Ecological dynamics

The reference plant community for the Loamy Bottomland site is a hardwoods savannah with tall and midgrass understory. The grasses are primarily little bluestem (Schizachyrium scoparium), Indiangrass (Sorghastrum nutans), big bluestem (Andropogon gerardii) and switchgrass (Panicum virgatum). Big bluestem and Indiangrass are the most commonly occurring grass species. Smaller amounts of little bluestem, switchgrass (Panicum virgatum), sideoats grama (Bouteloua curtipendula), Canada wildrye (Elymus canadensis), meadow dropseed (Sporobolus compositus var drummondii), Texas wintergrass (Nassella leucotricha), vine mesquite (Panicum obtusum), and tall dropseed (Sporobolus compositus var compositus) occur as well. Shrubs and trees consist of pecan (Carya illinoinensis), bur oak (Quercus macrocarpa), liveoak (Quercus fusiformis), hackberry (Celtis occidentalis), elms (Ulmus spp) and grapes (Vitis spp). Woody plants have increased on virtually all of the loamy bottomland sites over the past 100 to 150 years. Where the site was once cultivated, mesquite (Prosopis glandulosa) tends to be one of the first woody plants to appear. Where there is a seed source close by, juniper (Juniperus ashei and Juniperus virginiana) will invade the site. In certain areas juniper has become a significant problem species along with mesquite. The production potential of the site is high. Pre-settlement grazers included bison and deer. The grasses are fairly palatable and nutritious and the site provides year round grazing. Generally speaking, the soils on this site are the most fertile of the Major Land Resource Area (MLRA). When good rainfall is received, the site produces well.

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 in check. The grass species are considered fire neutral as far as their response to fire. Climate is 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 can 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 uses of fire on mature (larger) or dense stands of woody plants does not result in the same positive effects that burning has in tall/mid-grass communities.

With abusive grazing practices, the vigorous Indiangrass and big bluestem will become lower in vigor while little bluestem will increase then secondary successional species such as sideoats grama, tall and meadow dropseed, Texas wintergrass, vine mesquite and white tridens (Tridens albescens) will begin to increase along with an increase of woody plants such as mesquite, sumacs (Rhus spp) and persimmon (Diospyros virginiana). Western ragweed (Ambrosia psilostachya), crotons (Croton setigerus), nightshades (Solanum spp.) and cool-season annuals will quickly invade if the principal species are in a weakened condition.

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 woody plants, but a good cover of perennial grasses likely minimizes the seed to soil contact the needed 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 woody plants 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 until the brush canopy is so dense that the shade starts to interfere with photosynthesis. The loamy bottomland site can be abused to the point that the perennial warm season grasses thin out and lower succession 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 state 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 states of vegetation depend on the type of grazing management applied over many years, and the rate of invasion and establishment of woody species. After the site crosses the threshold to a lower ecological condition, the effects of seasonal moisture and short-term dry spells become more pronounced. Plant communities that consist of warm-season perennial grasses such as little bluestem 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.

State and Transitional Pathways: 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 as a result of 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 to 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 forbs 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.