Ask Mr. Smarty Plants

One limited source is a publication on palynological studies (identification of ancient pollen, etc.) of Texas (Pollen records of late-Quaternary North American sediments – edited by Vaughn M. Bryant and Richard G. Holloway).  This book is available for in-library review and checkout (to those with this privilege) at the Walter Geology Library at the University of Texas at Austin.  The book includes two or three sections that may prove helpful to you, but probably will not address the question fully. Unfortunately, there is no single comprehensive source document, and the little reliable information that does exist is scattered through a number of publications with very limited distribution.  Most of these publications are archeological survey reports that are not even in libraries, so it would be very difficult for anyone without direct connection to archeological investigations to gain access to them. Also, many of the reports are based on poor or highly localized records, with very little chronological control.  Even worse, virtually all of these reports reflect only limited understanding of plant associations, plant ecology, and environmental controls on floral distribution.  There are a few records utilizing evidence other than pollen, including plant tissues or impressions preserved in bogs, caves, and other contexts, but these, too, are highly site-specific and provide information regarding only a few taxa.  Sorry, there just is no broad-based database.

The Pleistocene Epoch, 2.6 million to 11,700 years ago, included periods that were wetter and cooler than the present climate of central Texas and intervals that were at least as warm and dry as current conditions.  The flora of the Pleistocene was, therefore, variable and included taxa that would not be adapted to the present climate as well as many species that have endured throughout, although with some differences in distribution and relative abundance. 

An interesting aspect of the modern flora of central Texas is that it includes relict plants formerly widespread during the Pleistocene, but which in this region are now confined to restricted habitats.  The Hill Country has a highly diverse flora today in part because there are relict plants from the climatic periods of the past.  For example, central Texas is the northern or eastern limit for many subtropical and dryland species such as Sabal mexicana (Mexican palm), Ehretia anacua (Anacua), Eysenhardtia texana (Texas kidneywood), Celtis ehrenbergiana (Desert hackberry), Diospyros texana (Texas persimmon) and Pinus remota (Papershell pinyon) that are thought to be holdovers from the warm or dry intervals.  Upland, exposed areas with thin soils afford the right habitats (i.e., those with xeric conditons) today, allowing the subtropical and dryland species to persist under the present climatic conditons. 

In contrast, some eastern hardwoods and the so-called Lost Pines, Pinus taeda (Loblolly pine), reach their current western limits in central Texas as representatives of the wet or cool episodes. The spring-fed streams and deep canyons of the Hill Country preserve wet, relatively cool climates (mesic conditions) presumably like those of much larger areas more than 11,000 years ago.  Species like the Cotinus obovatus (American smoke tree), Philadelphus texensis (Texas mock orange), the Lost Maples—Acer grandidentatum (Bigtooth maple), Tilia americana var. caroliniana (Carolina basswood), Lindera benzoin (Northern spicebush), and others are rare and/or highly localized in their distribution today, but may well have been present throughout the region.  The riparian environments along major streams probably have changed little, such that their modern woodland floras are at least representative of the Pleistocene woodlands.   

You can find a discussion of the modern vegetation of the Edwards Plateau of Central Texas related to the geography, geology, soils and climate of the area in Plant Communities of the Edwards Plateau:  An overview emphasizing the Balcones Escarpment zone between San Antonio and Austin with special attention to landscape contrasts and natural diversity by David H. Riskind and David D. Diamond in Balcones Escarpment edited by Patrick L. Abbott and C. M. Woodruff.

For further discussion of the possible historical presence of Sabal mexicana in the Austin TX area see "A Possible Return of Texas Palmettos to Waller Creek—Pure Speculation" by Bob Harms.