Ecological dynamics

The Midgrass/Shortgrass Community (1.1) is the Reference Plant Community. This plant community is almost exclusively (>95 percent) warm-season midgrass and shortgrass species. Few perennial forbs are present accounting for less than (<5 percent) of the total site production. Only a trace of woody shrubs will be found. Trees are seldom found on this upland site. Approximate total annual production on this site ranges from 1,200 lbs. in low rainfall years to 2,200 pounds per acre during high rainfall years. Productivity is moderate with most of the production coming from blue grama (Bouteloua gracilis), black grama (Bouteloua eriopoda), sideoats grama (Bouteloua curtipendula), cane bluestem (Bothriochloa barbinodis), plains bristlegrass (Setaria leucopila), Arizona cottontop (Digitaria californica), vine mesquite (Panicum obtusum), sand dropseed (Sporobolus cryptandrus), slim tridens (Tridens muticus) and buffalograss (Bouteloua dactyloides). Lesser amounts of hairy grama (Bouteloua hirsuta), perennial threeawn (Aristida wrightii), fall witchgrass (Digitaria cognata) and sand muhly (Muhlenbergia arenicola) could be found on the site. The few commonly found forbs were croton spp. (Croton spp.), dotted gayfeather (Liatris punctata), scarlet globemallow (Sphaeralcea coccinea), baby white aster (Chaetopappa ericoides), silverleaf nightshade (Solanum elaeagnifolium), yellow spiny daisy (Haplopappus spinulosus), woolly plantain (Plantago patagonica), tansy aster (Machaeranthera tanacetifolia), western ragweed (Ambrosia psilostachya), and annual forbs. The primary woody species that did occur on this site in trace amounts were yucca (Yucca glauca), broom snakeweed (Gutierrezia sarothrae), catclaw mimosa (Mimosa aculeaticarpa), plains pricklypear (Opuntia imbricata), lotebush (Ziziphus obtusifolia), vine ephedra (Ephedra antisyphilitica), and fourwing saltbush (Atriplex canescens).

The site consists of nearly level to gently undulating soils that occur on broad upland plains. Yucca has a tendency to increase on limy upland sites that have had regular spring and early summer deferment for many years with good yucca seed production. Yucca blooms are very palatable to deer, pronghorn and cattle.

Fire played a role in the ecology of this site as well as most other high plains sites. The general role of fire was to sustain the natural grassland and suppress shrubby species. Fire helped to keep a balance between the grasses, forbs and shrubs. However, in the shortgrass region, fire was probably secondary to climate in promoting the reference vegetative state. A drier climate (<20 inches annual precipitation) creates a situation where the subsoil is dry more often than it is wet. Plant roots grow in response to moisture and this dryer climate favors short grasses with fibrous root systems or short rhizomatous grasses. Yucca is a major increaser on this site and natural fire no doubt kept yucca suppressed significantly. Annual forbs are stimulated by fire and diversity is generally increased. Heavy grazing after a fire can have a negative effect if conditions are dry and remain so for an extended period.

Periodic overgrazing and trampling by migrating herds of bison and elk as well as resident herds of pronghorn antelope occurred during drought periods. Bison moved about in large herds over the region somewhat regulated by water sources and fire frequency. However, long rest periods followed once the large herds of bison moved out of the area, allowing the resilient grassland to re-establish and maintain the reference community structure and composition.

Variations in climatic factors, especially the amount and timing of precipitation, greatly influence the productivity of ecological sites and are largely responsible for the fluctuations in the amount of vegetative growth from one season to the next. It is not unusual for fluctuations of greater than 50 percent to occur from one year to another. These types of climatic variation are part of the overall environment in which the reference community developed. However, it needs to be pointed out that long-term drought (4 to 6 years of rainfall 50 percent below the mean) can act in concert with other forces to affect changes in plant communities. For instance, extended drought weakens plants and makes them more susceptible to the effects of overgrazing. Drought conditions coupled with fire can be damaging and need long periods of time to fully recover. Extremely dry summers followed by wet winters can favor cool season annual grasses at the expense of perennial warm season species. A well-adapted, healthy community could better withstand such rigors of drought. However, even they experience damage that would result in some departure from the former stable state. Usually, the departure would be temporary.

When domestic livestock were brought to the plains in the 1870’s, it was largely an open range situation. By 1890, however, most of the area had been fenced and livestock were confined to theses areas continually.

With continuous heavy grazing, no fire, no brush management and/or pest management this site will transition from the reference community to the Midgrass/Shortgrass/Shrub Community (1.2). As livestock and wildlife numbers increase and grazing use exceeds a plants ability to sustain defoliation, the more palatable and generally more productive species decline in stature, productivity and density. The tendency of this site is to become a shortgrass dominant site if long term grazing abuse occurs. This will lead to a decline in the vigor of sideoats grama and other desirable midgrass species. Low vigor blue grama, buffalograss, threeawns, and hooded windmillgrass will dominate the site. Yucca will increase on the site if the grass cover is weakened and the yucca plants make seed for several years. With the weakened grass cover, broom snakeweed will often gain a major foothold on the site. The production of vegetation has shifted from mostly herbaceous vegetation to increasing amounts of woody shrubs. Herbaceous vegetation is still the largest production in this state. Nutrient cycling, the water cycle, watershed protection and biological functions have changed somewhat. This state can transition back to reference with good management practices such as prescribed grazing, chemical brush and pest management. Prescribed burning could be used if the fuel load and conditions allow.

If long-term, heavy grazing continues with no fire or any form of brush and pest management, a threshold will be crossed to the Shortgrass/Shrub/Annuals Community (2.1). In this state, typical vegetation will be perennial threeawns and low vigor, sodbound blue grama and buffalograss. Low quality short grasses such as burrograss (Scleropogon brevifolius), hairy tridens (Erioneuron pilosum), gummy lovegrass (Eragrostis curtipedicellata), and tumble windmillgrass (Chloris verticillata) will become the dominant grass species as the more desirable grasses are removed. Bare areas will increase with annuals filling the voids. The loss of herbaceous cover and increased bare ground encourages accelerated erosion. Broom snakeweed and yucca will increase dramatically with mesquite invading and establishing on many of the limy upland sites. Nutrient cycling, the water cycle, watershed protection and biological functions have been severely reduced.

At this point, the plant community is so degraded that it cannot reverse retrogression without extensive energy and management inputs. Prescribed grazing with rest periods during the growing season, re-seeding with adapted native grass species, chemical and/or mechanical brush management, and some form of pest management. With the reduced amounts of grass fuel, prescribed burning is usually not an option in this state.

In the early 1930’s Lehman lovegrass (Eragrostis lehmanniana), a grass of African origin, was introduced in MLRA 77D as a drought tolerant, easy to establish, introduced grass species. This grass species was used in many grass mixtures and pasture plantings in an attempt to re-seed poor condition rangeland following mechanical brush management and to return old cropland fields to a perennial vegetative state for livestock grazing purposes. This grass is both invasive and persistent; published evidence indicates that variables such as elevation, summer precipitation, winter temperatures, and soils impact its abundance and distribution. Limy upland sites in a weakened state near established areas of Lehman lovegrass may become invaded by this grass. Presently, several thousand acres of loam, clay loam and sandy loam sites have been invaded in MLRA 77D to the point that Lehman lovegrass is the dominant grass species with few if any native species remaining. The resulting plant community is a Lehman Lovegrass/Shrub Dominant Community (3.1). Once this lovegrass has become well established, returning the site to the reference state would be expensive and generally not very successful or practical. Prescribed burning for seedbed preparation purposes may be necessary to remove excessive amounts of plant biomass. Moderate to heavy mechanical brush management, heavy seedbed preparation and re-seeding to a native grass mixture would be required. The application of herbicides can be effective to reduce competition from this lovegrass species, but there is only a narrow time of treatment opportunity. Since this grass species has become naturalized much like K.R. bluestem has in Central Texas, it is unlikely that it will disappear through any natural processes such as competition from native species.

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.