Ecological dynamics

The historic climax plant community (HCPC) for the Clay Loam site is best characterized as shortgrass dominant with a few midgrasses, few forbs, and scattered shrubs. Midgrasses occur in areas that receive extra moisture. Forbs vary greatly from year to year depending on amount and timing of precipitation. Shrubs are mostly scattered with isolated small pockets of thicker cover. The site would be classified as a grassland in the HCPC, and can shift toward a shrubland with the invasion of woody shrubs. The HCPC (1.1) consists of blue grama (Bouteloua gracilis) as the predominant grass species with a significant amount of galleta (Pleuraphis jamesii). Smaller amounts of buffalograss (Bouteloua dactyloides), sideoats grama (Bouteloua curtipendula), vine mesquite (Panicum obtusum), and other mid- and shortgrasses occur. The forb component consists of both perennial and annual species including rushpea (Hoffmannseggia jamesii), prairie coneflower (Ratibida columnifera), lyreleaf greeneyes (Berlandiera larata), wooly paperflower (Psilostrophe villosa), dotted gayfeather (Liatris punctata), plains zinnia (Zinnia grandiflora), baby white aster (Chaetopappa ericoides), scarlet globemallow (Sphaeralcea coccinea), wooly plantain (Plantago purshii), and annual broomweed (Amphiachyris dracunculoides). Suffrutescent half shrubs such as broom snakeweed (Gutierrezia sarothrae) and rayless goldenrod (Isocoma wrightii) are also present in small quantities and are included in the forb table in the plant community list. The major shrub species found on the site are winterfat (Krascheninnikovia lanata), Mormon tea (Ephedra sp.), mesquite (Prosopis glandulosa), four-wing saltbush (Atriplex canescens), and cactus species, namely, cholla (Cylindropuntia imbricata) and pricklypear (Opuntia phaecantha). In the HCPC, the mesquite is very scattered but has generally increased in many Clay Loam locations to a moderate canopy. The only tree species found on the site are occasional hackberry (Celtis laevigata) and scattered plants of oneseed juniper (Juniperus monosperma). Juniper will sometimes form small colonies of thicker cover. The shrub community does not exceed 10 percent of the total plant community production in the HCPC; and is usually less than 5 percent. The productive capacity of the site is moderate and it is grazed readily by domestic livestock.

The soils associated with this site are derived from red bed sediments and are usually quite high in silt content. This tends to make the soils susceptible to surface crusting and water erosion if a good vegetative cover is not maintained. On sites subjected to heavy continuous grazing for long periods where considerable cover has been removed, it is not uncommon to find numerous rills and small gullies forming. Infiltration on this site is moderate in the HCPC because overall vegetative cover is at a maximum, but evaporation and runoff can become excessive if cover decreases past a certain threshold. Gullies can be started very quickly by any major livestock trailing to and from water sources. As cover decreases, patchy bare areas become noticeable and annual weedy species will establish. Moderate levels of grazing with occasional rest usually maintain sufficient perennial cover to keep the site stable.
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In general, the shortgrass species present on the Clay Loam site are drought-tolerant and fairly resistant to grazing pressure, which are desirable characteristics from a site stability standpoint. Continuous heavy grazing will cause the blue grama to lose vigor and these plants may become somewhat sod-bound in appearance. Galleta may increase somewhat at the expense of the blue grama due to less grazing pressure occurring on the galleta. Less overall cover increases runoff and decreases infiltration, therefore increasing the droughty tendencies of the site. With further abusive grazing, some slight pedestalling of bunch grasses begins to occur.

The midgrasses such as sideoats grama and vine mesquite will decrease, leaving low-vigor shortgrass species with a large increase in annual forbs. Overall production will become less and eventually bare areas will appear and increase in size. Surface sheet erosion will increase and annual forbs will compete directly with perennial grasses for both moisture and nutrients. Finally, small gullies and rills become numerous as the low vigor short grasses and annuals offer poor soil protection.

Woody shrub encroachment on this site often occurs simultaneously with the above described decline in vigor and production of perennial grasses, but not always. It is fairly common for mesquite to invade the site with minimal deterioration in the grassland community. Mesquite encroachment is basically a function of seed dissemination, largely through cattle depositing the beans in their manure. Historically, there were small amounts of mesquite present in this region, but certainly it was not the widespread species dominating plant communities as is often the case today. The invasion of woody shrubs and cacti seems to be linked to long-term drought periods in conjunction with poor management practices. Woody plants seem to always increase during wet years following drought periods. During droughts, the grass cover is weakened by severe moisture stress and by over-utilization, and when effective rains finally occur, bare ground is exposed and the perennial grasses are slow to respond due to poor health and vigor. Species with invasive tendencies are better able to establish because of reduced competition. The stronger vigor and overall health of the perennial grasses in the historic plant community enabled these plants to better withstand the rigors of drought. Once woody shrubs such as mesquite establish, strict prescribed grazing along with measures that ac


tually kill or greatly suppress the encroaching shrubs will be necessary in order to shift the community back toward that of the HCPC. The effects of these remedial measures may take several years to be fully realized, and are not of a permanent nature. Once established, shrubs hold a definite competitive advantage over grasses and forbs in a semiarid climate. Most of the woody shrubs in this region are re-sprouters, which renders them difficult to control.

All the sites in this MLRA were historically grazed, and/or browsed by bison, elk, pronghorn, and mule deer (along with numerous small herbivores such as prairie dogs, rabbits, ground squirrels, etc.) in pre-settlement times. The habits of the larger herbivores were semi migratory and after grazing an area, they moved on to other localities where grazing resources were more attractive. Grazed areas received rest naturally and generally the recovery periods following grazing were adequate for vegetation to regain vigor and replenish reserves. The same locations might not have been grazed for several months, and perhaps even years, depending on rainfall patterns and animal movements. Grazing by these large native animals was often intense and animal impact on the range was significant, but the recovery periods were adequate for the healing of both the soil and the vegetative resources. The general lack of streams and surface water in this region may have also regulated grazing of animals such as bison. These herds required large amounts of water and needed to drink with some regularity. Regions to the east of this MLRA likely received more impact from bison because of more dependable water. In general, native grazers played a part in the ecology of the plant communities, providing a graze-rest scenario that allowed those communities to persist. Of course, climatic fluctuations, including drought periods influenced native vegetation as much as any one factor, and there was constantly some minor shifting of species taking place, and production varied from year to year. With settlement of the area, native grazers were replaced with domestic livestock.

The advent of the ranching industry saw a more continuous system of grazing implemented; and with barbed wire, the creation of smaller grazing units allowed for more intense use of the resources. Cattle being transported in from brush-infested regions began to furnish the mechanism for mesquite proliferation. Overstocking was a problem in the late 1800’s and early 1900’s and, unfortunately, it still is today in some areas. Much of the rangeland abuse that occurred was due to lack of knowledge of the capability of the resources. Given this history of past grazing management, an understanding of the capability the resource, and the establishment of a realistic and practical resource management plan should be the goal of all involved in rangeland decision making today.

Natural fire also played a part in the development of plains grasslands. It is generally accepted that fires occurred naturally at least every 8 to 10 years and possibly even more often on some sites. The majority of these natural fires were set by lightning. The sites most likely to burn were those where fuel was most abundant. The Clay Loam site in HCPC usually produces sufficient fine fuel to carry a fire, therefore it is assumed that fires periodically occurred. How sweeping these fires may have been could depend on the overall terrain locally – that is to say – all the sites collectively within a certain area. There are some sites in this MLRA that possess sparse cover and contain certain physical features such as rocky hills, bare soils, and gullied landscapes. These areas had some effect on the continuity of fires. Natural fires often tended to burn in a patchy pattern. The most far-reaching effect of natural fire on native vegetation was to suppress certain woody species and cacti and to help maintain a grassland state. Fires often stimulated forb growth, thereby adding to site diversity at times. Fire influenced wildlife movements as it affected grazing or browsing habits as they were attracted to the fresh growth following a burn. It must be pointed out that naturally occurring fire as it affected the plant communities of pre-settlement times is much different from the way prescribed (or natural) fire will affect a plant community that has deteriorated in ecological condition. Ranges in poor ecological condition may actually be damaged by fire, while healthy plant communities may respond positively. Prescribed fire is a useful tool in range and wildlife management in certain situations. Plant communities can certainly be manipulated by using fire. Most plains grass species are fairly tolerant of fire. Some species of woody plants can be suppressed by proper use of fire provided sufficient fuel is present. Greater plant diversity may result from a properly applied burn. On certain sites where coarser grasses such as tobosa (Pleuraphis mutica), galleta (Pleuraphis jamesii), alkali sacaton (Sporobolus airoides), and giant sacaton (Sporobolus wrightii) are found, fire can increase their palatability. Some species (and ecological sites) are more tolerant of fire than others, and this is a most important fact to realize in applying fire. Particular attention must be paid to environmental factors such as soil moisture, temperatures, and season, as well as wind speed, wind direction, and relative humidity. The inability to predict rainfall post-burn is the greatest problem faced in the use of prescribed fire in this MLRA.

Soils, climate, grazing history, and natural fire all worked together to shape the natural plant community on this site; and the interaction of these factors must be understood in order to best formulate an appropriate management scenario to sustain the soil and plant and animal resources.

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.