State 1
Savannah State - Reference

Community 1.1
Tallgrass/Oak Savannah Community

Figure 9. 1.1 Tallgrass/Oak Savannah Community

The Tallgrass/Oak Savannah Community (1.1) is the reference community and is characterized as a Texas oak and live oak savannah with 20 to 30 percent woody species canopy cover.

Little bluestem dominates the herbaceous component of the site, with big bluestem and Indiangrass present as significant components. Other important grasses are sideoats grama, tall grama (Bouteloua hirsuta var. pectinata), tall dropseed, slim tridens, silver bluestem, and green sprangletop (Leptochloa dubia). Forbs commonly found on the site include dotted gayfeather (Liatris punctata), awnless bushsunflower (Simsia calva), Maximilian sunflower (Helianthus maximiliani), Engelmann’s daisy (Engelmannia peristenia), halfshrub sundrop (Calylophus serrulatus), and bundleflower (Desmanthus spp.). Other shrub and tree species include species of sumac (Rhus spp.), bumelia (Sideroxylon spp.), elbowbush (Forestiera pubescens), juniper, ash (Fraxinus spp.), and cherry (Prunus spp.), along with Texas kidneywood (Eysenhardtia texana), agarito (Mahonia trifoliolata) and yucca (Yucca spp.). Ashe’s juniper (Juniperus ashei) was historically restricted to rocky ledges and shelves where it was protected from fire.

The reference savannah community will transition to the Midgrass/Oak Savannah Community (1.2) under the stresses of improper grazing. The first species to decrease in dominance will be the most palatable and/or least grazing tolerant grasses and forbs (Indiangrass, big bluestem, Engelmann’s daisy). This will initially result in an increase in composition of little bluestem. If improper grazing continues, little bluestem will decrease and midgrasses such as silver bluestem and sideoats grama will increase in composition. Less palatable forbs will also increase at this stage.

Without fire and/or brush control, woody species on the site will increase and the site will transition to the Shrubland State. This can occur with or without the understory transitioning to the Midgrass/Oak Savannah Community (1.2). This transition can occur without degradation of the herbaceous community from dominance by tallgrasses to dominance by midgrasses.

Because the woody species that dominate in the Shrubland State are native species that occur as part of the Savannah State, the transition to the Shrubland State is a linear process with shrubs starting to increase soon after fire or brush control ceases. Unless some form of brush control takes place, woody species will increase to the 35% canopy cover level that indicates a state change. This is a continual process that is always in effect. Managers need to detect the increase in woody species when canopy is less than 35% and take management action before the state change occurs if they want to maintain the historic plant community. Shrubs begin to increase soon after fire or treatment; slowly at first, followed by a rapid transition to the Shrubland State. The drivers of the transition (lack of fire and lack of brush control) constantly pressure the system towards the Shrubland State.

This site typically has heavy grazing pressure on lower slopes and less steep slopes. The steepest portion of the site will receive little grazing pressure from cattle, but may have significant browsing by deer and goats. Once slopes exceed 10%, especially when the surface is covered with rocks, cattle use decreases markedly.

The soils of this site are shallow and are usually gravelly or stony. In the absence of plant cover and litter, the soil crusts easily. Due to the slope, runoff is rapid, even under sufficient plant cover. Bare soil composes less than 5 percent of ground cover. Plant basal cover and litter make up the remainder of the ground cover. Soils are strongly calcareous and droughty, giving them low water holding capacity and fertility.

Community 1.2
Midgrass/Oak Savannah Community

Figure 12. 1.2 Midgrass/Oak Savannah Community

Figure 13. 1.2 Midgrass/Oak Savannah Community (2)

The Midgrass/Oak Savannah Community (1.2) typically results from improper cattle grazing management over a long period of time combined with a lack of brush control. Indigenous or invading woody species increase on the site (with or without fire). In the Tallgrass/Oak Savannah Community (1.1), repeated fires and competition from a vigorous grass component keep woody canopy cover restricted to mottes within the savannah and 30 percent or less woody canopy cover. When the Midgrass/Oak Savannah Community (1.2) is continually overgrazed and fire is excluded, the community crosses a threshold to the Shrubland State (2.1), which is dominated by woody plants.

Important grasses are little bluestem, sideoats grama, green sprangletop, silver bluestem, tall dropseed, and Texas wintergrass. Texas wintergrass, seep muhly (Muhlenbergia reverchonii), slim tridens, Wright’s threeawn (Aristida purpurea var. wrightii), low panicums, low gramas, annual bromes, and less palatable forbs (western ragweed and prairie coneflower) begin replacing the midgrasses. Some of the perennial forbs persist, but less palatable forbs will increase. Woody canopy varies between 30 and 50 percent, depending on the severity of grazing, fire interval, and availability of increaser species. Numerous shrub and tree species will encroach because overgrazing by livestock has reduced grass cover, exposed more soil, and reduced grass fuel for fire. Typically, trees such as oaks and ash will increase in size, while other tree and shrub species such as bumelia, sumacs, elbowbush, agarito, juniper, and pricklypear will increase in density. Brown and Archer (1999) concluded that even with a healthy and dense stand of grasses, woody species will populate the site and eventually dominate the community. To control woody species populations, prescribed grazing and/or browsing and fire can be used to control smaller shrubs and trees. Mechanical removal of larger shrubs and trees may be necessary in older stands.

Heavy continuous grazing will reduce plant cover, litter, and mulch. Bare ground will increase and expose the soil to erosion. Litter and mulch will move off-site as plant cover declines.

Increasing woody dominants are oaks and Ashe’s juniper. Once the tallgrasses have been eliminated from the site, woody species cover exceeds 30 to 50 percent canopy cover, and the woody plants within the grassland portion of the savannah reach fire-resistant size (about 3 feet in height), the site crosses a threshold into the Shrubland State (2) and the Mixed-Brush Shrubland Plant Community (2.1).

Until the Midgrass/Oak Savannah Community (1.2) crosses the threshold into the Shrubland State (2), this community can be managed back toward the Tallgrass/Oak Savannah Community (1.1) through the use of cultural practices including prescribed grazing, prescribed burning, and strategic brush control. It may take several years to achieve this state, depending upon climate and the aggressiveness of the treatment. Once invasive woody species are established, returning fully to the reference community is difficult, but it is possible to return to a similar plant community.

Potential exists for soils to erode to the point that irreversible damage may occur. If soil-holding herbaceous cover decreases to the point that soils are no longer stable, the shrub overstory will not prevent erosion of the A and B soil horizons. This is a critical shift in the ecology of the site. Once the A horizon has eroded, the hydrology, soil chemistry, soil microorganisms, and soil physics are altered to the point where intensive restoration is required to restore the site to another state or community. Simply changing management (improving grazing management, or controlling brush) cannot create sufficient change to restore the site within a reasonable time frame.

Pathway 1.1A
Community 1.1 to 1.2

Tallgrass/Oak Savannah Community

Midgrass/Oak Savannah Community

The Tallgrass/Oak Savannah Community requires fire and/or brush control to maintain woody species cover below 30%. This community will shift to the Midgrass/Oak Savannah Community when there is continued growing season stress on palatable grass species. These stresses include improper grazing management that creates insufficient critical growing season deferment, excess intensity of defoliation, repeated, long-term growing season defoliation, long-term drought, and/or other repeated critical growing season stress. Increaser species (midgrasses and woody species) are generally endemic species released by disturbance. Woody species canopy exceeding 30% and/or dominance of tallgrasses falling below 50% of species composition indicate a transition to the Midgrass/Oak Savannah Community. The reference community can be maintained through the implementation of brush management combined with properly managed grazing that provides adequate growing season deferment to allow establishment of tallgrass propagules and/or the recovery of vigor of stressed plants. The driver for community shift 1.1A for the herbaceous component is improper grazing management. The driver for the woody component is lack of fire and/or brush control.

Pathway 1.2A
Community 1.2 to 1.1

Midgrass/Oak Savannah Community

Tallgrass/Oak Savannah Community

The Midgrass/Oak Savannah Community will return to the Tallgrass/Oak Savannah Community with brush control and grazing management that provides sufficient critical growing season deferment in combination with proper grazing intensity. Favorable moisture conditions will facilitate or accelerate this transition. The understory and overstory components can act independently when canopy cover is less than 50%, i.e., an increase in shrub canopy cover can occur while proper grazing management creates an increase in desirable herbaceous species. The understory component may return to dominance by tallgrasses in the absence of fire (at least until shrub canopy cover reaches 50%). Reduction of the woody component to 30% or less canopy cover will require inputs of fire and/or brush control. The driver for community shift 1.2A for the herbaceous component is proper grazing management, while the driver for the woody component is fire and/or brush control.

State 2
Shrubland State

Community 2.1
Mixed-Brush Shrubland Community

The Mixed-Brush Shrubland Community (2.1) is the result of many years of fire exclusion and/or a lack of proper brush management. It may or may not have been accompanied by improper grazing management, although improper grazing management can increase the rate of degradation. Oaks, Ashe’s juniper, and/or honey mesquite dominate the Mixed-Brush Shrubland Community (2.1), which has greater than 50 percent woody canopy cover. It is now essentially a dense shrubland with remnant grasses under the canopy and within interspaces. Once the brush canopy exceeds 50 percent, annual production for the understory is limited and is generally made up of unpalatable shrubs, grasses, and forbs within tree and shrub interspaces. Common understory shrubs are pricklypear, agarito, sumacs, and elbowbush. With continued heavy cattle grazing and/or browsing and no brush control, the trees and shrubs can exceed 70 percent canopy cover, and potentially reach almost 100 percent cover. Excessive grazing by other deer or goats will create a community dominated by large trees. Few remnant midgrasses and opportunistic shortgrasses, annuals, and perennial forbs occupy the woody plant interspaces. Characteristic grasses are threeawns (Aristida spp.), buffalograss (Bouteloua dactyloides), cedar sedge (Carex planostachys), hairy grama (Bouteloua hirsuta), and fall witchgrass (Digitaria cognata). Texas wintergrass and annuals are found in and around tree/shrub cover. Grasses and forbs make up 40 percent or less of the annual herbage production. Common forbs include dotted gayfeather, western ragweed (Ambrosia psilostachya), sensitive-briar (Mimosa spp.), Mexican sagewort (Artemisia ludoviciana ssp. mexicana), and queen’s-delight (Stillingia sylvatica). At its most extreme, this community takes on a woodland appearance, large woody species with understory dominated by low production grasses, sedges, and forbs that have low palatability and high shade tolerance.

Sideoats grama and other desirable midgrasses decrease to the point that grasses no longer form the dominant component. Shortgrasses such as red lovegrass (Eragrostis secundiflora), purple threeawn (Aristida purpurea), tumblegrass (Schedonnardus paniculatus), and tumble windmill grass (Chloris verticillata) increase. Forbs such as curlycup gumweed (Grindelia squarrosa), western ragweed, silverleaf nightshade (Solanum elaeagnifolium), and thistles increase in species composition. Unpalatable forbs such as western ragweed increase in species composition. Annual herbage production decreases due to a decline in soil structure and organic matter. Biomass shifts toward the woody component. Plant vigor and productivity of the grassland component is reduced due to competition for nutrients and water from woody plants.

Without some form of brush control, the community loses its savannah appearance with invasive shrubs beginning to fill the open grassland portion of the savannah. The oak mottes remain, but are no longer the only areas with trees. There is a continued decline in diversity of the grassland component and an increase in woody species such as sumac. Unpalatable woody species have increased in size and density. Honey mesquite is an early increaser throughout the MLRA. Redberry juniper (Juniperus pinchotii) occurs only in the southern counties of the MLRA and eastern redcedar (Juniperus virginiana) occurs only in the northern portion. Ashe’s juniper occurs mostly in the southern portion, but can be found throughout the MLRA.

Excessive cattle grazing tends to create a different response and structure to the community than does excessive deer or goat grazing. Excessive cattle grazing tends to accelerate invasion of shrubs because it creates conditions where young shrubs increase in vigor and size while palatable grasses decrease in vigor and abundance. Excess deer or goat grazing tends to create a dominance of large trees by removing both young shrubs and the young growth that grow below the browse line on larger shrubs and trees. In areas where high deer densities occur, heavy browsing can decrease preferred woody plants. While large trees will continue to increase in size, they will have very little production below the browse line. The site becomes dominated by large trees with little forage available for livestock or wildlife. Large trees with little understory provide much less soil protection than do dense stands of grass. As soils erode, understory species have reduced potential to revegetate the site. The bare area under the browse line creates a situation that provides poor forage conditions and poor visual cover for wildlife.

If irreversible soil damage has occurred, it may be possible to remove brush and seed the site to a grassland community. However, it is very difficult and expensive to restore the site to reference conditions due to the loss of organic matter, soil horizons, soil microbes, and soil structure necessary to maintain the reference community.

The shrub canopy acts to intercept rainfall and increase evapotranspiration losses, creating a more xeric microclimate. Soil fauna and organic mulch are reduced, exposing more of the soil surface to erosion in interspaces. The percent of exposed limestone bedrock increases with erosion. However, within the woody canopy, hydrologic processes stabilize and soil organic matter and mulch begin to increase and eventually stabilize under the shrub canopy.

The Mixed-Brush Shrubland Community (2.1) provides good habitat cover for wildlife, but only limited forage or browse is available for livestock or wildlife. At this stage, highly intensive restoration practices are needed to return the shrubland to a grassland. Alternatives for restoration include brush control and range planting, proper stocking, prescribed grazing, and prescribed burning following restoration to maintain the desired community.

As the grassland vegetation declines, more soil is exposed, leading to crusting and erosion. Due to the shallow depth to limestone bedrock and steep slope, erosion can be severe. Higher rainfall interception losses by the increasing woody canopy combined with increased evaporation and runoff can reduce the effectiveness of rainfall. Soil organic matter and soil structure decline within the interspaces, but soil conditions improve under the woody plant cover. Soil loss can occur during rainfall events.

Annual primary production is approximately 1,000 to 2,000 pounds per acre. In this plant community, annual production is balanced between herbaceous plants and woody species. Browsing animals such as goats and deer can find fair food value if browse plants have not been grazed excessively. Forage quantity and quality for cattle is low. Unless brush management takes place, woody canopy will continue to increase until it exceeds 75 percent. This trend cannot be reversed with proper grazing management alone.

Transition T1A
State 1 to 2

Shrubs make up a portion of the plant community in the Savannah State, hence woody propagules are present. Therefore, the Savannah State is always at risk for the transition to the Shrubland State in the absence of fire. The mean fire return interval in the Savannah State is 7-10 years. Even with proper grazing and favorable climate conditions, lack of fire for 20-30 years will allow woody species to increase in canopy to reach the 50% threshold level. Introduction of aggressive woody invader species (i.e. juniper or mesquite) increase the risk and accelerate the rate at which this transition state is likely to occur. This transition can occur from any community within the Savannah State, it is not dependant on degradation of the herbaceous community, but on the lack of some form of brush control. Improper grazing, prolonged drought, and a warming climate will provide a competitive advantage to shrubs which will accelerate this process. Tallgrasses will decrease to less than 5% species composition.

Restoration pathway R2A
State 2 to 1

Restoration of the Shrubland State to the Savannah State requires substantial energy input. Mechanical or herbicidal brush control treatments can be used to remove woody species. A long-term prescribed fire program may sufficiently reduce brush density to a level below the threshold of the Savannah State, particularly if the woody component is dominated by species that are not fire sprouters. Chemical or hand brush control in combination with prescribed fire, proper grazing, and favorable growing conditions may be the most economical means of creating and maintaining the desired plant community. If remnant populations of tallgrasses, midgrasses, and desirable forbs are not present at sufficient levels, range planting will be necessary to restore a desirable herbaceous plant community. The driver for Restoration Pathway R2A is fire and/or brush control combined with natural restoration of the herbaceous community or active management of the herbaceous restoration process (range seeding). Restoration may require aggressive treatment of invader species.