Ecological dynamics

The historic aspect of this site is a hardwood savanna with a tall and midgrass understory with forbs. Undoubtedly though, the historic landscape also had some areas that were very thick and difficult for early travels to cross. Historical writings refer to this area as the “Cross Timbers”, because it was so difficult to cross and these areas were extensive.

Historically, this site was inhabited by grassland wildlife species such as bison, elk, white-tailed deer, bear, grassland birds, bob white quail, and small mammals. Also present were predators such as wolf, coyote, and lion.
Fire, set by lightning and Native Americans, along with herbivory were major disturbances that shaped and maintained this community. Fire played a role in the ecology of the site as is true for most of the grasslands of the Great Plains. The main effect of fire on this site was to reduce the less fire tolerant trees and woody shrubs and cactus (Opuntia spp.) and favor the grasses and forbs. Fire would improve nutrient quality of the plants, attract grazers, diversify the plant community and recycle nutrients.

It is perceived that bison would follow the fires which burned about every 3 – 5 years. They would heavily graze post-burn until forage or water was scarce, then move to other burned areas. The interaction of fire and grazing create diversity in this site for a year or two post burn. The site could burn any time of year that fuel was available and there was an ignition source. The heavy grazing impact would remove plant material permitting sunlight to reach the soil surface. This provided opportunity for many short lived forbs and grasses to germinate which diversified the landscape. Grazers would also recycle nutrients to the soil surface through dung. Even though the grazing would have been very heavy, it was of short term and followed by recovery period. The heavily grazed areas, because of healthy root systems and soil litter, would then recover to pre-burn/graze condition in a few years until burned again. Over time, the grasses would increase in dominance with the forbs and diversity decreasing.

Long-term droughts that occur only three to four times in a century will effect some change in historic 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.

The collective influence of fire, grazing, and weather extremes created a “shifting mosaic” in the site that would in all probability had mature grasses mixed in with low seral stage landscapes. Over time, the low seral stage landscapes would recover until a grazing/burning disturbance would occur again.

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.
When a brush canopy 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 sometimes reseeding.

A State and Transition Model for the Sandy Loam Savanna Ecological Site (R084BY174TX) is depicted in Figure 1. Descriptions of each state, transition, plant community, and pathway follow the model. Experts base this model on available experimental research, field observations, professional consensus, and interpretations. It is likely to change as knowledge increases.

Plant communities will differ across the MLRA because of the naturally occurring variability in weather, soils, and aspect. The Reference Plant Community is not necessarily the management goal. Other vegetative states may be desired plant communities as long as the Range Health assessments are in the moderate and above category. The biological processes on this site are complex. Therefore, representative values are presented in a land management context. The species lists are representative and are not botanical descriptions of all species occurring, or potentially occurring, on this site. They are not intended to cover every situation or the full range of conditions, species, and responses for the site.

Both percent species composition by weight and percent canopy cover are described as are other metrics. Most observers find it easier to visualize or estimate percent canopy for woody species (trees and shrubs). Canopy cover can drive the transitions between communities and states because of the influence of shade and interception of rainfall. Species composition by dry weight is used for describing the herbaceous community and the community as a whole. Woody species are included in species composition for the site. Calculating similarity index requires use of species composition by dry weight.

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.