Ecological dynamics

The information contained in the State and Transition Diagram (STD) and the Ecological Site Description was developed using archeological and historical data, professional experience, and scientific studies. The information presented is representative of a very complex set of plant communities. Not all scenarios or plants are included. Key indicator plants, animals and ecological processes are described to inform land management decisions.

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 has had a major influence on the plant community development. As a rule, these are developmentally very young soils. In fact, some areas of the Sand Hills site now supports a plant community which were bare dunes as recently as 50 to 60 years ago. On the other hand, some sites appear to be more mature and support a more diverse plant community that appears to be more stable. So, different stages of plant community development can be observed over the range of the site. Describing a reference plant community for this site is difficult because of the different ages of sediments, and the stage of development of the plant community. It may be that the age of soil deposition and soil stability are 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.
Tallgrass/Shrub/Forb Community (1.1) is a mixture of tallgrasses (50-60 percent), shrubs (40-50 percent) and perennial forbs (10-15 percent). A varying amount of annual forbs can be found depending on the moisture situation in a given year. The dominant tallgrasses were sand bluestem (Andropogon hallii), giant sandreed (Calamovilfa gigantea), little bluestem (Schizachyrium scoparium) and the taller dropseed species (Sporobolus spp.). Lesser amounts of switchgrass (Panicum virgatum) and Indiangrass (Sorghastrum nutans) can be found where moisture conditions are slightly more favorable. Usually Indiangrass occurs where finer textured lens occur in the subsoil. A good variety of forbs exists but the amount varies greatly from year to year. The more commonly found forb species are queen’s delight (Stillingia sylvatica), gaura (Gaura spp.), western ragweed (Ambrosia psilostachya), camphorweed (Heterotheca subaxillaris), sand lily (Mentzelia strictissima), and annual wild buckwheat (Eriogonum annuum). The major shrubs were sand sagebrush (Artemisia filifolia) and skunkbush sumac (Rhus aromatica). Lesser amounts of sand plum (Prunus angustifolia) and southwestern rabbitbrush (Chrysothamnus pulchellus) were usually present. 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 the 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 plant 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 somewhat 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.

Natural 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 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. Some post-burn erosion will certainly take place, especially if moisture is deficient and the soil is bare in early spring. Many plant bases and seedlings are cut off by blowing sand. However, if post-burn rainfall is adequate, erosion can be minimal, and recovery can and will take place. It is common for recovery to take 3 growing seasons and possibly 4 to reach equilibrium. Biomass is reduced for at least 3 years, but after that, a moderate shift toward grass vegetation with shrub suppression that last 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 (>80 percent)/Midgrass and Forb (<20 percent) Dominant Community (2.1). Tallgrasses will decline and midgrass species will increase along with a dramatic increase in the shrub component. 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, threeawns, and a wide variety of lower successional species. Sand sagebrush and skunkbush sumac will increase dramatically. Bare areas may become more numerous and wind erosion will increase.

During the grazing/browsing process, the shrubs may be utilized somewhat, but little pressure is placed upon them. The balance between grass, forbs and shrubs is rather delicate, and over utilization of the more palatable species will most assuredly favor the shrubs. The plant community will be less diverse. In this phase, ecological processes have changed somewhat, but the pathway back toward the reference community can be initiated through prescribed grazing, selective brush and pest management. The use of prescribed fire should be done with the utmost care on this site, as generally there is not enough fine fuel to carry an effective fire. There are other tools that can be used with less risk. Very careful use of herbicides can sometimes be used to 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 along with periodic droughts, retrogression will move the plant community towards the Shrub (>90 percent)/Annuals (>10 percent) Dominant Community (3.1). In extreme cases, shrubs will dominate with mainly annual forbs and grasses being present. Sand sagebrush and skunkbush sumac canopy will continue to increase, on some sites yucca (Yucca glauca) may increase to become a high percentage of the total plant community. Numerous bare areas will be scattered throughout the site. The rolling mini-dune areas may show signs of wind erosion from the lack of deep rooted grass cover. The plant community can become so degraded that it cannot reverse the transition without extensive energy and management inputs. At this point, a major threshold has been crossed. Restoration of the Shrub/Annuals Dominant Community (3.1) requires re-seeding, prescribed grazing (3 - 5 consecutive year’s deferment during the growing season) as well as extensive brush and pest management.

The shrub dominant community is generally a stable community but diversity of species must consider all of the ecological processes and in the management of the site for wildlife. Several species of wildlife frequent the site and the state of the plant community certainly influences the quality of habitat for quail, pronghorn and mule deer. All of these species prefer a mixture of grass, forbs and shrubs. A variety of shrubs, tall grasses and forbs will be more beneficial for wildlife habitat, 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)

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.

As a site changes in the structure and makeup of the plant community, the changes may be due to management, natural occurrences or both. At some point in time thresholds are crossed. Once changes have progressed to a certain point, the balance of the community has been altered to the extent that a return to the former state is not possible unless some form of energy is applied to make it happen. These changes take place on all ecological sites, but some sites support communities that are more resistant to change than others. Also, some sites are more resilient, and tend to 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 or mechanical brush management to decrease the amount of woody 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 upon the desired result.

The plant community balance of this site is more fragile than some of the shortgrass 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.