Ecoregions of central Texas
Austin itself falls into two major ecoregions: the Edwards Plateau (western side of Austin) and the Blackland Prairie (eastern side of Austin). The dividing line is the Balcones Fault (now inactive) which runs approximately north-south through Austin.
In general, the divisions between major ecoregions in central Texas are determined by geology. larger version of the vegetation map on the right
The Edwards Plateau (#7 in the map) is composed of flat layers of hard limestone, often deeply eroded into hills and valleys near Austin, and overlain by thin soils. Its vegetation was and is a mosaic of savannas and woodlands. The area has a long history of ranching but the soil is too shallow for plowing. more information about the vegetation and ecology of the eastern Edwards Plateau
East of the Balcones Fault, the topography is flat to rolling and the parent material is usually chalk, sand, or alluvium. It is believed that tallgrass grassland predominated where the bedrock is chalk (soft limestone), forming the Blackland Prairie (#4; named for its dark clay-rich mollisols), while sandier soils supported the Post Oak Savanna (#3). Most of the area was cultivated, mostly for cotton, and is now a mixture of fields, pastures, and secondary woodland.
North of Austin another ecoregion begins, the Cross Timbers and Prairies (#5). This was and is a mosaic of savannas and woodlands, although much altered by cultivation and ranching.
About 50 miles northwest of Austin a large granite dome is exposed, the Llano Uplift (locally pronounced la'-no; the dotted oval in region 7). Besides the visible exposures of rounded pink granite, you can tell that you have reached the Llano uplift when the ubiquitous live oak (Quercus fusiformis) is replaced by post oak (Q. stellata).
About 25 miles east of Austin, near Bastrop, sandy soils and a perched water table are responsible for the Lost Pines, the westernmost population of loblolly pine (Pinus taeda).
East of the Balcones Fault there are also extensive areas of alluvial soil and there are, or were, gallery forests along the rivers. From Longhorn Dam in east Austin to Buchanan Dam 50 miles northwest of Austin, the Colorado River is dammed to form a string of reservoirs: Ladybird/Town Lake, Lake Austin, Lake Travis, etc.
Click on the ecoregion map for a link to a larger image. For more information, here are links to TPWD description of Texas vegetation types and EPA maps and supporting text about Texas ecoregions.comments, questions, suggestions? email me at firstname.lastname@example.org