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 reference plant community consists of a mixture of short and midgrasses with tallgrasses being few. Some perennial forbs are present with a few annual forbs and a few scattered woody shrubs. Productivity is moderate with most of the production coming from blue grama (Bouteloua gracilis) and sideoats grama (Bouteloua curtipendula). Other commonly found grasses are: hairy grama (Bouteloua hirsuta), sand dropseed (Sporobolus compositus) and perennial three-awn (Aristida purpurea). Vine mesquite (Panicum obtusum) and western wheatgrass (Pascopyrum smithii) grow in runoff concentration areas, and small pockets of sand bluestem (Andropogon hallii) or Indiangrass (Sorghastrum nutans) are sometimes found. Little bluestem (Schizachyrium scoparium) will occur in small amounts where the soil becomes more shallow. The more commonly found forbs are: dotted gayfeather (Liatris punctata), scarlet globemallow (Sphaeralcea coccinea), slimleaf scurfpea (Psoralidium tenuiflorum), lyreleaf greeneyes (Berlandiera lyrata), Engelmanndaisy (Engelmannia peristenia), baby white aster (Chaetopappa ericoides), half shrub sundrop (Calylophus serrulatus), trailing ratany (Krameria lanceolata) and annual forbs. The main woody species found are yucca (Yucca spp.) and broom snakeweed (Guterrezia sarothrae), with occasional catclaw mimosa (Mimosa aculeaticarpa var. biuncifera) and occasional juniper (Juniperus spp.) where seed sources are close. Shrubs and trees are few. The site occurs on slightly to moderately sloping areas on upland plains where slight geologic erosion may have occurred and the soils are somewhat “thinner” than those of the associated deep hardland site that occurs on more level terrain. The increase in calcium carbonate content throughout the soil profile is a major factor in the amount of sideoats grama growing on this site. This is the major difference between the closely associated Deep Hardland site that is dominated by blue grama instead of sideoats grama. The forb component is more apparent in years of increased rainfall. Pronghorn antelope favor this site because of the variety of forbs present. Cryptogamic crusts are more common on this site than on the nearby hardland sites. The production of this site is quite close to that of the hardland sites. The two main indicator plants on limy upland sites are sideoats grama and yucca. Yucca has a tendency to increase on limy upland sites that have had regular spring and early summer deferment for many years because of the yucca seed production under these scenarios. Yucca blooms are extremely palatable to deer, pronghorn and cattle.

Fire played a role in the ecology of this site, as is true for practically all high plains sites. The general role of fire was to sustain grassland and keep shrubby species from becoming the dominant species. However, in the shortgrass region, fire was probably secondary to climate in shaping the reference community. A dryer climate (< 20 inches precipitation) means that the subsoil is dry more often than it is wet. Plant roots grow in response to moisture so naturally dryer climates favor shortgrasses with fibrous root systems or short rhizomatous grasses. Fire, no doubt, suppressed yucca, increased diversity and stimulated annual forbs. Heavy grazing after a fire can have a negative impact on the plant community if conditions are dry and remain dry for an extended period of time.

Grazing by large herbivores certainly played a major role in perpetuating the grasslands of the plains. Bison moved about in large herds over the region. Bison movement was somewhat defined by water availability and fire. Large areas of country were grazed closely; then the animals moved on to new grazing. Recovery time for the vegetation was sufficient for the grassland vegetation to complete a life cycle in most cases. Other large animals such as pronghorn antelope and elk also grazed the plains at will. 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 was confined within these fenced areas. With continuous pressure on the vegetation, the ranges began to decline in ecological diversity and productivity. Early day ranchers had little information on proper stocking rates, and in many cases, more animals were confined within areas that could not sustain them. Long-term grazing abuse, especially on Limy Upland sites, will lead to a decline in vigor of sideoats grama and other midgrass species. Blue grama will increase because it is more adapted to grazing pressure. With constant grazing pressure, the blue grama will become sod bound and loose its bunch grass appearance. Yucca will increase on the site if the grass cover is weakened and yucca is allowed to make seed for several years. Broom snakeweed is cyclic in nature and produces large quantities of seed that often germinate in the fall and winter under high moisture regimes (Sosebee, 1985). When densities of broom snakeweed become high enough on this site, it can digress from its cyclic nature and become both dominate and permanent. When sideoats grama and blue grama are perpetually in states of low vigor, bare areas open up and become vegetated with perennial three-awn, buffalograss and annuals forbs. There can be a shrub/half shrub dominant stage on the Limy Upland site. Dominant shrubs can be yucca, and in some cases broom snakeweed, providing a seed source for these species is available and the grass cover is opened up.

Variation in climatic factors, especially the amount and timing of precipitation, greatly influences 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 production fluctuations of greater than 50 percent to occur from one year to the next. These types of climatic variation are part of the overall environment in which the reference plant community developed. However, long-term drought (4 to 6 years of rainfall 50 percent below the mean) can act in conjunction 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. Extremely dry summers followed by wet winters can favor cool-season annual grasses that thrive while perennial warm-season species suffer. A well adapted, healthy community such as the reference plant community could better withstand such rigors of drought. However, even reference communities experienced damage that would occur on occasion which results in some departure from a former stable state. Usually, the vegetation would recover in time and the reference community would be restored.

Most of the plant species that grow on the site are drought tolerant warm-season grasses. Warm season grasses have C4 metabolism and are best adapted to hotter, dryer situations. The small component of cool-season grasses (Canada wildrye and western wheatgrass), are C3 plants. The herbaceous species may be either C3 or C4 metabolism plants. Shrubs are C3. C3 plants tend to grow best in lower light intensity and cooler temperatures. For the site to function properly, plant community integrity is essential.


PLANT COMMUNITIES AND TRANSITIONAL PATHWAYS (Diagram):
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.

As a site changes in the structure and makeup of the plant community, the changes may be due to management, natural occurrences, or both. At some point in time, thresholds may be crossed. This means that once changes have progressed to a certain point, the balance of the community has been altered to the extent that a return to the former state is generally not possible, that is, not possible without some form of external energy being applied in order to make the community respond in some specific way. These changes in plant communities occur on all ecological sites. Some sites are more resistant to change than others. Also, some sites seem to be more resilient and able to heal more easily than other sites. Usually, changes in management practices alone, such as altering grazing methods, will not result in restoration of former vegetative states. Once a threshold has been crossed, an example of an energy input that might be necessary to effect change might be the implementation of chemical brush management and complete growing season rest to reduce the domination of woody shrubs and increase the production of perennial grasses and forbs. This might have to be done more than once and could take several years. Such a vegetative shift could not be accomplished by regulation of grazing alone. The amount of energy required to effect a change would depend on the present vegetative state and the desired state.