Changes in Texas vegetation during the past 30,000 years offer us clues about climatic changes, about the animals that once lived here, and about the hardships the earliest Texans, the Paleo-Indians, had to face in their daily quest for food and shelter. The remains of ancient plants yield the most reliable information about past vegetation. Unfortunately, most soils in Texas are unfavorable to plant preservation. In most of the state the less resistant plant parts decompose quickly after they are buried and leave no visible traces. Through the techniques of phytolith (plant crystal) research, through palynological investigation (see POLLEN), and through evidence from other fields, however, we can now assemble a rough conjectural view of the major vegetational changes in Texas during the past 30,000 years.
The years 30,000–22,500 B.C. were an interlude between two major glacial periods in North America. During this time conditions in Texas were stable and favorable. Pollen records from deposits in West Texas reveal that at first most of the area north and west of Austin was covered by a large prairie and few trees. Grasses dominated the land, and pine, juniper, Douglas fir, and spruce trees were restricted mostly to the higher elevations of the Guadalupe, Davis, and Chisos ranges. The prairie of the Edwards Plateau probably supported stands of juniper and piñon in some higher and more protected habitats. The probable climate of West Texas in this period was cooler and wetter than today, with fewer temperature extremes. Pollen evidence suggests that minor climatic fluctuations occurred. These are reflected in the fossil record by cyclical increases and decreases in the proportion of tree pollen to pollen from other plants. Some cycles lasted several thousand years and suggest that at times large islands of pine and juniper invaded the grasslands. Prairie remained dominant in West Texas for this entire period, however, and provided grazing for many species of now-extinct animals. Knowledge about the early vegetation of the same period in Central, South, or East Texas is limited. An extensive oak-hickory-pine forest probably dominated East Texas, but its western limit is unknown. It probably extended as far west as Huntsville. Research indicates that during this early period much of the currently forested regions in the central United States was covered by vast prairies marked with patches of shrubs. It is possible that this vegetation pattern extended through Central Texas as far south as San Antonio. Other research, however, suggests that a vast oak-hickory-pine forest extended across the southern United States and terminated somewhere in East or Central Texas. South Texas from San Antonio west to Del Rio and south to Mexico was probably covered by a mosaic of grassland and prairie interspersed with islands of shrubby oaks. But even minor changes in the amount of rainfall could have changed the vegetation quickly.
Between 22,500 and 8,000 B.C. changes in world climates led to a buildup of large continental ice sheets in North America that reached their maximum growth around 20,000 years ago. The disruption of wind patterns and the cooling influence of such large masses of ice in North America affected the vegetation and climate of the whole continent. In Texas the average annual temperature dropped to about five degrees centigrade cooler than it is today. The difference that this made would be equivalent to the difference between the climate in Uvalde, in South Texas, where summers are hot and dry and where it rarely freezes, and that in Lubbock, in the Panhandle, where summers are cooler, winter days are freezing, and snows are frequent. Pollen records from West Texas suggest that as this period began, the cooling reduced evaporation and led to the formation of large playas in many areas of West Texas. The resultant cooler and wetter climate encouraged existing forests and produced parklands. The spruce, juniper, Douglas fir, and pine forests of the West Texas mountains expanded downward to lower altitudes and spread out onto the mountain flanks, where they mixed with grasslands to form parklands and savannas. These changes produced a perfect habitat for vast numbers of now-extinct giant herbivores, including mastodons, mammoths, horses, Pleistocene bison, and camels. The oak-hickory-pine forests of East Texas probably did not expand significantly during this period. However, the dominant species of trees probably changed somewhat. Pollen records show that from about 22,500 to 12,000 B.C. the cooler-weather oak, elm, spruce, maple, hazelnut, alder, and birch may have dominated the forests. Likewise, atmospheric conditions in East Texas during this period were like those in West Texas. The existing vegetation in South Texas during this period did not change significantly. Areas of oak and juniper parklands expanded in the northern portion of the region just south of San Antonio, and the drier regions near the Mexican border filled with lush grassland. Vegetational variations were caused primarily by cycles of drought and wet years.
During the last few thousand years of this period the large glaciers receded as the North American continent warmed. By 10,000 years ago the ice sheets were gone; the path of the jet stream probably moved northward to bring warmer and drier winds to much of Texas. In West Texas the large playas dried, and the forests of pine, Douglas fir, and juniper contracted until they were again confined only to the protected mountain summits. Spruce disappeared entirely. The pine and juniper trees that once formed the prairie parklands also disappeared. Once again the West returned to a giant, uninterrupted grassland. Around the lower Pecos area of Southwest Texas the canyons and protected hills lost most of their piñons and junipers, and many desert plants presently found in that region, such as cacti, agave, yucca, and sotol, were already becoming quite common.
The vegetation on the rolling hills of the Edwards Plateau in Central Texas changed during the last few thousand years of this period from a dominance of piñon and juniper to a dominance of scrub oak and juniper. East of the Balcones Escarpment the forests lost most of their cool-loving species between 14,000 and 8,000 B.C. Alder, maple, spruce, and hazelnut disappeared entirely from the East Texas forests. Most of the birch species also disappeared from these forests, leaving behind only small patches of the warm-tolerant river birch. Basswood, dogwood, chestnut, and a few other forest species that grow best in cooler, wet habitats did not disappear entirely but were reduced to minor components in the new deciduous forests. Expanding into these forests were the species that now dominate them-new species of oak, hickory, walnut, pecan, sweet gum, and elm. Loblolly pines also may have expanded their numbers around 8,000 B.C. Yet many scientists believe that the dominance of pines in East Texas is fairly recent.
By 8,000 B.C. the vegetation in South Texas probably looked much as it does today and as it had before 30,000 B.C.-a mosaic that changed in wet and dry years. Moisture controlled how much of the region became oak shrubs, grassland, or semidesert.
The fossil pollen record reveals that the Texas climate has become progressively hotter and drier through the last 10,000 years. During that time a number of minor climatic oscillations have occurred. Around 500 B.C. West and Southwest Texas underwent a notable cooling that allowed the forests of West Texas to expand downslope and also encouraged the southward expansion of the lush grasslands of the Southern High Plains. This expansion reached the Rio Grande and was widespread enough to encourage large herds of bison to range freely as far south and east as Langtry and Del Rio. Like most of the brief climatic reversals of the last 10,000 years, the one in West Texas was probably triggered by changes in the weather such as unusually strong hurricanes, which could have carried large volumes of water farther inland than usual; changes in ocean currents that could have caused phenomena like el niño off the coast of Chile; changes in the path of the continental jet stream that could have brought cooler or hotter surface temperatures; or localized weather changes that produced regional droughts. Each climatic oscillation during the last 10,000 years was brief, and most were restricted in area. By 8,000 B.C. West and Southwest Texas vegetation probably consisted of prairies extending in an unbroken wave north of a line running from the site of San Angelo to that of El Paso. South of this line the vegetation consisted of a graded mosaic beginning with mixed scrub and grasslands then changing to scrublands, and ending with patches of semidesert south of a line from Marathon eastward to the edge of the Edwards Plateau. As today, moisture was the single most critical influence on the vegetation of West Texas.
In South Texas little changed during the last 10,000 years, though various vegetational components expanded or contracted. The scrub oak parklands that for thousands of years had been the dominant vegetation in areas south of San Antonio began to disappear, for example, and soon were restricted to their present moist areas. Scrublands of mesquite, acacia, and cactus expanded into the areas once covered by oaks, and throughout South Texas desert succulents such as agave, yucca, sotol, and cacti became common. Grasses became scarce in some South Texas areas that dried and turned into semideserts.
The forests of East and Central Texas also changed. In Central Texas just east of Austin the oak-hickory forests became more open as fingers of grassland invaded. The fossil pollen records suggest that the forests probably persisted until between 3,000 and 1,500 years ago. After that time all that remained was pockets of oak and pecan isolated in a vast grassy savanna. This contraction of the Central and East Texas forests continued until the forests reached their present westward margin around Huntsville. Loblolly pines probably came to dominate the forests of East Texas at this time. Fossil pollen evidence suggests that even though the early forests of East Texas may have expanded as far west as Austin, they were primarily composed of deciduous trees. Records show that the large relic stands of loblolly pines in Bastrop State Park, in Central Texas, did not expand southward, northward, or westward during the past 15,000 years. They probably did not expand even during the height of the Wisconsin glacial period around 20,000 years ago.