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Vegetation dynamics and management
We don’t know much about what the vegetation ‘should’ be
This region was not settled by people of European and African descent until the mid 1800's or later. Between 1492 and settlement are 350 years in which the original inhabitants disappeared, horses and cattle formed large feral populations and the Apaches and Comanches developed a horse- and bison-based culture. We do not know to what extent these events affected the frequency of fires, the intensity of grazing, and so forth.
Because of the lack of natural lakes and other wetlands, there are no pollen cores to help us reconstruct past vegetation.
Most textbooks include this region either with the eastern deciduous forest or with the grasslands of the Great Plains. Nevertheless, it seems most likely that it has never been either forest or grassland. It also seems likely that it has always been a mosaic of woodlands (defined as vegetation dominated by trees of lower stature than true forests, with a more open canopy) and savannas (defined as areas with scattered woody plants in an herbaceous matrix). It is not known what the ratio of woodland to savanna was, the scale of this mosaic, or how permanent the spatial arrangement of woodland and savanna was. Likewise we do not know the scale or persistence of woody patches within the savannas. We assume that the species common today were also common in the past, but even this may not be true (see below). We assume that fire played a role in maintaining the savannas. The woodlands may also have experienced either surface or crown fires, or both.
We don’t even know these things about the vegetation at the time of settlement in the mid to late 1800's. Here a study of old survey records, photographs, etc. might be very useful.
In some ways we are fortunate, however. Very little of the Plateau was ever plowed, in contrast to the Blackland Prairie to the east of the Plateau, whose vegetation is nothing like its original tallgrass grassland. There is a long history of cutting Juniperus ashei, first for fence posts and to improve grazing. The Edwards Plateau has been, and continues to be, heavily grazed, but it had bison, so the native grasses are relatively grazing tolerant, unlike those of the Great Basin and central California. Other than Bothriochloa ischaemum (King Ranch bluestem) and Solenopsis invicta (fire ants), all the common species are still natives. Compared to much of the rest of the United States, the vegetation of this region is probably relatively unaltered.
A brief overview of present vegetation
We can identify several different plant communities that grade into one another on the eastern Edwards Plateau, while remembering that there are many sites whose vegetation is intermediate between two or more of these.
- mixed woodland, dominated by Quercus buckleyi (Texas oak, Spanish oak), especially on slopes just below hilltops, Juniperus ashei (Ashe juniper, cedar), Ulmus crassifolia (cedar elm), Celtis spp. (hackberry), etc. This is the habitat of the endangered golden-cheeked warbler.
- tree savanna, dominated by Quercus fusiformis (live oak), Juniperus ashei (Ashe juniper), Berberis trifoliolata (agarita), Diospyros texana (persimmon), etc. Mesquite (Prosopis glandulosa) is occasionally present where the soil is deep enough. The herbaceous layer may be dominated by shortgrasses, especially Bouteloua rigidiseta (Texas grama grass), Aristida longiseta (three-awn grass), Hilaria belangeri (mesquite grass), and/or Buchloe dactyloides (buffalo grass), or by midgrasses, especially Schizachyrium scoparium (Andropogon scoparius) (little bluestem grass), Nassella (Stipa) leucotricha (Texas winter grass), Bouteloua hirsuta (hairy grama grass), Aristida glauca/purpurea complex (three-awn grasses), and/or Muhlenbergia reverchonii (muhly grass). Taller grasses are more common on hillsides and shorter grasses in flatter areas because grazing pressure is usually less on hillsides than in flatter areas.
- shrub savanna or shrubland, dominated by Rhus spp. (sumac), Quercus sinuata (shin oak), and other shrubby species, with or without small individuals of the larger woody species. This is the habitat of the endangered black-capped vireo.
- cedar brake, dominated by Juniperus ashei (Ashe juniper), often with a few remaining tall Quercus spp. trees (oaks).
- bottomland forest, dominated by Carya illinoensis (pecan). Rare, since the few bottomland sites are now mostly cultivated or under reservoirs.
- riparian forest, dominated by Taxodium distichum (bald cypress), Platanus occidentalis (sycamore), and/or Salix nigra (willow). Grows as a ‘gallery forest’ along some creeks and rivers.
- highly disturbed sites, especially roadsides. Baccharis neglecta (Roosevelt weed) is found almost exclusively in dry sites where the soil has been intensively disturbed. Yucca rupicola (twist leaf yucca) is often found growing with it on roadside banks. Roadside areas that receive road runoff water are often significantly wetter then the rest of the landscape and often have tall grasses, including Sorghum halapense (Johnson grass).
For a generalized vegetation map of Texas, see http://www.csdl.tamu.edu/FLORA/taes/tracy/regecoNF.htm
For a more detailed vegetation map of Texas, go to http://www.tpwd.state.tx.us/landwater/land/maps/gis/map_downloads/ and click on one of the maps labeled ‘vegetation types’. The actual link to the smallest pdf file is http://www.tpwd.state.tx.us/landwater/land/maps/gis/map_downloads/media/pwd_mp_e0100_1070n_08.pdf.
Vegetation dynamics and management
Under present conditions, Juniperus ashei (Ashe juniper, cedar) rapidly increases in number and cover in every kind of site in this region except bottomland and riparian sites. J. ashei and other woody species are cleared by land managers to maintain or increase forage for cattle and other stock, for aesthetic or recreational reasons, and to maintain savanna vegetation. Savanna sites in which this species is not removed usually become cedar brakes.
Shrub savannas and shrublands are also not stable vegetation types under present conditions. They tend to become cedar brakes or perhaps mixed woodlands.
The potential relationship between cedar brakes and mixed woodlands is not clear. Under present conditions there is little or no regeneration of oaks due to deer browsing. Casual observations suggest that other woody angiosperm species are also not regenerating.
Herbicides are not sufficiently effective on Juniperus ashei, so brush clearing is done mechanically. Sometimes all woody plants are cleared; sometimes the oaks are left. J. ashei does not re-sprout if cut close to the ground or uprooted. After clearing, the brush is usually piled up and burned.
Fire is beginning to be used for J. ashei control in savannas in some areas. However grazing must be reduced or eliminated long enough to allow sufficient fine fuel to accumulate, and the mortality rate of small J. ashei plants in winter fires was found to be only about 40%, so the usefulness of fire is limited. Presumably summer fires would be more effective, but most counties have a burn ban in place all summer.
Under present conditions fires are uncommon. Lack of fine fuel, fire suppression, and the firebreaks created by roads all probably help make surface fires uncommon. Cedar brakes and mixed woodlands can support crown fires, although they are rare. These fires are very intense and very dangerous. The possibility of a crown fire in western Austin and in the urban/rural interface west of Austin is a serious safety concern.
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