State 1
Savannah

Community 1.1
Midgrass Savannah

This community represents the reference plant community. It is a fire-climax, midgrass plant community with less than a five percent canopy of shrubby woody plants. Modern reports (Mann 2004) discuss the importance of human caused fire as an important factor in keeping open grasslands and savannahs prior to European settlement. It is assumed that prior to European settlement, the Midgrass Savannah Community (1.1) occurred over the majority of this ecological site in a dynamically shifting mosaic over time with the Mixedgrass/Mixedbrush Community (2.2) in the Shrubland State (2). Dominant grasses include false Rhodesgrass (Chloris crinita), multiflower Rhodesgrass (Trichloris pluriflora), little bluestem, Arizona cottontop, pink pappusgrass, sideoats grama, buffalograss (Bouteloua dactyloides), curly-mesquite (Hilaria belangeri), perennial threeawn (Aristida spp.), plains bristlegrass (Setaria spp.), Texas wintergrass (Nassella leucotricha), and hooded windmillgrass (Chloris cucullata). Forbs are Engelmann’s daisy (Engelmannia peristenia), bundleflower (Desmanthus spp.), sensitive briar (Schrankia spp.), orange zexmenia (Wedelia hispida), Mexican sagewort (Artemisia ludoviciana), bushsunflower (Simsia lagasceformis), lazy daisy (Aphanostephus spp.), and annual forbs. The woody species are mesquite, whitebrush, snakewood, spiny hackberry (Celtis pallida), cacti, wolfberry, vine jointfir (Ephedra spp.), desert yaupon (Schaefferia cuneifolia), and guayacan (Guaiacum augustifolium). Because the woody species that dominate the Shrubland State (2) are native species that occur as part of the Savannah State (1), the transition 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 gain in stature and increase to the 20 percent canopy cover level that indicates a state change. This is a continual process. Managers need to detect the increase in woody species when canopy is less than 20 percent and take management action before the threshold is crossed. There is not a five-year window before shrubs begin to increase 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 to cross threshold T1A. Once woody canopy exceeds approximately 20 percent and is taller than three feet (an indication of reproduction capability), threshold T1A has been crossed to the Shrubland State (2). Energy in the form of heavy equipment and/or herbicides will be required along with prescribed grazing to shift the plant community back to the Savannah State (1). The Savannah State (1) can be converted to the Converted State (4) by controlling brush while seeding to native or introduced grasses. It may also be plowed and converted to cropland (T1C). There is essentially no bare soil in this community. Plant basal cover and litter comprise all of the ground cover.

Community 1.2
Shortgrass Savannah

The Shortgrass Savannah Community (1.2) of the Savannah State (1) retains the savannah plant structure with the woody species canopy being as high as 20 percent, but the majority are less than three feet tall and not yet capable of reproduction. The increase in woody canopy results from lack of fire or effective brush control. The shift in the herbaceous component typically results from heavy continuous grazing which removed many midgrasses, which are replaced by shortgrasses such as buffalograss, curlymesquite, threeawn, tumblegrass (Schedonnardus paniculatus), and red grama (Bouteloua trifida). Other common increasers to the site are leatherstem (Jatropha dioica), huisache (Acacia farnesiana), ragweed (Ambrosia spp.) guajillo (Acacia berlandieri), and tasajillo (Cylindropuntia leptocaulis). Implementation of proper grazing management in combination with brush management will transition (1.2A) the Shortgrass Savannah Community (1.2) back to the Midgrass Savannah Community (1.1) Since most shrubs and trees are younger than reproductive age in this state, use of fire can be effective in maintaining the open portions of the savannah. A sound grazing management plan will be essential to reverse the trend toward shortgrass dominance. Continued abusive grazing can reduce vigor, cover of mid, and shortgrasses, which will result in increased bare soil. This increases risk of soil erosion and indicates risk of crossing the threshold (T1B) to the Sparsely Vegetated State (3). 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. The Shortgrass Savannah Community (1.2) will cross the threshold to the Shrubland State (2) without effective brush control. Important shortgrasses are buffalograss, curly mesquite, three awns, red grama and Texas wintergrass. Unpalatable, shade-tolerant grasses and forbs begin replacing midgrasses. Examples include ragweed and annual forbs. Shaded conditions favor cool-season grasses such as Texas wintergrass (Nassella leucotricha). Woody canopy varies between 5 and 20 percent, depending on the severity of grazing, fire interval, amount of brush control, and availability of woody increaser species. Numerous shrub and tree species will encroach if overgrazing has reduced grass cover, exposed more soil, and reduced fine fuels (grasses). Typically, trees such as mesquite and whitebrush increase in size, while other shrub species such as spiny hackberry, wolfberry, yaupon, and cacti increase in density.

Pathway 1.1A
Community 1.1 to 1.2

The Midgrass Savannah Community (1.1) requires fire and/or brush control to maintain woody species cover below five percent. This community will shift to the Shortgrass Savannah Community (1.2) when there is continued growing-season stress on midgrasses. These stresses include improper grazing management that creates insufficient critical growing-season deferment, excess intensity of defoliation, repeated, long-term growing-season defoliation, and long-term drought. Increaser species (shortgrasses and woody species) are generally endemic species released by disturbance. Native woody species canopy exceeding 10 percent and/or dominance of midgrasses falling below 50 percent of species composition indicate a transition to the Shortgrass Savannah Community. The Midgrass Savannah Community (1.1) can be maintained through the implementation of brush management combined with properly managed grazing that provides adequate growing-season deferment to allow establishment of midgrass 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, while the driver for the woody component is lack of fire and/or brush control.

Pathway 1.2A
Community 1.2 to 1.1

The Shortgrass Savannah Community (1.2) will return to the Midgrass Savannah Community (1.1) with brush control and proper grazing management that provides sufficient critical growing-season deferment in combination with proper grazing intensity. Favorable moisture conditions will facilitate or accelerate this transition. Reduction of the woody component will require inputs of fire and/or brush control. The understory and overstory components can act independently when canopy cover is less than 20 percent, meaning an increase in shrub canopy cover can occur while proper grazing management creates an increase in desirable herbaceous species. 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

Community 2.1
Midgrass/Mixed Brush

This Midgrass/Mixed Brush Community (2.1) is a result of a transition from the Savannah State (1) to the Shrubland State (2). This threshold is passed when the woody canopy shades the ground to the point where insufficient fuels are produced to carry a fire that will control the woody canopy and when shrubs reach three feet in height, meaning they reach reproductive age. The understory is very limited in production due to the competition for sunlight, water and nutrients. There is an increase in tasajillo, prickly pear (Opuntia spp.), yucca (Yucca spp.), annual grasses, and forbs. Shortgrasses produce less fuel load than midgrasses, therefore fire frequency and intensity is decreased when shortgrasses dominate the site. When fires are no longer hot enough to kill woody species, woody plants proliferate and eventually dominate the site. Once woody canopy cover exceeds 20 percent, the community has crossed a threshold that will require some form of management to reduce the brush back to its original state. The area will probably need to be seeded with natives and a good grazing management program established to maintain the health and vigor of the forage component.

Community 2.2
Mixed Brush

This Mixed Brush Community (2.2) is a phase of the Shrubland State (2) characterized by over 50 percent woody plant canopy. It is the culmination of continued heavy grazing and a lack of effective brush control (fire or brush management). Woody species dominate the site with little understory production. The community loses its savannah appearance with native shrubs beginning to fill the open grassland portion of the savannah. Shade from overstory is the driving factor. Annual herbage production decreases due to a decline in soil structure and organic matter. Production of the overstory canopy has increased by a similar amount to the decrease in herbaceous production. All unpalatable woody species have increased in size and density. Bare ground may be present that has crusted to the point that there is little water infiltration and little seedling emergence. Water infiltration does occur directly under some of the woody species, such as mesquite as it moves down the trunk of the tree to the base. During the growing season, light showers are captured in the canopy of the trees and evaporate. Energy flow and nutrient use is predominantly through the shrubs. Winter rains can produce understory forage from cool-season annual forbs and grasses and perennials such as Texas wintergrass. This community is highly resilient. Intensive treatment is required to return to communities with less woody cover. Brush treatment tends to be short-lived. Treated areas rapidly return to the Mixed Brush Community (2.2) due to the presence of propagules on and adjacent to treated areas. Observation shows that even effective treatment will require constant maintenance to suppress brush reestablishment. Without maintenance, canopy cover may exceed 50 percent in three to five years. Annual production is dominated by woody species. Browsing animals such as goats and deer can find fair food value if browse plants have not been grazed excessively and created browse lines. Forage quantity and quality for cattle is low. Prescribed fire is not a viable treatment option for conversion of this site back to a semblance of the Midgrass Savannah Community (1.1). Chemical brush control may be necessary for large-scale treatments. Individual plant treatment with herbicides on small patches may be a viable economical option and may help restore the savannah appearance. Mechanical treatment combined with seeding of native species may have the greatest chance of success in returning the reference community, although it may not be economical.

Pathway 2.1A
Community 2.1 to 2.2

Without some form of brush control, woody density and canopy cover will increase in the Midgrass/Mixed-Brush Community (2.1) until it converts into the Mixed-Brush Community (2.2). Improper grazing management and/or long-term drought (or other growing-season stress) will accelerate this transition. Woody canopy cover exceeding 50 percent indicates this community shift. Midgrasses will further decrease as the result of the increase in shade thus resulting in an increase in shortgrasses. Improper grazing or other long-term growing-season stress can increase the composition of less productive grasses 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 decline in production and shift in composition toward sedges, grasses, and forbs suited to growing in shaded conditions with reduced 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

Brush management and/or fire can reduce the woody component of the Mixed-Brush Community (2.2) to below the community shift level of 50 percent woody canopy cover. Continued fire and/or brush management will be required to maintain the woody canopy cover level below 50 percent. It may be difficult to shift back to the Grass/Mixed-Brush Community (2.1) with fire alone. The canopy reduces understory growth to carry intense fires. Once woody species become tall enough to not be killed by understory fires, fire is likely to only remove small woody plants. Large trees will dominate over an herbaceous understory. This is amplified if the understory transitions to cool-season grasses, which further reduce opportunity for prescribed fire. If the woody component has been invaded, fire remains an option to create transition 2.2A. If the herbaceous component has transitioned to shortgrasses and low forbs, proper grazing management (combined with favorable moisture conditions and adequate seed source) will be necessary to facilitate the shift of the understory component in the Mixedgrass/Brush Community (2.2) to the Midgrass/ Mixed-Brush Community (2.1). Range planting may accelerate the transition of the herbaceous community, particularly when combined with favorable growing conditions. Range planting is more commonly associated with restoration efforts associated with Restoration Pathway R2A. The driver for community shift 2.2A is fire and/or brush control. Transition 2.2A is difficult to create with management. Due to the large size of trees present, brush control may require selective removal of large trees along with brush control aimed at smaller trees and saplings.

State 3
Sparsely Vegetated

Community 3.1
Sparsely Vegetated

Continued lack of fire and brush management along with abusive grazing results vegetation loss and increase of bare ground. If the fertile A-horizon erodes, the site transitions to the Sparsely Vegetated Community (3.1), dominated by low producing annual forbs and grasses. The decline may be exacerbated by extended drought conditions. Annual forbs such as broomweed are abundant. Stunted mesquite, lotebush, and pricklypear are scattered across the site. In the lowest stages of degradation, there is a significant amount of bare ground, and scalded areas are obvious. Some of the scalds are the result of geologic erosion while others are the result of long-term abuse and mismanagement. This plant community is a terminal state that will not return to reference plant communities because of total degradation of the soil, and complete loss of most of the higher successional native plant species. 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 period.

State 4
Converted

Community 4.1
Converted Land

The Converted Land Community (4.1) occurs when the site, is cleared and plowed for planting to cropland, hayland, native grasses, tame pasture, or use as non-agricultural land. The Converted State (4) includes cropland, tame pasture, hayland, rangeland, and go-back land. Agronomic practices are used with non-native forages in the Converted State (4) and to make changes between the communities in the Converted State (4). The native component of the savannah is usually lost when seeding non-natives. Even when reseeding with natives, the ecological processes defining the past states of the site can be permanently changed. The site is frequently converted to cropland or tame pasture sites because of its deep fertile soils, favorable soil/water/plant relationship, and level terrain. Hundreds of thousands of acres have been plowed up and converted to cropland, pastureland, or hayland. The Clay Loam site can be an extremely productive forage producing site with the application of optimum amounts of fertilizer. Crop and pasturelands require weed and shrub control because seeds remain present on the site, either by remaining in the soil or being transported to the site. Converted sites require continual fertilization for crops or tame pasture to perform well. Common introduced species include coastal bermudagrass, buffelgrass, kleingrass, and Old World bluestems (Bothriochloa spp.) which are used in hayland and tame pastures. Wheat, oats, forage sorghum, grain sorghum, cotton, and corn are major crop species. Cropland and tame pasture require repeated and continual inputs of fertilizer and weed control to maintain the Converted State. Without agronomic inputs, the site will shift to the Abandoned Land Community (4.2).

Community 4.2
Abandoned Land

The Abandoned Land Community (4.2) occurs when the Converted Land Community (4.1) is abandoned or mismanaged. Mismanagement can include poor crop or haying management. Pastureland can transition to the Abandoned Land Community (4.2) when subjected to lack of brush management, fire and improper grazing management (typically long-term overgrazing). Long-term cropping can create changes in soil chemistry and structure that make restoration to the reference state (1) very difficult and/or expensive. Return to a near native savannah community on the Clay Loam Site is more likely to be successful if soil chemistry, microorganisms, and structure are not heavily disturbed and if aggressive invasive plants are not present. Preservation of favorable soil microbes increases the likelihood of a return to reference, or near reference, conditions. Restoration to native savannah will require seedbed preparation and seeding of native species. Protocols and plant materials for restoring savannah communities are a developing portion of restoration science. Sites can be restored to the Savannah State (1) in the short-term by seeding mixtures of commercially available native grasses. With proper management (prescribed grazing, weed control, brush control), these sites can come close to the diversity and complexity of Midgrass Savannah Community (1.1). It is unlikely that abandoned farmland will return to the Savannah State (1) without active brush management because the rate of shrub increase will exceed the rate of recovery by desirable grass species. Without active restoration, the site is not likely to return to reference conditions due to the presence of introduced forbs and grasses. The native component of the savannah is usually lost when seeding non-natives. Even when reseeding with natives, the ecological processes defining the past states of the site can be permanently changed.

Pathway 4.1A
Community 4.1 to 4.2

The Converted Land Community (4.1) will transition to the Abandoned Land Community (4.2) if improperly managed as cropland, hayland, or pastureland. Each of these types of converted land is unstable and requires constant management input for maintenance or improvement. This community requires inputs of tillage, weed management, brush control, fertilizer, and reseeding of annual crops. The driver of this transition is the lack of management inputs necessary to maintain cropland, hayland, or pastureland.

Pathway 4.2A
Community 4.2 to 4.1

The Abandoned Land Community (4.2) will transition to the Converted Land Community (4.1) with proper management inputs. The drivers for this transition are weed control, brush control, tillage, proper grazing management, and range or pasture planting.

Transition T1A
State 1 to 2

Shrubs make up a portion of the community in the Savannah State (1), hence woody propagules are present. Regardless of grazing management, without some form of brush control, the Midgrass Savannah Community (1.1) will transition to the Shrubland State (2) even if the understory component does not shift to dominance by shortgrasses. Therefore, the Savannah State (1) is always at risk for shrub and tree dominance and the transition to the Shrubland State (2) in the absence of fire and brush management. The driver for Transition T1A is lack of fire and/or brush control. The mean fire return interval in the Savannah State (1) is three to seven years. Most fires will burn only the understory. Even with proper grazing and favorable climate conditions, lack of fire for 5 to 10 years will allow trees and shrubs to increase in canopy to reach the 20 percent threshold level. The introduction of aggressive woody invader species increases the risk and accelerates the rate at which this transition is likely to occur. Transition to the Shrubland State (2) can occur from any community within the Savannah State (1), it is not dependent 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.

Transition T1B
State 1 to 3

The Savannah State (1) transitions to the Sparsely Vegetated State (3) if soil loss continues or increases to the point that total plant canopy cover is less than 25 percent or total annual aboveground biomass production is less than 1,000 pounds per acre. This could occur due to overgrazing (failure to adjust stocking rate to declining forage production due to increased dominance of unpalatable forbs or inaccessible shrubs), long-term lack of fire, warming climate, or extensive drought. The trigger for this transition is the loss of vegetation. This creates open spots with bare soil. If the A-horizon erodes, the soil fertility decreases sharply and the site transitions to the Sparsely Vegetated State (3). Other key factors signaling approach of transition T1B are increases in soil physical crusting, decreases in cover of cryptogamic crusts, decreases in soil surface aggregate stability, and/or evidence of erosion including water flow patterns, development of plant pedestals, and litter movement. The driver for this transition is improper grazing management in combination with long-term drought.

Transition T1C
State 1 to 4

The transition to the Converted State from the Savannah State occurs when the savannah is plowed for planting to cropland or hayland. The threshold for this transition is the plowing of the savannah soil and removal of the woody plant community. The Converted State (4) includes cropland, tame pasture, and go-back land. The site is considered “go-back land” during the period between cessation of active cropping, fertilization, and weed control and the return to the “native” states. Agronomic practices are used to convert rangeland to the Converted State (4) and to make changes between the communities in the Converted State (4). The driver for these transitions is management’s decision to farm the site.

Restoration pathway R2A
State 2 to 1

Restoration of the Shrubland State (2) to the Savannah State (1) requires substantial energy input. The driver for this restoration pathway is removal of woody species, restoration of native herbaceous species, and ongoing management of woody species. Without maintenance, woody species are likely to increase again.

Transition T2A
State 2 to 4

The transition to the Converted State from the Shrubland State occurs when the site is plowed for planting to cropland or hayland. The size and density of brush in the Shrubland State (2) will require heavy equipment and energy-intensive practices (i.e. rootplowing, raking, rollerchopping, or heavy disking) to prepare a seedbed. The threshold for this transition is the plowing of the savannah soil and removal of the woody plant community. The Converted State (4) includes cropland, tame pasture, and go-back land. The site is considered “go-back land” during the period between cessation of active cropping, fertilization, and weed control and the return to the “native” states. Agronomic practices are used to convert rangeland to the Converted State (4) and to make changes between the communities in the Converted State (4). The driver for these transitions is management’s decision to farm the site.

Restoration pathway R3A
State 3 to 1

This state has lost soil or vegetation attributes to the point that recovery to the Savannah State (1) will require reclamation efforts, such as soil rebuilding, intensive mechanical treatments, and/or reseeding in recommended areas only. The driver for this restoration pathway is reclamation efforts.

Transition T3A
State 3 to 4

The transition to the Converted State from the Sparsely Vegetated State occurs when the site is plowed for planting to cropland or hayland. The threshold for this transition is the plowing of the savannah soil and removal of the woody plant community. The Converted State (4) includes cropland, tame pasture, and go-back land. The site is considered “go-back land” during the period between cessation of active cropping, fertilization, and weed control and the return to the “native” states. Agronomic practices are used to convert rangeland to the Converted State (4) and to make changes between the communities in the Converted State (4). The driver for these transitions is management’s decision to farm the site.

Restoration pathway R4A
State 4 to 1

Restoration from the Converted State (4) can occur in the short term through active restoration or over the long-term due to cessation of agronomic practices. Cropland and tame pasture require repeated and continual inputs of fertilizer and weed control to maintain the Converted State (4). If the soil chemistry and structure have not been overly disturbed (which is most likely to occur with tame pasture) the site can be restored to the Savannah State (1). The level of disturbance while in the converted state determines whether the site restoration pathway is likely to be (R4A, R4B, or R4C). Return to native savannah communities in the Savannah State (1) is more likely to be successful if soil chemistry and structure are not heavily disturbed. The presence of residual introduced forage plants may preclude a full return to native grasses. Preservation of favorable soil microbes increases the likelihood of a return to reference, or near reference, conditions as does remnant seed sources. Converted sites can be returned to the Savannah State (1) through active restoration, including seedbed preparation and seeding of native grass and forb species. Protocols and plant materials for restoring savannah communities is a developing part of restoration science. The driver for both of these restoration pathways is the cessation of agricultural disturbances.

Transition T41
State 4 to 2

Restoration from the Converted State (4) can occur in the short term through active restoration or over the long-term due to cessation of agronomic practices. Cropland and tame pasture require repeated and continual inputs of fertilizer and weed control to maintain the Converted State (4). Heavily disturbed soils are more likely to return to the Shrubland State (2) or the Sparsely Vegetated State (3). Without continued disturbance from agriculture, the site can eventually return to either the Savannah (1) or Shrubland State (2). The level of disturbance while in the converted state determines whether the site restoration pathway is likely to be (R4A, R4B, or R4C).

Restoration pathway R4C
State 4 to 3

Restoration from the Converted State (4) can occur in the short term through active restoration or over the long-term due to cessation of agronomic practices. Cropland and tame pasture require repeated and continual inputs of fertilizer and weed control to maintain the Converted State (4). Heavily disturbed soils are more likely to return to the Shrubland State (2) or the Sparsely Vegetated State (3). Without continued disturbance from agriculture, the site can eventually return to either the Savannah (1) or Shrubland State (2). The level of disturbance while in the converted state determines whether the site restoration pathway is likely to be (R4A, R4B, or R4C).