State 1
Savannah State - Reference

Community 1.1
Oak/Tallgrass Savannah Community

Figure 9. 1.1 Oak/Tallgrass Savannah Community

The Oak/Tallgrass Savannah Community (1.1) is the reference community and is characterized as a live oak savannah with less than 20 percent live oak canopy cover. Live oak will be the most abundant along water courses, ledges, and escarpments.

Little bluestem dominates the herbaceous component of the site. Other important grasses are Indiangrass, big bluestem, switchgrass, Canada wildrye (Elymus canadensis), silver bluestem (Bothriochloa laguroides var. torreyana), sideoats grama (Bouteloua curtipendula), tall dropseed (Sporobolus compositus), and Texas wintergrass. Forbs commonly found on the site include awnless bushsunflower (Simsia calva), Engelmann’s daisy (Engelmannia peristenia), orange zexmenia (Wedelia texana), dotted gayfeather (Liatris punctata), and halfshrub sundrop (Calylophus serrulatus). Other shrub and tree species found in this plant community(1.1) include species of bumelia (Sideroxylon spp.), sumac (Rhus spp.), hackberry, juniper, and elm, along with elbowbush (Forestiera pubescens), saw greenbrier (Smilax bona-nox), and Texas kidneywood (Eysenhardtia texana). 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 Oak/Midgrass 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.

The soil surface of this site is characterized by up to 50 percent cover by hard, honeycombed limestone. Bare soil composes 10 percent or less of the ground cover. Plant basal cover and litter make up the remainder of the ground cover. Infiltration is high and runoff is low. Although the limestone on the soil surface increases runoff and leads to the development of water flow patterns, runoff is eventually intercepted by plants within the site. Soils are high in organic matter and the heavy plant cover contributes to increasing organic matter and soil building.

Community 1.2
Oak/Midgrass Savannah Community

The Oak/Midgrass Savannah Community (1.2) typically results from improper cattle grazing management over a long period of time. Indigenous or invading woody species increase on the site (with or without fire). In the Oak/Tallgrass Savannah Community (1.1), repeated fires and competition from a vigorous grass component keep woody canopy cover restricted to mottes within the savannah and 20 percent or less woody canopy cover. When the Oak /Midgrass Savannah Community (1.2) is continually overgrazed and fire is excluded, the community crosses a threshold to a state that is dominated by woody plants, the Mixed-Brush/Midgrass Community (2.1).

Important grasses are big bluestem, Indiangrass, little bluestem, sideoats grama, silver bluestem, composite dropseed, and Texas wintergrass. More grazing-resistant shortgrasses, such as Texas wintergrass and buffalograss (Bouteloua dactyloides), and less palatable forbs begin replacing the midgrasses. Some of the reference community perennial forbs persist, but less palatable forbs will increase. Woody canopy varies between 20 and 35 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 (Quercus spp.), elms, hackberry, and hawthorn (Crataegus spp.) will increase in size, while other tree and shrub species such as bumelia, sumacs, elbowbush, agarito (Mahonia trifoliolata), honey mesquite, juniper, and pricklypear (Opuntia spp.) 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. The time frame for woody species to dominate a healthy community with proper grazing management is not precisely known, but reference sites indicate this will take over 50 years (and possibly hundreds of years).

Heavy continuous grazing will reduce plant cover, litter, and mulch. Bare ground will increase and expose the soil to erosion. Some mulch and litter movement may occur during rainstorms, but little soil movement occurs due to gentle slopes in this vegetation type. Litter and mulch will move off-site as plant cover declines.

Increasing woody dominants are oaks and honey mesquite. Once the tallgrasses have been eliminated from the site, woody species cover exceeds 35 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/Midgrass Community (2.1).

Until the Oak /Midgrass Savannah Community (1.2) crosses the threshold into the Mixed-Brush/Midgrass Community (2.1), this community can be managed back toward the tallgrass 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 treatment. Once woody species begin to establish, returning fully to community 1.1 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

The Oak Savannah/Tallgrass Plant Community will shift to the Oak Savannah/Midgrass Plant Community when there is continued growing season stress on palatable grass species. These stresses include insufficient critical growing season deferment, excess defoliation intensity, repeated, long-term growing season defoliation, and/or long-term drought. Increaser species (midgrasses and woody species) are generally endemic species released from competition. Woody species canopy exceeding 20% and/or dominance of tallgrasses falling below 50% of species composition indicate a transition to the Oak Savannah/Midgrass Plant Community. Reference vegetation can be maintained through implementation of managed grazing that provides adequate growing season deferment to allow establishment of tall grass propagules and/or the recovery of vigor of stressed individual plants. Proper grazing management may be combined with fire and/or brush management to create a shift towards or maintain the reference community.

Pathway 1.2A
Community 1.2 to 1.1

The Oak Savannah/Midgrass Plant Community will return to the Oak Savannah/Tallgrass Plant Community under grazing management that provides sufficient critical growing season deferment in combination with proper grazing intensity as long as the seedbank or seed source is still present. Favorable moisture conditions will facilitate or accelerate this transition. The understory component may return to dominance by tallgrasses in the absence of fire, however, reduction of the woody component to 20% or less canopy cover will require inputs of fire and/or brush control.

State 2
Shrubland State

Community 2.1
Mixed-Brush/Midgrass Community

The Mixed-Brush/Midgrass Community (2.1) presents a 35 to 50 percent woody plant canopy. Live oak is the dominant species within the oak mottes with honey mesquite and juniper invading former grassland areas. The community loses its savannah appearance with invasive shrubs beginning to fill the open grassland portion of the savannah. Oak mottes remain, but are no longer the only areas with trees. This community type is the result of continuous improper grazing by livestock and a lack of fire. In areas where high deer densities occur, heavy browsing can decrease preferred woody plants. There is a continued decline in diversity of the grassland component and an increase in woody species such as sumac. Unpalatable forbs such as western ragweed (Ambrosia psilostachya) increase in species composition. Annual herbage production decreases due to a decline in soil structure and organic matter and has shifted toward the woody component. All 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. Many of the reference community (1.1) shrubs are still present.

Sideoats grama and other midgrasses decrease to the point that grasses no longer form the dominant component. Shortgrasses such as buffalograss and Texas wintergrass increase. Remnants of the palatable grasses and forbs along with unpalatable invaders occupy the interspaces between trees and shrubs. Cool-season species such as Texas wintergrass, plus other grazing-resistant species, can be found under and around woody plants. Plant vigor and productivity of the grassland component is reduced due to competition for nutrients and water from woody plants. Common herbaceous species include threeawns (Aristida spp.), hairy grama (Bouteloua hirsuta), and Mexican sagewort (Artemisia ludoviciana ssp. mexicana). Buffalograss, western ragweed, and curly-mesquite (Hilaria belangeri) are persistent increasers until shrub density reaches maximum canopy.

As the grassland vegetation declines, more soil is exposed, leading to crusting and erosion. Due to the shallow depth to the limestone bedrock, 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 1000 to 3000 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 and proper grazing management are applied at this stage, woody canopy will increase until it exceeds 50 percent, indicating a conversion to the Mixed-Brush Shrubland Community (2.2). The trend for increased shrub cover cannot be reversed with proper grazing management alone.

Community 2.2
Mixed-Brush Shrubland Community

The Mixed-Brush Shrubland Community (2.2) is the result of many years of improper grazing, lack of periodic fires, and/or a lack of proper brush management. Oaks, honey mesquite, and/or juniper dominate the Mixed-Brush Shrubland Community (2.2), 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 very 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 browsing by 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 curly-mesquite, buffalograss, and fall witchgrass (Digitaria cognata). Texas wintergrass and annuals are found in and around tree/shrub cover. Grasses and forbs make up 35 percent or less of the annual herbage production. Common forbs include dotted blazing star (Liatris punctata), orange zexmenia (Wedelia texana), croton (Croton spp.), western ragweed, Mexican sagewort, and sensitive-briar (Mimosa spp.). 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.

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. 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. The resulting grassland will not look or function like the reference community (1.1). Instead, it is likely to be dominated by few introduced midgrasses and produce less biomass. 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. Rangeland health functions will depart substantially from reference conditions.

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 (2.2) 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.

Pathway 2.1A
Community 2.1 to 2.2

Without fire (natural or human-caused) and/or brush control, woody density and canopy cover will increase in the Mixed-Brush/Midgrass Plant Community until it converts into the Mixed-Brush Shrubland Plant Community. Improper grazing and/or long-term drought (or other growing season stress) will accelerate this transition. Woody species canopy exceeding 50% indicate this transition. Improper grazing or other long-term growing season stress can increase the composition of shortgrasses and low-growing (or unpalatable) forbs in the herbaceous component. Even with proper grazing, in the absence of fire the woody component will increase to the point that the herbaceous component will shift in composition toward shortgrasses and forbs suited to growing in shaded conditions with little available soil moisture.

Pathway 2.2A
Community 2.2 to 2.1

Brush management and/or fire can reduce the woody component below the transition level of 50% brush canopy. Continued fire and/or brush management will be required to maintain woody density and canopy below 50%. If the herbaceous component has transitioned to shortgrasses and low forbs, proper grazing (combined with favorable moisture conditions) will be necessary to facilitate the shift of the understory component to the midgrass-dominated Mixed-Brush/Midgrass Plant Community. Range planting may accelerate the transition of the herbaceous community, particularly when combined with favorable growing conditions.

Transition T1A
State 1 to 2

The Savannah State is resistant to shrub dominance. However, shrubs make up a portion of the plant community in this state, therefore propagules are present. 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 50-100 years will allow woody species to increase in canopy to reach the 35% threshold level. Improper grazing, prolonged drought, and 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. 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 the reference plant community.