Ecological dynamics

This is a riparian site occurring on low terraces and floodplains along major rivers and streams in this region of the plains. The site does not occur along the dry sandy draws that are associated with more upland terrain. The soils are composed of sandy and silty alluvium that has been deposited as a result of flooding and associated overland flow. Soil development processes are generally limited. The frequency of flooding events varies from periodic to frequent and these events have played a major role in the development of the historic natural plant community. Deposition of sandy alluvium has occurred over time and continues on portions of many Sandy Bottomland delineations. Some of the fluvial terraces located further upland from the streambed are no longer regularly flooded and have become more stable. The development of the plant community depends upon the frequency of major flooding, as well as soil development and the presence or absence of a high water table.

In historic times, most ecologists agree that these prairie river riparian sites in the plains produced scattered timber, mainly cottonwoods (Populus spp.) and a few willows (Salix spp.) along with some shorter shrubs such as plum (Prunus spp.), sumac (Rhus spp.), and willow baccharis (Baccharis salicina). Cottonwoods, willows, and willow baccharis are classified as phreatophytes and make up greater than 60% of all the woody species. Tallgrasses such as sand bluestem (Andropogon hallii), switchgrass (Panicum virgatum), Indiangrass (Sorghastrum nutans), and little bluestem (Schizachyrium scoparium) were the dominant class of herbaceous vegetation in the historic climax plant community (HCPC). Production in the HCPC was moderately high with annual production approaching 3000 pounds per acre of air dry plant material. It should be noted that annual air dry production does not mean standing biomass, as the amount of standing biomass is usually considerably more than the current year’s growth referred to in this description.

In the periods between flooding events, vegetation would establish and increase, but with a major flood, vegetation was often at least partially destroyed and erosion and deposition left a more sparse plant population, mainly in the areas closest to the channel. The actual channel of these streams would shift in location with different areas being more subjected to flooding over time. Under this cycle of flooding and stable periods, fluvial terraces were formed. Over time, some areas were less frequently flooded and became more stable. With stability, the plant community would increase in both diversity and density. Generally, smaller streams have less-developed terraces that are narrower in width. Major rivers may have stream terraces several hundred feet in width.

There is some speculation as to whether flooding events were influenced by overgrazing of the surrounding watershed, contributing to greater runoff. This is possible, but difficult to say definitely. One of the Spanish explorers of the 1600’s described the Canadian River as a rather narrow stream of live water with scattered groves of cottonwoods and with shrubs and tallgrasses growing to the banks. This description is different from the one of the early day cattle drivers who had to cross this treacherous stream in the 1870’s and 80’s. These cowboys often encountered a swiftly running river up to one hundred yards wide, or a wide, sandy bottomed stream with a meandering channel with numerous pockets of quicksand. But in either case, there had not been chronic overgrazing of domestic livestock occurring as no ranching operations were in place in the region before about 1877. Of course, the bison often did graze the range closely, but their migratory habits usually allowed for recovery of the native vegetation.

If the watershed burned off prior to some heavy rains, then increased runoff would surely occur. It is likely that there was considerable variation in the appearance of the larger streams as they meandered eastward. The flooding events were probably quite variable from year to year.

In some cases, impoundments (reservoirs) have been constructed on the major streams which have interrupted the natural frequency of flooding. This has caused a major change in the amount and the nature of the vegetation on this site. If flooding is taken out of the picture almost entirely, then vegetation rapidly becomes established on the floodplain down to the actual channel with woody vegetation often leading the way. Downstream from dams, the amount of all types of vegetation has usually increased, and in many cases, invasive woody species such as saltcedar (Tamarix ramosissima) have invaded and become dominant. Over time, with no major flooding, vegetation of all types will usually increase as the site stabilizes. This is a natural process, but is one that is definitely influenced by human activities such as dams and channels.

The fact that there is quite often a high water table present beneath these streams and the associated fluvial terraces also affects the type and amount of native vegetation present. Generally, willows and cottonwoods take advantage of this presence of water, as do some of the deeper-rooted shrubs and tallgrasses. In general, the presence of phreatophytic woody species coincides with a shallow aquifer. If the ground water happens to be somewhat saline or alkaline, then species (both shrubs and herbaceous plants) tolerant of this characteristic will tend to dominate. Generally speaking, a plant community dominated by a variety of grass species with fewer water-loving shrubs indicates a greater depth to ground water. The depth to water will have just as much or more influence on the plant community makeup as grazing management or the presence or absence of fire.

Grazing of domestic livestock has had some influence on native plant communities on this site. The vegetation on this site is not necessarily more preferred by livestock, but is often utilized heavily because of the protection offered by landscape position and cover, shade in summer, and sometimes because of proximity to live water. If the tallgrasses which often dominate this site are continually over-utilized, they will decrease and mid and short grass species such as dropseeds (Sprorobolus spp.), inland saltgrass (Distichlis spicata), scratchgrass (Muhlenbergia asperifolia), threeawns (Aristida spp.), and annual forbs such as camphorweed (Heterotheca subaxillaris) and the perennial forb western ragweed (Ambrosia psilostachya) will increase. Woody shrubs such as sandbar willow (Salix exigua) and willow baccharis (Baccharis salicina) may also increase on portions of the site where the tallgrasses offer little or no competition. Saltcedar is an introduced shrub/tree that has invaded bottomlands along many of the western streams in the U.S., and occupies significant portions of many Sandy Bottomland delineations in this region. Russian olive (Elaeagnus angustifolius) and common bermudagrass (Cynodon dactylon) may also invade this site. This site usually has a few cool-season grasses such as Canada wildrye (Elymus canadensis) and western wheatgrass (Pascopyrum smithii), along with some winter and early spring forbs which attract livestock during the dormant period for warm-season plants. Increased animal impact on this site can disturb the fragile sandy soil and stimulate annual forbs and sandburs. Inappropriate grazing methods can increase the possibility of downstream pollution. Maintaining good vegetative cover on this site has a positive effect on water quality downstream by reducing sedimentation and increasing aquifer recharge. This site should be carefully grazed and regular deferment periods are recommended.

Natural fire was also an important factor in shaping and maintaining the historic plant community on the Sandy Bottomland site. Because there was often a significant fuel load of tallgrasses present on portions of the site, along with some accumulated dead woody material from shrubs and trees, fire was a predictable visitor. Periodic fires kept the presence of shrubs and trees limited in historic times. There is no doubt that cottonwoods have increased dramatically on many sites due to fire being controlled (as well as flooding events being limited). If the presence of cottonwoods is desired, which is often the case for aesthetic and wildlife reasons, the use of prescribed fire on this site must be carefully planned and judiciously applied, as these trees are quite susceptible to damage by fire. The larger cottonwoods must be protected by mowing underneath the canopy, removing accumulated dead limbs, and otherwise reducing fuel loads immediately adjacent to the trees. Most of the shrubs present on the site are re–sprouting species and are only suppressed by fire. However this suppression may last several years depending upon the species of shrub involved. Occasional burning removes old accumulated plant residues common to the tallgrass species, which renders these plants more palatable to grazing animals. Since this site is adjacent to streams that often have a largely bare sandy streambed, the movement of fire can sometimes be more easily managed.

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.