Ecological dynamics

The reference plant community for the site is primarily shortgrasses with a few midgrasses, a few forbs and few shrubs. It is a Rolling Red Plains grassland site. Blue grama (Bouteloua gracilis) and buffalograss (Bouteloua dactyloides) are the most commonly occurring species. Smaller amounts of vine mesquite (Panicum obtusum), common curlymesquite (Hilaria belangeri), sideoats grama (Bouteloua curtipendula), tobosa (Pleuraphis mutica), and Arizona cottontop (Digitaria californica) occur as well. Shrubs are few in the reference community. They consist of prickly pear (Opuntia spp.), tasajillo (Cylindropuntia leptocaulis), lotebush (Ziziphus obtusifolia), wolfberry (Lycium berlandieri) and scattered mesquite (Prosopis glandulosa). Mesquite are more confined to areas along drainages and tend to be single-stemmed plants.

Tobosa and common curlymesquite occur more in the southern counties of the Major Land Resource Area (MLRA), as does Texas wintergrass (Nassella leucotricha). Mesquite has increased on virtually all of the clay loam sites over the past 100 to 150 years. Where there is a seed source close by, juniper (Juniperus spp.) will invade the site. In certain areas, juniper has become a significant problem species along with mesquite. The production potential of the site is moderate and it is a preferred site for grazing of domestic livestock. Pre-settlement grazers included pronghorn, bison and elk. The shortgrasses 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 rainfall is received, the site produces well.

Fire has played a role in the ecology of the site as is true for most of the rolling red plains grasslands. The main effect of fire on this site was to hold woody shrubs and cactus 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. Prescribed fire is sometimes used as a tool to promote diversity, mainly for wildlife. Fire will usually not produce much mortality in woody plants, especially mesquite. After mesquite control with herbicides, fire can sometimes be used effectively to suppress re-growth. Small juniper can sometimes be killed by fire. 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.

Since this site is much preferred as a grazing resource, it has a tendency to be abused perhaps more than some other associated sites. With abusive grazing practices, the vigorous blue grama and buffalograss will become lower in vigor and secondary successional species such as sand dropseed (Sporobolus cryptandrus), and silver bluestem (Bothriochloa laguroides) will begin to increase. Tobosa will often increase on this site if long term abuse continues. Tobosa tends to be found more in the southern portion of the MLRA. The blue grama and buffalograss are tough, resistant species and these species are tolerant of some fairly heavy grazing for long periods, but at some point, a threshold is crossed and the ground cover is opened up resulting in bare places where weedy species can establish. Western ragweed (Ambrosia psilostachya), crotons (Croton spp.), and cool-season annuals will quickly invade if the principal species are in a weakened condition. Mesquite will usually increase on this site but it is slowed by good management.

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. Selective individual removal of mesquite and/or juniper is easy and economical when a few plants begin to show up on the site, but the increase may be fairly rapid and the number of woody plants per acre will soon become too numerous for individual control to be feasible. Prescribed grazing with a reasonable stocking rate can sustain the grass species composition and production at a near reference level. This site is generally quite tolerant of grazing due to the short grass species that dominate. Generally, with long-term moderately heavy grazing pressure, the blue grama will tend to loose its semi-bunch grass appearance and become sod bound resembling buffalograss at a glance. The site can exist in this lower vigor stable state indefinitely if grazing abuse is not severe. The clay loam site can be abused to the point that the perennial warm-season grasses thin out and lower successional grasses along with annual forbs begin to dominate. This process of degradation usually takes many years and is further exacerbated by summer droughts and above average winter moisture.

Long-term droughts that occur only three to four times in a century can effect some change in historic plant communities, when coupled with abusive grazing. Short-term droughts are common and usually do not have a lasting effect in changing stable plant communities, although production will be affected. With a seed source and invasion, a heavy mesquite canopy often becomes established which shades the ground sufficiently so as to favor cool season annual species. Once a state of mesquite and cool-season annuals is reached, recovery to a good perennial grass cover is unlikely without major input with brush management and reseeding. In summary, the change in states of vegetation depend on the type of grazing management applied over many years, and the rate of invasion and establishment of woody species. While the ecological site crosses the threshold to a lower ecological condition, the effects of seasonal moisture and short-term dry spells become more distinct. Plant communities that consist of warm-season perennial grasses such as blue grama and the associated species of historic climax are able to persist and withstand climatic extremes with only minor shifts in the overall plant community.

Over the years, some sites have changed to a more mixed grass and shrub community, more wildlife species have come to utilize it for habitat. Woody plants provide cover for white-tailed deer and bob-white quail. These wildlife species have both increased along with the brushy plants due to the cover that these plants provide. According to most wildlife biologists, both species prefer a lower successional plant community than the reference community. More forbs are needed to meet these species food requirements and woody plants for browse are important for deer. It is often the objective of many land owners to strike a balance in plant community so that these wildlife species can exist along with domestic livestock. This can be accomplished by a carefully thought out grazing and brush management program. It must be realized that managing at a lower successional level may meet some wildlife species requirements very well, but may not be nearly as productive for grazing purposes, and may not be as capable of satisfying functions such as nutrient cycling, hydrologic protection, plant community stability or soil protection. A proper balance can be achieved with careful planning that considers all resources.

Hydrologically, the site contributes runoff to the various draws, creeks, and streams that are common in the MLRA. If the perennial grass cover is maintained in good vigor, then maximum infiltration occurs and runoff is reduced. More water getting into the ground means a healthier, more productive plant community. If infiltration is minimal, then the effect is an artificially shallow soil with plant roots retreating to near the soil surface. More perennial grass cover means less runoff may result but the runoff that does occur is less laden with sediment. Overall watershed protection is enhanced by a healthy grassland community, as is nutrient cycling.

State and Transitional Pathways (Diagram) Narrative:
The following diagram suggests some pathways that the vegetation on this site might take in response to various treatment or natural stimuli over time. There may be other states that are not shown on this diagram. This information is to show that changes in plant community do occur due to management and natural factors; and can be changed by implementing certain practices. The plant communities described are commonly observed on this site. Before making plans for plant community manipulation for specific purposes, consult local professionals.

As a site changes in plant community makeup, the changes may be due many factors. Change may occur slowly or in some cases, fairly rapidly. As vegetative changes occur, certain thresholds are crossed. This means that once a certain point is reached during the transition of one community to another, a return to the first state may not be possible without the input of some form of energy. This often means intervention with practices that are not part of natural processes. An example might be the application of herbicide to control some woody species in order to reduce its population and encourage more grass and forb growth. Merely adjusting grazing practices would probably not accomplish any significant change in plant community once certain thresholds are crossed. The amount of energy required to effect change in community would depend on the present vegetative state and the desired change.