Granite Gravel 25-32 PZ

Description

The Savannah State consists of two communities: the Midgrass Savannah Community (1.1) and the Oak Savannah Community (1.2). The Midgrass Savannah Community occurred on this ecological site in a dynamically shifting mosaic over time with the Oak Savannah Community.

The majority of production occurs in late spring and early summer when temperatures and moisture are typically most suitable for growth. As conditions become warmer and drier, grasses become dormant and substantial litter accumulation occurs, making the site prone to fire. Recurrent fire (less than 10 years intervals) favors the dominant grasses and suppresses woody plants. In years without fire, leaf litter decomposes and adds organic matter to the soil, thus enhancing its fertility and water holding capacity. Prescribed grazing accelerates the process. The dominant grasses are productive below ground and are deeply rooted. Extensive root systems bind the soil to minimize erosion while enabling the dominant grasses to access stored soil moisture.

Prior to settlement, Granite Gravel sites had a savannah appearance with open areas dominated by midgrasses (little bluestem and sideoats grama) interspersed with scattered mottes dominated by oaks. The Midgrass Savannah (1.1) may have up to 20 percent canopy cover while the Oak Woodland will have more than 20 percent woody canopy. Relatively frequent fires (7-12 year mean fire return interval) (Frost 1998) maintained the open areas by killing shrubs that were not yet to a fire resistant height. Mature hardwoods found in the mottes were long-lived and resistant to ground fires.

Fires may have been natural or human-induced. When fires were frequent on the savannah, most fires burned only the understory and pruning, leaving mottes of trees. Even with proper grazing and favorable climate conditions, lack of fire for 8-15 years will allow trees and shrubs to increase in canopy to reach the 20 percent level that indicates the shift to the Oak Savannah Community (1.2). This transition is not dependent on degradation of the herbaceous community, but on the lack of some form of brush control.

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 to slow the establishment of young brush and trees but not of sufficient quantity to create crown fires that would scorch 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, this fire would slow the growth of young shrubs and trees without reducing the cover of mature trees. This allows for returning the savannah appearance and shifting species composition back to dominance by little bluestem and other grasses.

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 reduce the cover of 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 is resistant to fires and a very resilient woodland community would develop.

The absence of fire, the Oak Savannah Community (1.2) dominated the site with a near closed canopy stand of hardwoods, including oak (Quercus spp.) and pecan (Carya spp.). The two communities in the Savannah State shifted between one another depending on the frequency and intensity of fire, grazing, and drought. The primary influence on the understory is grazing management and the primary influence on the overstory is fire. This allows the understory and overstory to react independently, i.e., trees can increase to the point where they dominate a site even if the understory component remains vigorous and intact. Grazing management alone cannot maintain the site in the Midgrass Savannah Community (1.1).

It was rare that a dense woodland community would shift to a grassland or savannah community. A die-off could occur due to disease or to a very hot fire that spread to the tree crowns and killed mature trees, 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.

Shrubs and trees comprise a portion of both plant communities in the Savannah State (1.0), hence woody propagules are present. The Savannah State always has the potential for shrub dominance without fire. Mann (2004) discussed the importance of human-caused fire as an important factor in maintaining open grasslands before European settlement.

The relationship between the two communities in the Savannah State remains similar post-settlement. However, natural fires become less frequent and less widespread as human population density increases. “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 community 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 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). The savannah is more often observed with mowing or haying than with grazing management. There are examples of this site being maintained as a savannah with introduced hay fields and mottes of trees.

Dominant plant species

1. post oak (Quercus stellata), tree

2. little bluestem (Schizachyrium scoparium), grass

3. sideoats grama (Bouteloua curtipendula), grass

Submodel

Description

The Shrubland State is characterized by trees, a significant shrub cover, and a shortgrass- understory. Two communities represent this state and are distinguished by the amount of shrubs present. The Shortgrass Savannah Community (2.1) is characterized by having less than 25% canopy cover by woody species. The Shrubland Community (2.2) is characterized by having more than 25% woody canopy cover. The understory may be similar in both. Both communities in the Shrubland State have typically lost the savannah appearance.

The hardwoods that made up a portion of the plant community in the Savannah State (1.0) may or may not be present in the Shrubland State (2.0). The transition to the Shrubland State will not cause a decrease in the number of hardwoods. However, the Shrubland State often occurs on lands that have been cleared of brush and trees at some point in the past. Trees were removed for lumber or firewood, in some cases to clear the land for pasture or farming. Rootplowing had the same effect as tillage, converting the site to a grassland immediately following plowing but leaving the site subject to rapid invasion by fast-growing shrub species. This transition may respond like agricultural conversion and may have been accompanied by shifts in soil chemistry and structure. Rootplowing is likely to shift the community to the Oak Savannah Community (2.2). 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.

The understory of the Shrubland State tends to be dominated by shortgrasses and lower-palatability forbs. The communities in the Shrubland State have a degraded herbaceous community when compared to the Savannah State. This is generally a result of long-term improper grazing management.

One factor that leads to overgrazing on this site is the failure to reduce the stocking rate as woody cover increases (loss of grazeable acres). Increased woody cover results in 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.

Dominant plant species

1. mesquite (Prosopis), shrub

2. juniper (Juniperus), shrub

3. red grama (Bouteloua trifida), grass

4. hairy grama (Bouteloua hirsuta), grass

Submodel

Description

This state is characterized by a single community, the Eroded Community (3.1).

The Eroded State has the potential to be a terminal state. Due to the relatively high risk of severe soil erosion of the soil type, this site can erode to the point where there is a loss of soil functionality. When this level of erosion occurs, the site loses soil structure, soil fertility, organic matter, and/or soil microflora. There are examples of the loss of the A and B horizons and some with the soil eroded to bedrock. Once the site loses soil horizons or soil functions, it is very difficult or impossible to return the site to one of the other States, resulting in State 3 being a terminal state.

Steeper portions of this site are subject to severe erosion when the plant cover is disturbed. Some examples of the site have eroded to the granite bedrock.

Dominant plant species

1. mesquite (Prosopis), shrub

2. sumac (Rhus), shrub

3. whitebrush (Aloysia gratissima), shrub

Submodel