Ecological dynamics

The information contained in the State and Transition Diagram (STD) and the Ecological Site Description was developed using archeological and historical data, professional experience, and scientific studies. The information presented is representative of a very complex set of plant communities. Not all scenarios or plants are included. Key indicator plants, animals and ecological processes are described to inform land management decisions.

The Draw Ecological Site is found on second bottoms or on wide, gently concave drainage ways. The site often receives additional moisture as runoff from adjacent slopes and rarely overflow of the drainage way. The deep, fine textured soils often have alluvium or colluvial layers of soils on the surface which have been deposited more often by runoff from adjacent sites but sometimes overflow of the drainage way.

The reference plant community found on the site by settlers in the early 1800’s is assumed to have been a Midgrass Prairie Community (1.1) with occasional trees and shrubs scattered throughout the drainage way. This “Reference” plant community evolved under the influence of occasional flooding, periodic long droughts, grazing by bison, pronghorn antelope and other herbivores and frequent wildfires. It is postulated that fires occurred as often as at four to six-year intervals in this region (Frost 1998) prior to European settlement. Fire was likely the most influential disturbance in shaping the vegetation into an open Midgrass Prairie Community (1.1). It interacted with the climatic regime and intermittent heavy grazing to favor grasses over woody plants and forbs (Brown and Smith 2000). Midgrasses, such as vine-mesquite , sideoats grama, Arizona cottontop characterized the Midgrass Prairie Community. Texas wintergrass and western wheatgrass were also common midgrasses. Tobosa occurred in small amounts in the western or drier sides, of the MLRA but was absent east of the 25-inch rainfall line. Sedges and tallgrasses including little bluestem, sand bluestem and switchgrass occupied the infrequent depressions and wetter areas along the drainage. Trees, such as hackberry, American elm, Osage orange, western soapberry and shrubs and vines were scattered along the drainage or clumped where depressions provided extra water and protection from fire. See the “Species Composition List” table below for the estimated species composition and production values.

The Midgrass Prairie Community (1.1) was relatively stable and resilient within the climate, multi-species grazing and fire regime until European settlement in the 1850s (Milchunas 2006). By the early 1800s horses, cattle and sheep that had escaped from early Spanish missions had begun putting increased grazing pressure on the plant community. European settlement in the mid 1800’s brought elimination of the bison herd, reduction of the antelope and deer herds, removal of the American Indian and a large increase of domestic livestock. The development of the windmill and barbed wire fencing during the 1870s helped spread overgrazing by livestock throughout the region. Overstocking by domesticated livestock induced a reduction of more palatable grasses and forbs and allowed less palatable, or more grazing resistant, grasses and forbs increase. Total herbage production declined as grazing resistant shortgrasses and forbs began replacing the midgrasses and forbs. Texas wintergrass, meadow dropseed and buffalograss were early increasers under grazing. There was a concomitant decline in vegetative ground cover, mulch and soil organic matter and an increase in bare soil and runoff. The shift in composition of the plant cover and decline in soil properties favored woody plant encroachment. This, along with the reduction in intensity and frequency of fires, allowed invasion of species from adjacent sites or the increase of more grazing resistant endemic species. Under the above scenario, the Midgrass Prairie Community (1.1) transitioned into a Mixed-grass Prairie Community (1.2) where grasses dominated but the encroaching woody species contributed an increasing amount to the annual production of the plant community.

If the Mixed-grass Prairie Community is continuously overgrazed and fire is excluded the transition toward woody plant dominance continues. The primary encroaching woody species are mesquite and pricklypear. Salt cedar and juniper can also become invasive on this site. Tobosa (in the west), buffalograss and other unpalatable or more grazing resistant grasses increase while palatable historic forbs and midgrasses decline. Grass cover, litter and soil organic matter decline as bare ground, erosion and other desertification processes increase. When the woody plant component reaches approximately 15% percent of the annual production, grazing management strategies, such as rest from grazing alone, generally will not reverse the transition to shrubland. A combination of proper grazing, timely prescribed burning and selective control of fire resistant woody species can be successful in maintaining the grass dominant community, however. With continued livestock grazing and no brush management the Mixed-grass Prairie Community (1.2) transitions into a Shortgrass/Mixed-brush Community (2.1), where shortgrass and cool-season grasses persist but woody plants begin to dominate the structure and function of the plant community.

Mesquite and pricklypear usually characterize the woody cover of the Shortgrass/Mixed-brush Community (2.1), but other woody species are present. There may be occasions where western soapberry, hackberry or elm form thickets. Bumelia, pricklyash and wolfberry are characteristic understory shrubs. American elm and hackberry trees increase in size, but are infrequent. The grass component is a mixture of shortgrasses, midgrasses, cool season grasses and low quality forbs, initially. Buffalograss and blue grama are persistent initially but with continued livestock overgrazing they are gradually replaced by less palatable species such as threeawn, white tridens and meadow dropseed. Cool-season grasses, such as Texas wintergrass, are persistent. Japanese brome and other winter annuals occupy the increasing amount of bare ground, especially following wet winters. During this stage, the transition to shrubland can be reversed, with considerable effort, by mechanical and/or chemical brush control methods and prescribed grazing management that provides fine fuel loadings necessary for prescribed burning. Prescribed burning generally does not kill mesquite once plants reach >2 years of age, but fire can suppress mesquite of any age if the fire can cause top kill. Slick-bark mesquite is more susceptible. Prescribed burning/chemical control systems have been developed to aid in enhancing and utilizing this vegetation type.

If abusive grazing by livestock continues and brush control practices are not applied, the woody component will increase in size and density. The grassland component will not produce enough fine fuel for fires to effectively suppress the woody plants when the woody plant canopy cover reaches about 30 to 35 percent. At this point, the site completes the transition into a new plant community type, the Mixed-brush/Shortgrass/Annuals Community (2.2). This plant type is dominated by mesquite and mixed-brush to the exclusion of most reference herbaceous species. Buffalograss, threeawns, tobosa, alkali sacaton and annuals persist in the woody plant interspaces. Texas wintergrass is common under protection of shrubs and trees. Once canopy cover exceeds 35 to 50 percent woody plant cover, forage production is very limited except in wet periods when annuals provide extra forage. Shortgrasses and cool-season grasses and forbs are present but sparse due to shading and competition from the woody plants. Mesquite and understory brush continue to increase in size and density regardless of grazing management. Desertification, including soil and fertility erosion, continues in the interspaces until maximum ground cover by woody species is approached. Once shrub cover reaches potential, the hydrologic processes, energy flow and nutrient cycling stabilize under the woody vegetation environment (Thurow 1991).

Major expense and energy are required to restore the Mixed-brush/Shortgrass/Annuals Community (2.2) back to a Grassland state. Restoration of site in this stage is very difficult to accomplish because of soil characteristics. An integrated approach is required. Mechanical or herbicidal treatments such as dozing, individual plant treatments (IPT), herbicide spraying and range planting followed by grazing deferment, prescribed grazing and prescribed burning, are essential for the site to return to near the postulated reference community. The brushy species, namely mesquite, are hard to control with herbicides on this site. Re-invasion occurs due to the residual seed bank. Mechanical control such as grubbing or root plowing can destroy the perennial grass cover and more often than not, annuals prevail for two or three years, even with reseeding. The restoration process may take several years of repeated treatments. Therefore, maintaining the site in at least the Shortgrass/Mixed–brush Community (2.1) stage, or better, the Mixed-grass Prairie (1.2) stage, through proper stocking and brush management, including the use of prescribed burning, is recommended.

State and Transition Diagram:
A State and Transition Diagram for the Draw (R078CY099TX) site is depicted below. Thorough descriptions of each state, transition, 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 natural 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.

Composition by dry weight and percent canopy cover are provided to describing the functional groups. Most observers find it easier to visualize or estimate percent canopy for woody species (trees and shrubs).

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.