Ecological dynamics

The Reference Plant Community for this ecological site is a Shortgrass/Midgrass Community (1.1) with less than 5 percent woody shrubs and only a limited variety of forbs. The heavy clay loam textured soils in a rainfall regime of less than 20 inches favor short and midgrasses. The dominant grasses are blue grama (Bouteloua gracilis), buffalograss (Bouteloua dactyloides), sideoats grama (Bouteloua curtipendula), plains bristlegrass (Setaria macrostachya), and tobosa (Pleuraphis mutica). Lesser amounts of black grama (Bouteloua eriopoda), sand dropseed (Sporobolus cryptandrus) and perennial threeawn (Aristida sp.) are scattered throughout the site. Other grass species that can be found in and around small depressional areas include vine mesquite (Panicum obtusum), Arizona cottontop (Digitaria californica) and cane bluestem (Bothriochloa barbinodis). The few major forb species present include gaura spp. (Guara spp.), croton spp. (Croton spp.), plains greenthread (Thelesperma filifolium), western ragweed (Ambrosia psilostachya) and wooly plantain (Plantago patagonica). Shrub/half-shrub species lightly scattered (<5 percent) throughout the site are broom snakeweed (Gutierrezia sarothrae), catclaw mimosa (Mimosa aculeaticarpa var. biuncifera), lotebush (Ziziphus obtusifolia), yucca (Yucca glauca), agarito (Mahonia trifoliolata), fourwing saltbush (Atriplex canescens) and wolfberry (Lycium berlandieri). Occasional shrubby mesquite (Prosopis glandulosa) may occur as well.

The production potential of the site is moderate and it is a preferred site for grazing of domestic livestock. The short grasses are palatable and nutritious and the site provides year round grazing. Generally speaking, the soils on this site are perhaps the most fertile of any in the MLRA. The most limiting soil factor is the tenacity with which soil water is held by the fine textured soil particles. The soils store maximum amounts of water but yield it to plants somewhat sparingly. In very dry periods, the soils can appear rather droughty. When good moisture occurs, the site produces well.

Pre-settlement influences included grazing or browsing by pronghorn antelope, deer and migratory bison, re-occurring droughts and frequent fires. In general, it is thought that wildfires occurred in this region every 8-15 years on the average. Since European settlement in the late 1800s, grazing pressure from domestic livestock and climatic factors has interacted with reduced fire frequency and intensity to give the competitive advantage to woody plant species. The interaction of these disturbances has changed the composition and structure of the dramatically on many sites. Mesquite has increased on virtually all of the clay loam sites over the past 100 to 150 years.

Fire played a role in the ecology of the site as is true for most of the plains grasslands. The main effect of fire on this site was to hold woody shrubs in check. The shorter grass species such as blue grama and buffalograss are considered fire neutral as far as their response to fire. Climate and soils are the most important and limiting factors affecting grass vegetation on the site. Fire stimulated forb growth if the timing was right and the fires of pre-settlement days were probably more severe due to more fuel being available which could have been more damaging to woody plants. Fire usually creates more diversity in this site for a year or two post-burn. Then the grasses tend to crowd out the forbs and diversity decreases. Forbs also need spring moisture which is perhaps the major factor in creating diversity in the plant community.

Since this site is much preferred as a grazing resource, it has a tendency to be abused perhaps more than some of the other associated sites. With abusive grazing practices, the vigorous blue grama and sideoats grama will become lower in vigor and secondary successional species such as black grama, buffalograss, sand dropseed, and tobosagrass will begin to increase. Broom snakeweed, prairie broomweed (Amphiachyris dracunculoides), western ragweed, crotons, and cool-season annuals will quickly increase if the principal species are in a weakened condition. The shift in plant cover to less palatable and generally shorter grasses and decline in soil and litter cover, favors woody plant encroachment. The hydrological functions have been reduced slightly at this point.

Mesquite, catclaw, yucca, and broom snakeweed are quick to increase on the site under continuous heavy grazing. The greatest contributor to the increase of mesquite is the domestic cow. The seed is consumed by animals after the seed pods ripen in late summer and when passed through the digestive system and excreted in the manure, the seed finds an excellent seedbed complete with moisture and nutrients. Some wildlife species rely heavily on mesquite beans and juniper berries for food and contribute to the spread of these species. It is possible for mesquite beans to lay dormant in the soil for many years and then germinate when ideal conditions occur. Grazing management probably has minimal effect on the proliferation of mesquite, but a good cover of perennial grasses likely minimizes the seed to soil contact the mesquite needs to establish.

Excessive grazing pressure combined with the lack of fire will result in a Shortgrass/Shrub Community (1.2). Grass vegetation still dominates annual herbage production, but the encroaching woody species production has increased. The more palatable grasses such as blue grama and sideoats grama will be in a low vigor state with perennial and annual forbs increasing. The percentage of bare soil will also increase at this point. This plant community may be returned to the reference community by prescribed grazing, chemical and/or mechanical brush management on an individual plant basis and possibly with prescribed burning if adequate fine fuel loads are present. Fuel loads are often the most limiting factor for the effective use of prescribed fire on this site. In general, the use of fire on shortgrass communities just does not result in the same positive effects that burning has in tall/midgrass communities. Woody plant suppression using safe approved herbicides is generally more practical, although prescribed fire may have a place in some circumstances.

When heavy grazing continues for extended periods on the Shortgrass/Shrub Community (1.2), and the lack of frequent and intense fires on the site, a transition from the Shortgrass/Shrubs Community into the Shrub/Shortgrass/Annuals Community (2.1) occurs. Droughts, which occur at approximately 20-year intervals in this region, magnify this situation. This plant community will generally be dominated by mesquite and broom snakeweed with lesser amounts of yucca, pricklypear and catclaw. During the transition the more grazing resistant plants such as tobosa, perennial threeawns, hooded windmillgrass, hairy grama, dropseeds and annuals will dominate the site. At this point, a threshold has been crossed and major, high cost and energy inputs will be required to return this site back to the reference community. Heavy mechanical brush management and chemical brush management along with other conservation practices such as range planting, and prescribed grazing will be necessary.

In the early 1930’s Lehman lovegrass (Eragrostis lehmanniana), a grass of African origin, was introduced in MLRA 77D as a drought tolerant, easy to establish introduced grass species. This grass species was used in many grass mixtures and pasture plantings in an attempt to re-seed poor condition rangeland following mechanical brush management and to return old cropland fields to a perennial vegetative state for livestock grazing purposes. This grass is both invasive and persistent; published evidence indicates that variables such as elevation, summer precipitation, winter temperatures, and soils impact its abundance and distribution. Clay loam sites in a weakened state near established areas of Lehman lovegrass may become invaded by this grass. Presently, several thousand acres of clay loam, loam, and sandy sites have been invaded in MLRA 77D to the point that Lehman lovegrass is the dominant grass species with few if any native species remaining. The resulting plant community is a Lehman Lovegrass/Shrub Community (3.1). Once this lovegrass has become well established, returning the site to the reference community would be expensive and generally not very successful or practical. Prescribed burning for seedbed preparation purposes may be necessary to remove excessive amounts of plant biomass. Moderate to heavy mechanical brush management, heavy seedbed preparation and re-seeding to a native grass mixture would be required. The application of herbicides can be effective to reduce competition from this lovegrass species, but there is only a narrow time of treatment opportunity. Since this grass species has become naturalized much like K.R. bluestem has in Central Texas, it is unlikely that it will disappear through any natural processes such as competition from native species.

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.