Ecological dynamics

This site is unique and somewhat variable in the range and diversity of the native plant community it supports. Climate, soils, parent material, slope, aspect, and rock cover are the major factors influencing plant communities on the site. Natural fire is also an influencing factor. Herbivory is of minimal influence on most Breaks areas. In order to describe a historic climax plant community for the site as a whole, it is necessary to use a broader range when addressing species and plant groupings because of the variations found from one location to another, and the different aspects of the Breaks site. Plant communities are generally similar, but there are some subtle differences that need to be defined.

In most instances, the HCPC (Historic Climax Plant Community) would fit within the categories of grassland/shrub with few to moderate forbs and few trees. In some cases, the production of shrubs may be almost equal with that of grasses. For the purposes of this ecological site description, short-growing woody species including the shrubby growth form of mesquite are considered to be shrubs. Oneseed juniper is considered a tree.

There is definitely a more xeric as well as a more mesic community phase for the Breaks site. The phase depends mostly on aspect, and perhaps to a lesser extent, amount of rock cover and degree of slope. When observing this site, it is evident that there is a larger shrub component present on the north- and northeast-facing slopes (mesic phase) than on south- and southwest-facing slopes (xeric phase). Also, the diversity of species is greater on the more mesic slope, and the total community production is also somewhat greater there. These differences in plant community are aspect-driven. Mesic slopes have less exposure to the intense rays of summer sunlight, and this translates to lower soil temperature, lower heat reflection from the soil surface back to the plant, and lower rates of evaporation. Mesic slopes are shaded for a greater percent of the daylight hours during the hottest part of the year.
The presence of large and medium sized rock or boulders and their orientation creates a micro environment of its own. Moderate cover of larger rocks increases shading effect on individual plants, traps moisture underneath the rocks, and acts as a mini watershed for the area that is immediately adjacent. There also exists an element of heat transfer wherein the rock itself absorbs and reflects some of the heat that would normally be absorbed directly into the soil. And lastly, the rock cover decreases soil erosion by absorbing raindrop energy and slowing the runoff process by interrupting flow patterns.
Slopes with a south and west facing aspect (xeric) tend to have sparser cover, more shortgrass species, and more drought-tolerant half shrubs. The degree of slope is also a factor and works in conjunction with aspect and rock cover. Steeper slopes tend to have less soil formation and more rapid runoff. The plant communities found on the breaks site can best be described by addressing both the mesic and the xeric phases independently.

The plant community composition typical of the mesic (north- and east-facing slopes) portion of the Breaks site is grasses shrubs or = forbs trees. The main grasses are sideoats grama (Bouteloua curtipendula), little bluestem (Schizachyrium scoparium), sand bluestem (Andropogon hallii), Canada wildrye (Elymus canadensis), blue grama (Bouteloua gracilis), New Mexico feathergrass (Hesperostipa neomexicana), hairy grama (Bouteloua hirsuta), sand dropseed (Sporobolus cryptandrus), and occasionally Indiangrass (Sorghastrum nutans). The major forb species include Mexican sagewort (Artemisia ludoviciana), halfshrub sundrop (Calylophus serrulata), dotted gayfeather (Liatris punctata), broom nailwort (Paronychia jamesii), blacksamson echinacea (Echinacea angustifolia), plains actinia (Tetraneuris scaposa), baby white aster (Chaetopappa ericoides), and broom snakeweed (Gutierrezia sarothrae). Shrubs most commonly found are skunkbush sumac (Rhus trilobata), feather dalea (Dalea formosa), Mormon tea (Ephedra sp.), and occasionally grape (Vitis sp.), plains greasebush (Glossopetalum planitierum), and winterfat (Krascheninnikovia lanata). Trees are limited to hackberry (Celtis laevigata), western soapberry (Sapindus saponaria), and oneseed juniper (Juniperus monosperma). Trees are infrequent with the exception of juniper, which occurs in pockets or in scattered stands depending on local situations. In general, grasses make up 40 to 50 percent of total production; forbs make up 15 to 25 percent; shrubs make up 20 to 35 percent, and trees make up 5 to 10 percent.

The plant community composition typical of the xeric (south- and west-facing slopes) portion of the Breaks site is grasses forbs = shrubs trees. The main grasses are sand dropseed (Sporobolus cryptandrus), hairy grama (Bouteloua hirsuta), black grama (Bouteloua eriopoda), slim tridens (Tridens muticus), perennial threeawn (Aristida wrightii), hairy tridens (Erioneuron pilosum), blue grama (Bouteloua gracilis), galleta (Pleuraphis jamesii), New Mexico stipa (Hesperostipa neomexicana), and sideoats grama (Bouteloua curtipendula). The majority of the grasses are drought-tolerant short-grass species. The major forb species include baby white aster (Chaetopappa ericoides), plains actinia (Tetraneuris scaposa), sand lily (Mentzelia nuda), Wright’s buckwheat (Eriogonum wrightii), plains zinnia (Zinnia grandiflora), Gordon’s bladderpod (Lesquerella gordonii), and broom snakeweed (Gutierrezia sarothrae). Shrubs most commonly found are feather dalea (Dalea formosa), catclaw mimosa (Mimosa biuncifera), occasional skunkbush (Rhus trilobata), prickly pear (Opuntia phaecantha), mesquite (Prosopis glandulosa), plains greasebush (Glossopetalum planitierum), and occasional winterfat (Krascheninnikovia lanata). The only tree species found is oneseed juniper (Juniperus monosperma), which may occur in scattered stands. In general, grasses make up 55 to 65 percent of total production; forbs make up 10 to 20 percent; shrubs make up 10 to 20 percent; and trees make up 5 to 10 percent. There are some breaks sites where juniper has become dominant over many decades, but even in these cases, the xeric phase will usually not have as thick a stand of juniper as will the mesic phase of community. Historical records indicate that in the HCPC the juniper was not nearly as extensive as it is now in many localities. The amounts of juniper referred to in this section are ranges thought to have been present in the HCPC.

The increased total production potential and the increased diversity for the mesic community phase can be attributed to the moisture related factors addressed in previous paragraphs.

Unlike most of the ecological sites in this MLRA, historic grazing by domestic livestock is not a major issue in the development of and change within plant communities over time on the Breaks site. Steep slopes often render the site largely inaccessible to cattle, although some light use of the foot slopes occurs. Sheep (and/or goats) would be more likely to access this site than cattle. There was periodic grazing of sheep by Mexican pastores for a time in the late 1800’s but it is not likely that this grazing has left any visible effects on the site. There is little grazing by sheep or goats in this region at present. Browsing use by mule deer can be observed, but is generally light.

If the deer population is locally heavy, it is possible that some preferred browse species might receive enough browsing pressure to affect the composition of the shrub component. In pristine times, elk probably grazed in this region, but only token amounts of elk remain on a local basis. Although herbivory has probably had some minor effect on plant communities over time, it is unlikely that this was a major influence.

Natural fire did influence the historic plant community development on many of the breaks sites in this MLRA. It is believed that fire suppressed some of the woody vegetation that is now dominant on many portions of this site. Oneseed juniper is especially susceptible to damage by fire, and this species has greatly increased from HCPC populations, largely due to natural fire being suppressed by various factors and for various reasons. Although the terrain characterizing this site is such that some local areas may have been protected from naturally occurring fire, it is certainly likely that portions of this site did burn with some regularity. The orientation of the slopes, amount of fuel, and the direction of the wind during a given fire would be determining factors. In observing wildfires in rough country, it can be noted that although the burns may appear somewhat patchy, some very rough areas often burn. Juniper has increased over widespread areas in this MLRA and now occupies many sites that are basically smooth terrain. When fires regularly occurred, the juniper tended to be more confined to the rougher terrain having sparse fuel loads where protection from fire was more likely. When considering prescribed fire as a management tool, it is essential to plan well in advance of the burn – and to consider all environmental factors and necessary safety preparations.

NOTE: Rangeland Health Reference Worksheets have been posted for this site on the Texas NRCS website (www.tx.nrcs.usda.gov) in Section II of the eFOTG under (F) Ecological Site Descriptions.

STATE AND TRANSITIONAL PATHWAYS : ( DIAGRAM)

Narrative:
The following diagram suggests some pathways that the vegetation on this site might take. There may be other states not shown on the diagram. This information is intended to show what might happen in a given set of circumstances; it does not mean that this would happen the same way in every instance. Local professional guidance should always be sought before pursuing a treatment scenario.