State 1
Grassland

Community 1.1
Midgrass Dominant

The dominant grasses for this site are sideoats grama, little bluestem, feather bluestem, bristlegrass species, and Arizona cottontop. Arizona cottontop and plains bristlegrass are the more opportunistic species on this site. Silver bluestem is prevalent in the eastern portion of the region, with tanglehead and Arizona cottontop becoming more abundant in the drier western region. Guajillo is the most common woody shrub, but others include mesquite and various acacias. These would be small and obscured by grasses. Fire did not play as important a role on this site as in deeper more productive sites due to lower production of grasses to burn. More than likely, fire occurred after a year of good rainfall followed by a dry season.

Community 1.2
Shortgrass Dominant

This phase (1.2) of the Grassland State still exhibits a grassland plant structure with a shift to weaker, less palatable shortgrasses. Heavy continuous grazing removes many of the midgrasses from the community. Annual and perennial forbs are more common as weaker plants give way to more bare ground. With continued grazing pressure, increaser grasses become more common across the site. Plant production becomes more erratic. Drought interacts with grazing to trigger mid-to-shortgrass transitions. Termite activity often increases during low rainfall periods to further decrease production and ground cover.

The shortgrass and forb communities are less productive than the midgrass communities they replace. Reductions in above-ground cover and root biomass make this community more prone to runoff, erosion, and prolong the effects of drought. A reduction in ground cover leads to higher soil temperatures that, in conjunction with the reduction of leaf and root biomass inputs, can cause declines in soil organic matter. This reduces soil water holding capacity and fertility that further affects species composition and production. Fire frequency and intensity in this community is low because of low fine fuel load and continuity. As a result, woody plants are free to increase in size, density, and total cover.

When removing grazing pressure, midgrasses can regain dominance on the site and undesirable trends in soil organic matter, fertility, temperature, and erosion can be arrested and reversed. However, this process is very difficult to predict. Restoration of fine fuel biomass and continuity enable use of prescribed fire to reduce the stature and cover of established woody plants. The extent to which the original midgrass community can be re-established will depend on the extent to which soil physical and chemical properties were altered during retrogression.

Community 1.3
Mixed-grass Savannah

In the absence of fire, savannahs with 5 to 20 percent woody cover develop from the shortgrass community. Guajillo, blackbrush, and leatherstem are some of the woody shrubs that increase. Mesquite can establish in a matrix of competitive, late seral grasses, but establishment and growth rates will be greater on retrogressed and grazed sites. As established woody plants develop, they modify soils and microclimate to facilitate establishment of other shrubs such as brasil, lime prickly ash and algerita (Berberis trifoliolata). Discrete mixed-brush clusters begin to develop in the grassy matrix, giving the landscape a parkland appearance. However, woody plants in this state are not of sufficient size, leaf area or density to affect herbaceous plants. Ground cover remains mid or shortgrass dominated, depending on grazing pressure.

Pathway 1.1A
Community 1.1 to 1.2

A shift to the Shortgrass Dominant Community occurs if the Midgrass Dominant Community is weakened by excessive leaf removal. Drought hastens the process. A reduction in midgrass also corresponds in a reduction of fuel loading needed for fire to effectively suppress woody species.

Pathway 1.2A
Community 1.2 to 1.1

This plant community can still be managed back to the Midgrass Dominant Community (1.1). Prescribed Grazing will be essential to reverse the trend toward the Mixed-grass Savannah (1.3). Prescribed Grazing will give the midgrasses a chance to restore vigor and compete. In years when rainfall provides extra grass fuel, burns can be used to suppress the brush.

Pathway 1.2B
Community 1.2 to 1.3

If heavy continuous grazing continues, the plant community will transition to a Midgrass Savannah Community (1.3) with a woody canopy of 5 to 20 percent.

Pathway 1.3A
Community 1.3 to 1.2

Conversion of these savannahs to grassland can be achieved with prescribed burning (individual plant treatments could be considered, but woody cover is too low to warrant conventional large-scale chemical or mechanical brush management). Conversion of shortgrass savannah to midgrass dominated grassland requires long-term relaxation of grazing pressure in conjunction with prescribed burning when the opportunity for adequate grass fuel exists.

State 2
Shrubland

Community 2.1
Shrubland Complex

Figure 14. 2.1 Shrubland Complex

Lack of fire and continued heavy grazing causes a shift from the Grassland State with 0 to 20 percent cover to shrublands with 20 to 50 percent cover. The transition may be abrupt, triggered by losses of grass cover during drought and rapid establishment and growth of woody plants in post-drought periods. As the density, height and canopy area of mesquite, guajillo, and acacia are maximized, understory shrubs such as brasil, lime prickly ash, spiny hackberry, and blackbrush continue to grow and become co-dominant. This community develops a pronounced parkland appearance with scattered shrub clusters. Herbaceous composition and production in zones between shrub clusters depends on grazing history and is comparable the reference community. However, extensive bare ground occurs beneath shrub canopies where herbaceous production is dramatically reduced due to shading and competition for water and nutrients by shallow-rooted woody plants.

At this point, prescribed grazing alone will not restore this community back to the Grassland State (1). During the growing season, light showers are captured in the canopy of the shrubs and evaporate. Energy flow and nutrients are predominately used by the shrubs. Cool-season annual forbs and grasses are produced by fall and winter rains. With these conditions, prescribed fire is a very limited option due a lack of fine fuel load. With continued heavy grazing and no brush management, woody cover will increase to more than 50 percent.

Use of prescribed fire can be very difficult, as shrub clusters disrupt fine fuel continuity, making it difficult for fires to spread. Low productivity of herbaceous patches translate into low fuel loads and fires may not be hot enough to carry through shrub patches. Furthermore, relaxation of grazing does not guarantee that prescribed fire can be used. In some years, there may not be enough rainfall to generate sufficient fuel; in other years, fuel production may be high, but warm temperatures may keep plants green and too moist for effective prescribed winter burns. Prescribed summer fires burn hotter and therefore can be more effective than winter burns, but are also more difficult to control.

Community 2.2
Woodland

Figure 17. 2.2 Woodland Community

In the absence of fire and brush management, a highly stable woodland develops as shrubs and trees increase in abundance and coalesce. This phase of the Shrubland Complex (2) features a woody shrub canopy greater than 50 percent. Blackbrush, leatherstem, condalia species, mesquite, and the other acacia species may begin to die due to natural causes, leaving a diverse mixed-shrub community. Ground cover and herbaceous production beneath shrub canopies is minimal, but soil organic carbon and nitrogen levels are enhanced. Due to the increase in shrub cover, more nutrients are used by the shrub cover than the herbaceous plants. Forbs and legumes tend to persist on the site, but in this deteriorated condition palatable perennials are replaced by a wide variety of annual forbs. Shortgrasses such as red grama, hairy tridens, gummy lovegrass, and red lovegrass may be the only species present.

Pathway 2.1A
Community 2.1 to 2.2

Shrubland Complex

Woodland

Transition to the 2.2 Woodland Community occurs with continued heavy grazing and lack of brush management. Woody cover is greater than 50 percent.

Pathway 2.2A
Community 2.2 to 2.1

Woodland

Shrubland Complex

Brush management is required to reduce the woody canopy below 50 percent. Mechanical or chemical options exist depending on landowner goals and cost effectiveness.

State 3
Converted Land

Community 3.1
Converted Land

This plant community is developed by applying brush management and seeding. The conversion can actually come from any of the previously mentioned communities where brush needs to be reduced and a seed source added to establish a desired plant community. In some instances, an adequate seed source may already exist in the soil.

When rootplowing is applied as brush management on this site, long term forb and woody plant diversity will be greatly reduced. Previous attempts at native seeding in this region were met with mixed results because of the seed source not being locally adapted to the region. Many of the grass species listed in the reference plant community are commercially available from collections made in south Texas. The locally adapted species are expected to be more successful in seeding efforts as compared to seed developed several hundred miles outside the region. However, proper seedbed preparation, planting techniques, and timely rainfall are essential for success.

The most common introduced grass species seeded is buffelgrass (Cenchrus ciliare). Seeding this species should be cautiously considered due to its aggressive nature to dominate plant communities and reduce herbaceous diversity. Once planted, conversion of buffelgrass dominated areas back to native grass is extremely difficult and rarely successful. The decision of which species to seed is a management decision based on clearly defined goals for livestock and wildlife. Careful consideration should be taken prior to seeding introduced species. Once introduced species are seeded, it is often difficult or impractical to remove them should objectives change. Because of the residual seed source of woody plants, encroachment is inevitable. To help maintain this plant community, prescribed grazing along with fire and some integrated brush management will be needed.

Community 3.2
Converted Land with Woody Seedlings

This plant community develops from the Converted Land Community (3.1). Abandoned croplands and land seeded with exotic or native grasses are prone to encroachment by woody plants and with heavy grazing or the absence of fire, can revert to shrublands. These changes are triggered by recruitment and growth of shrub plants in periods following drought. The shrub seedlings that appear in pastures establish by wind, water, animals, or in the soil from the seed bank. Other seedlings may be re-sprouts arising from woody plant stems, roots, burls, and lignotubers that remain following brush management. These tend to grow faster and have higher establishment rates than true seedlings. Nearly all shrubs on this site have this capability of vegetative regeneration; hence it is the primary source of woody plants that re-establish following brush management.

Pathway 3.1A
Community 3.1 to 3.2

A shift to the 3.2 Converted Land with Woody Seedlings occurs when no management activities such as prescribed grazing, brush management, or fire are accomplished as brush invades. Drought worsens the process. A reduction in planted grasses also corresponds in a reduction of fuel loading needed for fire to effectively suppress woody species.

Pathway 3.2A
Community 3.2 to 3.1

Brush management to control invading woody species will transition the site back to the 3.1 Converted Land Community.

Transition T1A
State 1 to 2

If heavy grazing continues and prescribed fire is not used, this phase will transition to a Shrubland State (2) with woody cover greater than 20 percent.

Transition T1B
State 1 to 3

The transition to the Converted Land State is triggered by major ground disturbing mechanical treatment and planting to native or introduced forages (usually following brush management).

Restoration pathway R2A
State 2 to 1

Aggressive brush and grazing management is required to revert the system back to the Grassland State. Re-seeding may be necessary if the grassy matrix is dominated by shortgrasses and annual forbs. Herbaceous production following brush management can be elevated by allowing them more resources. However, most shrubs are capable of re-sprouting, so treatments are often short lived. Allowances for follow-up treatments should be made. In the absence of follow-up treatments, woody cover and density may increase relative to pre-treatment conditions with adverse effects on forage production.

Transition T2A
State 2 to 3

The transition to the Converted Land State is triggered by major ground disturbing mechanical treatment and planting to native or introduced forages. Planting is usually done following brush management.

Transition T3A
State 3 to 2

Without follow-up management, seedlings of shrubs establish themselves and spread. If the seedlings are not controlled, the plant community will transition to the Shrubland Complex State (2) and will require machinery or herbicides to reduce the canopy and arrest the spread. Production of the grasses depends on the grazing management that has been applied and the canopy of the shrubs invading the site. As the canopy of the shrubs expands, grass and forb production will be reduced accordingly. Proper grazing and brush management are required to prevent woody plant ‘seedlings’ from dominating the site.