Loamy Sand 25-32 PZ

Description

Prior to settlement, the Loamy Sand sites had a savannah appearance with open areas dominated by tall and midgrasses interspersed with scattered mottes of oaks. The Savannah State may have up to 20 percent canopy cover. Relatively frequent fires (7-12 year mean fire return interval) (Frost 1998) maintained the open areas by minimizing shrub cover that was not yet to a fire resistant height. Mature single stemmed hardwoods found in the mottes were long-lived and resistant to ground fires.

Fires were natural or human-induced. When fires were frequent on the savannah, most fires burned only the understory, leaving mottes of trees. Even with proper grazing and favorable climate conditions, lack of fire for 7-12 years will allow trees and shrubs to increase in canopy to reach the 20 percent level that indicates the shift to the Woodland State. This transition is not so much dependent on the degradation of the herbaceous community as on the lack of some form of brush control and the reduction in fire. A major role of grazing management is to build and preserve fine fuel to carry maintenance fires.

Shrub species would increase within the grassland portion of the savannah and within the understory of the mottes following fire. Fine fuels were continuous and of sufficient quantity to allow fire minimizing cover of young brush and trees but not of sufficient quantity to create crown fires that would single stemmed mature trees. Therefore, the savannah would be relatively open for a short period following a fire, shrubs would begin to reestablish reducing the savannah appearance, fire would return in 10 years or less.

Occasionally a site would not burn for a period long enough for trees to grow to a fire resistant stage within the grassland portion of the savannah. As these trees matured, the fine fuel understory would decrease, reducing the ability of fires to grow large enough (and hot enough) to harm mature trees. This long-term lack of fire (25 - 50 years) would allow large trees to fill in open areas shifting the site to a woodland appearance. Once the site had dense tree cover, the site would be resistant to fires and a very resilient woodland community would develop. Grazing management alone cannot maintain the site in the Savannah State.

It was rare that a dense woodland community would shift to a grassland or savannah community. In order to do so, something would have to cause a widespread die-off of mature trees. This could occur due to disease or to a very hot fire that spread to the tree, events that typically only occur every 300 to 1,000 years. Following a severe fire, the site would have a grassland appearance for a few years as shrubs and trees resprouted or grew from seed.

The Savannah State always has the potential for shrub dominance without fire. Mann (2004) discussed the importance of human-induced fire as an important factor in maintaining open grasslands before European settlement.

“Cool”, slow-burning natural fires have become basically non-existent, because they are relatively easy to put out using modern firefighting equipment and techniques. Without fire, the reference Savannah State becomes less resilient. Unless managers practice some method of brush control, shrub species will increase in the grassland portion of the savannah and in the understory of the oak mottes.

Brush control can play the role that natural fires played pre-settlement. However, it is difficult to manage in an ecological and economic matter on a small scale, as this site is rapidly repopulated by shrubs and trees without fire or brush management. Brush control may be prescribed fire, mechanical, chemical or biological control, or targeted grazing (generally by goats, although some instances exist in the Central Basin where exotic wildlife species or overpopulated white-tailed deer reduce woody cover). There are examples of this site being maintained as a savannah with introduced hay meadows and mottes of trees.

Submodel

Description

The Woodland State is characterized by trees, a significant shrub cover, and a shortgrass- understory. The Woodland State often occurs on lands that have been cleared of brush and trees at some point in the past and has regrown. Once invasive woody species begin to establish, returning fully to the native community is difficult, but it is possible to return to a similar plant community.

One factor that creates overgrazing on this site is the failure to adjust the stocking rate downward as woody cover increases. Increased woody cover results in fewer grazeable acres and less forage being available. Unless stocking rates are reduced, the stocking pressure on the remaining forage increases, which increases the likelihood of palatable plants being overgrazed, losing vigor, and being grazed out of the community. At the same time, less palatable plants gain a comparative advantage and increase their representation in species composition.

If retrogression is cattle induced, big and little bluestem, Indiangrass, sand lovegrass, and the more palatable forbs decrease. Silver bluestem, tall and sand dropseed, Texas wintergrass and woody plants are the principal increasers.

If the plant community continues to degenerate, fall witchgrass, hooded windmillgrass, red and tumble and gummy lovegrass, tumblegrass, and threeawns which originally occurred in trace amounts increase to dominating proportions. In lower successional stages, woody plants such as blackjack and post oak, greenbrier, and shinoak, may increase and dominate the site. Mesquite, juniper and catclaw invade the site.

Description

The information for this community comes from experienced field specialists.

Abandoned cropland areas, or cleared areas, are often seeded to historic native species or exotic naturalized grass species such as bermudagrass (Cynodon spp.) or Kleingrass (Panicum coloratum). Herbage production on those seeded to adapted introduced grasses or native grasses reach peak production within a few years if a full stand is established. In this case, herbage production will equal the reference state if species such as big bluestem or switchgrass are seeded. Adapted introduced species plantings may surpass reference production. The practice of including adapted legumes or other forbs will enhance productivity and usefulness, especially for wildlife.

Invasion of the seeded fields by brush species such as mesquite, pricklypear (Opuntia spp.), condalia (Condalia spp.), willow baccharis (Baccharis spp.), Texas persimmon and juniper is common. Drought and reduced soil cover due to cropping and grazing and a nearby seed source trigger the invasions. The shrub ‘seedlings’ that appear in seeded or abandoned fields are true seedlings established by seeds brought in by animals, water or wind. The invading brush must be controlled with grazing management, prescribed burning or other brush management methods or the woody invaders will again dominate.

Many fields, however, have been abandoned and let ‘go back’ to native range or to invading exotic grasses for pasture.