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 Site is unique in many respects. The site developed in wind worked alluvial or eolian deposits. Lack of soil development and recent climatic conditions have been major influences on plant community development. As a rule, these are very young soils developmentally. In fact, some areas of the Sand Hills site now supporting a plant community 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 from one place to another. Describing the reference plant community for this site is difficult because of the different age of sediments, and the stage of development of the plant community. It may be that age of deposition and soil stability are major factors in this process. For purposes of a site description, it is assumed that the more diverse plant community is more indicative of the reference community that will develop over time.

The reference plant community is a mixture of tallgrasses, perennial forbs and shrubs. A varying amount of annual forbs can be found depending on the moisture situation in a given year. The dominant tallgrasses are sand bluestem (Andropogon hallii), giant sandreed (Calamovilfa gigantea), little bluestem (Schizachyrium scoparium) and the taller dropseed (Sporobolus spp.) species. Lesser amounts of switchgrass (Panicum virgatum) and Indiangrass (Sorghastrum nutans) can be found where moisture conditions are slightly more favorable. 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 texana), gaura (Gaura spp.), western ragweed (Ambrosia psilostachya), camphorweed (Heterotheca subaxillaris), prairie spiderwort (Tradescantia occidentalis), sand lilly (Mentzelia nuda), and annual wild buckwheat (Eriogonum annuum). The major shrubs are sand sagebrush (Artemesia filifolia) and skunkbush sumac (Rhus tribolata). Lesser amounts of sand plum (Prunus angustifolia), yucca (Yucca spp.), and southwestern rabbit brush (Chrysothamnus pulchellus) are 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. There are just a scattering of tree species present on the site. Western soapberry (Sapindus saponaria) and hackberry (Celtis spp.) can occasionally be found in the lower lying areas. 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 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.

Heavy, and sometimes even moderate, rates of grazing with domestic livestock for prolonged periods of time will severely pressure the most palatable plants. Disturbance to the soil surface causes a great increase in annual species. During the grazing and browsing processes, shrubs may be utilized somewhat, but little pressure is placed upon them. The balance between grasses, forbs and shrubs is rather delicate, and one would think that over utilization of the more palatable species would most assuredly favor the shrubs on this site. However, a 20 year study at Woodward, Oklahoma on pastures that were a mixture of Sand Hills and Deep Sand ecological sites found that the canopy cover of sand sagebrush fluctuated more due to weather than to stocking rate (Figure 1). The study found that sand sagebrush canopy cover fluctuations were similar at all stocking rates. An 11 - year study in Colorado found no difference in sagebrush canopy cover between moderate and heavy stocking rates. On the Sand Hills ecological site, stocking rate affects grass and forb availability and diversity but it does not appreciably favor an increase in shrub canopy.

Species diversity is usually desirable for ecological processes. Species diversity is also an important consideration when applying management programs for wildlife and cattle grazing. There are several species of wildlife that frequent this site. The state of the plant community certainly influences the quality of habitat for quail, lesser prairie chicken, turkey and deer. All of these species prefer a mixture of grass, forbs and shrubs.

Natural fire, no doubt, played a part in the sites’ ecology. This is true of nearly all plains sites. Since cover is sparse, and bare areas exist throughout the site, especially in dry years, fuel continuity may not have been sufficient for complete burns. There are historical accounts of fires burning up to the Sand Hills and then dying out. There were undoubtedly times when the site did burn and wildfires have been observed during present days with the results noted. These results are mixed. Some active erosion may take place post-burn, especially if moisture is lacking and the soil is bare in early spring. Many plant bases and seedlings can be cut off by blowing sand. If post-burn rainfall is adequate, erosion can be minimal and recovery can occur. Sand sagebrush strongly sprouts after burning and few plants are killed. Sagebrush canopy cover returns to pre-burn levels in about 3 years, so it is doubtful that a single fire will cause brush suppression that will last over 3 years. The use of prescribed fire should be done with the utmost care on the Sand Hills ecological site. In fact, unless the site is in very good ecological condition, prescribed burning should be avoided. 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. A variety of shrubs, tall grasses and forbs will be more beneficial for wildlife habitat and livestock grazing and will aid in the function of ecological processes such as nutrient cycling and the hydrological cycle.

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 may be crossed. This means that once changes have progressed to some critical point, the balance of the community has been altered to the extent that a return to the former state is not possible, that is, not possible unless some significant form of energy is applied to make the return happen.

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 shrubs and increase the amount of grasses and forbs. This shift in the 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 upon the present state of the site and upon the desired resultant community. This process occurs on all ecological sites, but some sites are more resistant to change than others.

State and Transition Diagram:
A State and Transition Diagram for the Sand Hills (R077EY063TX) site is depicted below. Thorough descriptions of each state, transition, and pathway follow the model. Experts base this model on available experimental research, field observations, professional consensus, and interpretations. It is likely to change as knowledge increases.
Plant communities will differ across the MLRA because of the natural variability in weather, soils, and aspect. The Reference Plant Community is not necessarily the management goal; other vegetative states may be desired plant communities as long as the Range Health assessments are in the moderate and above category.

The biological processes on this site are complex. Therefore, representative values are presented in a land management context. The species lists are representative and are not botanical descriptions of all species occurring, or potentially occurring, on this site. They are not intended to cover every situation or the full range of conditions, species, and responses for the site.

Composition by dry weight and percent canopy cover are provided to describing the functional groups. Most observers find it easier to visualize or estimate percent canopy for woody species (trees and shrubs).

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.