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 live oak savannah with up to 20 percent woody species canopy cover. The live oaks in this community are medium-sized and scattered throughout the site with a tall and midgrass understory.

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.

Little bluestem dominates the herbaceous component of the site, with sideoats grama, silver bluestem, tall grama (Bouteloua hirsuta var. pectinata), and tall dropseed (Sporobolus compositus) present as significant components. Big bluestem (Andropogon gerardii) and Indiangrass are present in this community. Other important grasses are hairy dropseed (Sporobolus compositus var. drummondii), and Texas wintergrass, plains lovegrass (Eragrostis intermedia), and sand dropseed (Sporobolus cryptandrus). Forbs commonly found on the site include orange zexmenia (Wedelia texana), greenthread (Thelesperma spp.), catclaw sensitive-briar (Mimosa nuttallii), dotted gayfeather (Liatris punctata), and western ragweed (Ambrosia psilostachya). Shrub and tree species found include live oak, Texas oak (Quercus texana), species of hackberry, bumelia (Sideroxylon spp.), and elbowbush (Forestiera pubescens), along with prairie flameleaf sumac (Rhus lanceolata), prickly ash (Zanthoxylum hirsutum), and saw greenbrier (Smilax bona-nox).

The reference Oak Savannah/Tallgrass Community (1.1) will transition to the Oak Savannah/Midgrass Community (1.2) under the stresses of improper grazing management. The first species to decrease in dominance will be the most palatable and/or least grazing tolerant grasses and forbs (e.g. Indiangrass and Engelmann’s daisy). Little bluestem will initially increase in composition on this site. 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 soils of this site are variable and moderately deep. Limestone or sandstone fragments are common on the surface. Infiltration is high and runoff is low. Bare soil composes 10 percent or less of the ground cover. Plant basal cover and litter make up the remainder of the ground cover. The soils are droughty and have a low water holding capacity. This site produces plants that are low in phosphorus and palatability. These soils respond rapidly to proper management practices, but deteriorate rapidly when abused.

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 long-term improper cattle grazing management and/or long-term lack of fire. Indigenous or invading woody species increase on the site (with or without fire). 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 20 percent or less woody canopy 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/Shortgrass Community (2.1).

Important grasses are little bluestem, sideoats grama, silver bluestem, hairy grama, and Texas cupgrass. Texas wintergrass, buffalograss (Bouteloua dactyloides), and less palatable forbs begin replacing the midgrasses. Some of the 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 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 agarito, honey mesquite, juniper, catclaw acacia, 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 approximately 25 years.

Prior to human disturbance the site transitioned between communities 1.1 and 1.2 in response to variation in fire, drought, and grazing impacts. Given time, proper management, and favorable growing conditions community 1.2 will transition back to 1.1.

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 plants 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 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

Improper Grazing Management, Lack of Fire, Lack of Brush Control, Long-Term Drought or Other Growing Season Stress. 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 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. The driver for community shift 1.1A for the herbaceous component is improper grazing management on sites with grazing. Increased competition from invader midgrasses and forbs and increasing shrub cover can play a role in driving herbaceous shifts. The driver for the woody component is lack of fire and/or brush control.

Pathway 1.2A
Community 1.2 to 1.1

Oak Savannah/Midgrass Community

Oak Savannah/Tallgrass Community

Proper Grazing Management, Fire, and Brush Management. 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 20% 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. The driver for the woody component is fire and/or brush control. Brush control can also benefit tallgrasses and drive community shift 1.2A for the herbaceous community.

State 2
Shrubland State

Community 2.1
Mixed-Brush/Shortgrass Community

Figure 15. 2.1 Mixed-Brush/Shortgrass Community

The Mixed-Brush/Shortgrass Community (2.1) presents a 40 to 60 percent woody plant canopy, with live oak dominating the oak mottes and honey mesquite and Ashe’s juniper (Juniperus ashei) invading the former grassland areas. 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. 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 (Rhus spp.). 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.

Shortgrasses such as buffalograss (Bouteloua dactyloides), purple threeawn (Aristida purpurea), and curly-mesquite (Hilaria belangeri) dominate the grass component, but grasses no longer form the dominant species component. Remnants of the community (1.1) grasses and forbs along with unpalatable invaders occupy the interspaces between trees and shrubs. Cool-season species such as Texas wintergrass and sedges (Carex spp.), 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 queen’s-delight (Stillingia sylvatica), Mexican sagewort (Artemisia ludoviciana ssp. mexicana), western ragweed, buckwheat (Eriogonum spp.), and dotted gayfeather (Liatris punctata). As tallgrasses decrease, forbs will increase to approximately 20 percent of the species composition.

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.

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.

Annual primary production is approximately 900 to 1500 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 is applied at this stage, woody canopy will continue to increase and perennial grasses and forbs will be replaced by annual grasses and forbs, causing the community to convert to the Mixed-Brush Shrubland Community (2.2). The trend 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 management, 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 for nutrients and shading and is generally made up of unpalatable shrubs, grasses, and forbs within tree and shrub interspaces. Common understory shrubs are pricklypear, agarito (Mahonia trifoliolata), sumac, elbowbush (Forestiera pubescens), 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 community conditions due to the loss of the organic matter, soil horizons, soil microbes, and soil structure necessary to maintain the that community.

Pathway 2.1A
Community 2.1 to 2.2

Lack of Fire, Lack of Brush Control, Improper Grazing Management, and Severe Drought. Without fire (natural or human-caused) and/or brush control, woody density and canopy cover will increase in the Mixed-Brush/Shortgrass 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. The driver for community shift 2.1A is lack of fire and/or brush control.

Pathway 2.2A
Community 2.2 to 2.1

Fire and Brush Management. Brush and/or fire management 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/Shortgrass Plant Community. Range planting may accelerate the transition of the herbaceous community, particularly when combined with favorable growing conditions. The driver for community shift 2.2A is fire and/or brush control.

Transition T1A
State 1 to 2

Transition to Shrubland State. 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 to maintain a Savannah State is 3-5 years. Even with proper grazing management and favorable climate conditions, lack of fire for 10-15 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 management, 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. This transition can occur from any of the Grassland State Communities. The driver for Transition T1A is lack of fire and/or brush control. The Grassland State is always at risk for this transition because woody species are present in the grassland plant community. Introduction of aggressive woody invader species (i.e. juniper) increase the risk that this state transition will occur and accelerate the rate at which it is likely to occur.

Restoration pathway R2A
State 2 to 1

Restoration Pathway to Savannah State. 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. The driver for Restoration Pathway R2A is fire and/or brush control combined with restoration of the herbaceous community and proper grazing management. Restoration may require aggressive treatment of invader species.