), mesa dropseed (Sporobolus flexuosus), cane bluestem (Bothriochloa barbinodis) and trace amounts of sideoats grama (Bouteloua curtipendula) and needle & thread (Heterostipa comata). The few shortgrass species found on this site include hooded windmillgrass (Chloris cucullata), tumble windmillgrass (Chloris verticillata), fall witchgrass (Digitaria cognata), sand paspalum (Paspalum setaceum) and Wright threeawn (Aristida purpurea var. wrightii). A good variety of forbs exist but the amount varies greatly from year to year. The more commonly found forbs are gaura species (Gaura spp.), primrose species (Oenothera spp.), sandhill penstemon (Penstemon ambiguus), annual wild buckwheat (Eriogonum annuum), halfshrub sundrop (Calylophus serrulatus), trailing ratany (Krameria lanceolata), dotted gayfeather (Liatris punctata), sand lily (Mentzelia decapetala), western ragweed (Ambrosia psilostachya), woollywhite (Hymnopappus filifolius), and common sagewort (Artemisia campestris). The major shrubs are sand shinoak (Quercus harvardii) accounting for approximately 10 percent of the site production with lesser amounts of sand sagebrush (Artemisia filifolia), southwestern rabbitbrush (Chrysothamnus pulchellus) and yucca (Yucca glauca). The woody shrubs are more stable from year to year and can better withstand the climatic extremes. 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 the 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.

Fire no doubt played 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 a sand hills site and dying out. There were undoubtedly times when the site did burn and present day observations bear this out and 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 quickly. 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.

Heavy or even moderately heavy continuous 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 Shortgrass/Shrub Community (1.2). Tallgrasses and midgrasses decline as shortgrass species and the shrub component increase. Heavy disturbance by hoof action to the soil surface can and usually do cause an increase in annual species. There will be a marked increase in western ragweed, perennial threeawn, and a wide variety of lower successional species. Bare areas will increase with wind erosion common throughout the site. The production of vegetation has shifted from mostly herbaceous vegetation to increasing amounts of woody shrubs. Herbaceous vegetation is still the largest production in this state.

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 the reference community. In this state, 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 heavy continuous grazing continues for extended periods, along with periodic droughts, retrogression will move towards a Shrub Community (2.1). In extreme cases, shrubs will dominate with annuals filling the bare interspaces. Sand shinoak will be the dominant shrub with sand sagebrush common throughout the site. Some sand hills sites may see a major increase in yucca plants. Many of the sand hills sites in this MLRA have seen an invasion of mesquite (Prosopis glandulosa) and broom snakeweed (Gutierrezia sarothrae) on the flatter slopes.

Invading grasses such as fringed signalgrass (Urochloa ciliatissima), red lovegrass (Eragrostis secundiflora), tumblegrass (Schedonnardus paniculatus), sand muhly (Muhlenbergia aronicola), and gummy lovegrass (Eragrostis curtipedicellata) appear on the site. Numerous bare areas will be scattered throughout the site. Under extreme conditions, this site deteriorates to active dunes and blowouts which are very difficult to re-vegetate.

The plant community will become so degraded that it cannot recover without extensive energy, economic and management inputs. At this point, a major threshold has been crossed. Restoration of the Shrub Community (2.1) requires prescribed grazing (3 - 5 consecutive years with deferment during the growing season), extensive brush and pest management and re-seeding with adapted native grass species. The success of re-seeding is totally dependent on timely and effective rainfall.

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 deer frequenting the site. A good 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.