State 1
Savannah State - Reference

Community 1.1
Oak Savannah/Tallgrass Community

Figure 9. 1.1 Oak Savannah/Tallgrass Community

The Oak Savannah/Tallgrass Community (1.1) is the reference community and is characterized as a mixed oak savannah with up to 20 percent woody species canopy cover. Oak species include live oak, post oak, scrub oak, Texas oak and Bigelow oak. Elm and hackberry species are also present in the tree mottes of this community.

Both percent species composition by weight and percent canopy cover are used in this ESD. Most observers find it easier to visualize or estimate percent canopy for woody species (trees and shrubs). Canopy cover drives the transitions between community and states because of the influence of shade and interception of rainfall. Species composition by weight remains an important descriptor of the herbaceous community and of the community as a whole. Woody species are included in species composition for the site. Calculating Similarity Index requires use of species composition.

Tallgrasses that characterize this community include little bluestem, which dominates the herbaceous component of the site, Indiangrass and big bluestem. Other important grasses are sideoats grama, tall dropseed, cane bluestem, silver bluestem, Texas wintergrass, slim tridens, and Canada wildrye (Elymus canadensis). Forbs commonly found on the site include awnless bushsunflower (Simsia calva), Engelmann’s daisy (Engelmannia peristenia), dotted gayfeather (Liatris punctata), blacksamson (Echinacea angustifolia), Mexican sagewort (Artemisia ludoviciana spp. mexicana), and halfshrub sundrop (Calylophus serrulatus). Other shrub and tree species include eastern redbud (Cercis canadensis), bumelia (Sideroxylon spp.), species of sumac (Rhus spp.), elbowbush (Forestiera pubescens), agarito (Mahonia trifoliolata), and saw greenbrier (Smilax bona-nox).

The reference savannah community will transition to the Oak Savannah/Midgrass Community (1.2) under the stresses of improper grazing and where fire does not prevent the establishment of juniper. The first species to decrease in dominance will be the most palatable and/or least grazing-tolerant grasses and forbs (including Indiangrass, big bluestem, and 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.

In reference condition, the canopy cover of shrubs and trees will increase as slope increases. When considering only the steepest portions (0-8% of the total area) of the Redland site, woody species cover may exceed 20%. The site will have overall woody species canopy cover of 10-20%, but tree mottes will have higher cover in localized areas.

The soil surface of this site is often characterized by limestone fragments, cherts, cobbles, and stones. The soils of this site are shallow, noncalcareous clays, clay loams, or loams underlain by limestone. Bare soil composes 10 percent or less of the ground cover. Large fragments make up 25 to 50% of ground cover. As steepness increases, the amount of cover from large fragments also increases (from 25% on level sites to 50% on steep sites). Plant basal cover and litter make up the remainder of the ground cover. Infiltration is slow and runoff is low to moderate. The limestone rocks on the soil surface reduce evaporation, contribute additional moisture to the soil, and protect palatable grasses and shrubs from overuse. The soils are high in organic matter and desirable minerals, and the heavy plant cover contributes to increasing organic matter and soil building.

Community 1.2
Oak Savannah/Midgrass Community

Figure 12. 1.2 Oak Savannah/Midgrass Community

The Oak Savannah/Midgrass Community Phase (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, except for non-sprouting juniper). In the Oak Savannah/Tallgrass Plant Community (1.1), repeated fires and competition from a vigorous grass component keep woody canopy cover restricted to mottes within the savannah and at 20 percent or less of total cover. When the Oak Savannah/Midgrass 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 little bluestem, sideoats grama, Indiangrass, big bluestem, silver bluestem, tall dropseed, hairy grama (Bouteloua hirsuta), and Texas wintergrass. Texas wintergrass, 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 increases to between 20 and 40 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 the grass cover, exposed more bare soil, and reduced grass fuel for fire. Typically, trees such as oaks and elms will increase in size, while shorter stature tree and shrub species such as bumelia, sumacs, elbowbush, agarito, honey mesquite, juniper, pricklypear, and yucca (Yucca 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, and 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 unknown, but reference sites indicate this will take over 50 years (and possibly hundreds of years).

Continued site degradation will lead to increases in plant bare ground, which will increase soil erosion. Litter and mulch will decrease for two reasons. First, there will be less litter as herbaceous production declines. Second, the litter present will be moved off site as the amount of runoff and length of water flow patterns increase because the number of plant bases interrupting flow will decrease.

The site crosses a threshold into the Mixed-Brush/Midgrass Plant Community (2.1) within the Shrubland State (2) once tallgrasses have decreased to less than 5% composition and woody species canopy cover exceeds 40 percent. At this point, woody shrubs within the grassland portion of the savannah will have reached a fire-resistant size (over 3 feet in height).

Until the Oak Savannah/Midgrass Plant Community (1.2) crosses the threshold into the Mixed-Brush/Midgrass Community (2.1), this community can be managed so that it transitions back to the 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 woody species become 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 the management of the site (improving grazing management or controlling brush) cannot create sufficient change to restore the site within a reasonable time frame. This level of erosion indicates that a threshold has been crossed to the Shrubland State.

Pathway 1.1A
Community 1.1 to 1.2

Oak Savannah/Tallgrass Community

Oak Savannah/Midgrass Community

The Oak Savannah/Tallgrass Plant Community will shift to the Oak Savannah/Midgrass Plant Community when there is continued growing season stress on palatable tallgrass species. These stresses include 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 from competition as the vigor of tallgrasses declines. 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. Community 1.1 can be maintained through the implementation of managed grazing that provides adequate growing season deferment to allow establishment of tallgrass propagules and/or the recovery of the vigor of stressed 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

Oak Savannah/Midgrass Community

Oak Savannah/Tallgrass Community

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. 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 reference conditions of 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 40 to 60 percent woody plant canopy. Live oak and post oak are the dominant species within the oak mottes with honey mesquite and Ashe’s juniper invading the former grassland areas. The community loses its savannah appearance as invasive shrubs fill open grassland portions of the savannah. The 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.

The overstory becomes increasingly dominated by the woody species present in the reference community(1.1) and low shrubs (particularly juniper, pricklypear, and yucca). This is accompanied by a continued decline in diversity of the grassland component and transitions of the understory to midgrasses (such as silver bluestem and Texas wintergrass) and unpalatable forbs (such as broomweed (Amphiachyris spp.) and western ragweed (Ambrosia psilostachya)). Bare ground, erosion, rilling, and the length of water flow patterns increase while litter, soil stability, and annual biomass production decrease.

As the grassland vegetation declines, more soil is exposed leading to crusting and erosion. In this vegetation type, erosion can be severe. Higher rainfall interception losses by the increasing woody canopy combined with evaporation and runoff can reduce the effectiveness of rainfall. Soil organic matter and soil structure decline within the interspaces, but soil conditions may improve again under the canopy of trees and large shrubs.

Annual herbage production decreases, due to a decline in soil structure and organic matter, and has shifted toward the woody component. Sideoats grama and other palatable midgrasses decrease to the point that grasses no longer form the dominant vegetative component. Shortgrasses such as buffalograss, purple threeawn (Aristida purpurea), sedges (Carex spp.), Texas wintergrass, and curly-mesquite (Hilaria belangeri) increase. Remnants of the reference community grasses and forbs along with unpalatable invaders occupy the interspaces between trees and shrubs. Cool-season species such as Texas wintergrass and sedges, 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. Likely herbaceous species include Mexican sagewort, annual broomweed (Amphiachyris dracunculoides), western ragweed, upright prairie coneflower (Ratibida columnifera), snow-on-the-mountain (Euphorbia marginata), and silverleaf nightshade (Solanum elaeagnifolium).

Unpalatable woody species increase in size and density. Honey mesquite is an early increaser throughout the MLRA; on this site it can increase to the point of forming mesquite thickets, particularly after severe damage to the site resulting in removal of competing trees and tall grasses (i.e. failed cultivation or severe grazing to the point of soil loss). 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. Birds and small mammals contribute to the increase of juniper as its seeds are deposited under trees the animals use for perching, nesting, and feeding.

Annual primary production is approximately 1000 to 2000 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 60 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, Ashe’s juniper, and/or honey mesquite dominate the Mixed-Brush Shrubland Community (2.2), which has greater than 60 percent woody canopy cover. It is now essentially a dense shrubland with remnant grasses under the canopy and within interspaces or a mosaic of shrub and tree mottes interspersed among low production open areas. Once the brush canopy exceeds 60 percent, annual production for the understory is very limited due to both competition and shading and is generally made up of unpalatable shrubs, grasses, and forbs within tree and shrub interspaces. Common understory shrubs are pricklypear, agarito, sumacs, elbowbush, and catclaw mimosa (Mimosa aculeaticarpa).

With continued heavy cattle grazing and/or browsing and no brush control, the trees and shrubs can exceed 75 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 include: Texas wintergrass, buffalograss, curly-mesquite, threeawns (Aristida spp.), sedges, hairy grama, Texas grama (Bouteloua rigidiseta), and annual bromes (Bromus spp.). 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 western ragweed, annual broomweed, upright prairie coneflower, snow-on-the-mountain, silverleaf nightshade, milkweeds (Asclepias spp.), Leavenworth’s eryngo (Eryngium leavenworthii), twinleaf senna (Senna bauhinioides), and Evax (Evax spp.). At its most extreme, this community takes on a woodland appearance interspersed with large areas of almost denuded rock where soil has eroded away. Particularly if there has been heavy browsing (by deer or goats), the site will be dominated by large trees with a marked browse line and an understory dominated by unpalatable, low production grasses, sedges, and forbs.

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 grows 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.

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. 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 grassland. Alternatives for restoration include: brush control and range planting, proper stocking, prescribed grazing, and prescribed burning following restoration to maintain the desired community.

If irreversible soil damage has occurred, it will be very difficult to remove brush and seed the site to a grassland community. Furthermore, it will be very difficult and expensive to restore the site to reference conditions due to the loss of the organic matter, soil horizons, soil microbes, and soil structure necessary to maintain the reference 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 management and/or long-term drought (or other growing season stress) will accelerate this transition. Woody species canopy exceeding 60% indicates this transition. Improper grazing management 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 60% brush canopy. Continued fire and/or brush management will be required to maintain woody density and canopy below 60%. If the herbaceous component has transitioned to shortgrasses and low forbs, proper grazing management (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 and trees 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 3-7 years. Even with proper grazing management and favorable climate conditions, lack of fire for 10-20 years will allow woody species to increase in canopy to reach the 40% threshold level. An infusion of invasive species (i.e. juniper or mesquite) will speed up the process. 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 management, 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.