Ecological dynamics

The Tight Sandy Loam ecological site evolved and was maintained by the grazing and herding effects of native wild large and small ungulates, extreme climatic fluctuations, and periodic fires. The reference plant community for this site is a post oak savannah with a tallgrass understory. If oaks are removed, open areas are often invaded by mesquite (Prosopis glandulosa). The herbaceous understory is dominated by tallgrasses such as little bluestem (Schizachyrium scoparium), Indiangrass (Sorghastrum nutans) and small amounts midgrasses such as sideoats grama (Bouteloua curtipendula). With continuous heavy grazing on perennial grasses (decreasers) such as Indiangrass and little bluestem, tallgrasses will decrease and be replaced by increasers such as silver bluestem, sideoats grama, Texas wintergrass (Nassella leucotricha) and sand dropseed (Sporobolus cryptandrus). Further regression will cause undesirable grasses, or invaders, such as three-awns (Aristida spp.) and annual grasses to increase. Forbs will make up a larger percentage of the plant composition and grasses would make up a smaller percentage.

Fire played a role in the ecology of the site as is true for most of the grasslands. The main effect of fire on this site was to hold woody shrubs and cactus in check. The grass species such as little bluestem and Indiangrass 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 forbs 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. Fire will usually not produce much mortality in older woody plants. After brush has been controlled with herbicides or mechanically, fire can sometimes be used effectively to suppress regrowth. Small juniper (Juniperus spp.) can be killed by fire. Fuel loads are often the most limiting factor for the effective use of prescribed fire on this site.


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 Indiangrass and big bluestem (Andropogon gerardii) will become lower in vigor while little bluestem will increase then secondary successional species such as sand dropseed and silver bluestem (Bothriochloa laguroides) will begin to increase along with an increase of woody plants. The little bluestem is a tough, resistant grass species 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 when present 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 and other woody plants, but a good cover of perennial grasses likely minimizes the seed to soil contact the mesquite needs to establish. Prescribed fire where it can be safely carried out provides a much better method to control the spread of woody plants. 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 of number of plants can 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 historic climax level until the brush canopy is so dense that the shade starts to interfere with photosynthesis. The Sandy 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 drought and above average winter moisture.

Long-term droughts that occur only three to four times in a century can effect some change in plant communities. Short-term droughts are common and usually do not have a lasting effect in changing stable plant communities, although production will be affected. When a brush canopy becomes established which shades the ground sufficiently it tends to favor cool-season annual species. Once a plant community consisting of brush 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 various plant communities of vegetation depend on the type of grazing management applied over many years and the rate of invasion and establishment of woody species. The effects of seasonal moisture and short-term dry spells become more pronounced after the site crosses thresholds to a lower ecological condition. Plant communities that consist of warm-season perennial grasses such as little bluestem and the associated species of the reference community are able to persist and withstand climatic extremes with only minor shifts in the overall plant community.

This site was inhabited by grassland wildlife species such as bison, grassland birds and small mammals. Over the years, as the site has 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. 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.