Changes in Plant Communities of the San Antonio Missions National Historical Park
O.W. Van Auken, Ph.D. and J.K. Bush, Ph.D.
Division of Life Sciences
The University of Texas at San Antonio
San Antonio, Texas 78285
When European man first came to South Texas he established a series of missions. Those missions were important in many ways to the spiritual and cultural development of the people of South Texas. The occupants of the missions did many things including changing the natural environment, which is the topic of this paper.
Early man found extensive grasslands in many parts of Texas, he did not find the thickets of brush that cover so much of Texas today. Nor did he find extensive areas of mesquite thickets or broad tracts of acadia. He did find brush, but it was in much more limited areas. He also found mesquite, but usually along creeks and streams (Inglis, 1962).
Changes occurred in many of these plant communities between the coming of the Europeans to North America and today. Basically, these changes included the disappearance of a great deal of grassland or prairie, which was accompanied by the appearance of extensive thickets of brush and small trees. The causes of these changes were actually man and his animals. Specifically, too many cattle and too little fire. Fire suppresses brush development and maintains grasslands or prairies. Overgrazing reduces the dry grass fuel required for fire, consequently reducing the frequency and extent of fires and thus allowing brush species to develop and take over grasslands (Smith, 1977). Brush for the most part is not eaten by cattle, therefore it can outcompete the grasses when fire is suppressed.
We studied a series of changes in plant communities in Bexar and Wilson Counties, Texas, caused by overgrazing or farmland abandonment coupled with a lack of fire (Van Auken, 1982). These changes have probably occurred time and time again throughout this area and have been caused by the agricultural activities of man. The key to understanding the changes or the sequence of events, called secondary plant succession, is to understand the growth and development of a single species, the huisache (Acadia smallii) (Fig. 1A). It is a small tree or shrub that has beautiful yellow or golden flowers that bloom in early spring (Correll and Johnston, 1970). Huisache, a key species, is biologically very important because it is thought to be a nitrogen fixer. Huisache will invade an area that is overgrazed or abandoned, grow up, and finally be eliminated from that area by competition with other species (Van Auken and Bush, 1984).
A plant community that was examined near Mission San Juan (San Antonio, Texas) had a series of dead huisache trees under a Texas sugarberry (Celtis Laevagta) canopy (Fig. 1E). The dead huisache was the real clue to understanding the role of huisache in the successional process. The succession apparently proceeds from the open grassland stage, which occurs for about 5 years after abandonment (Fig. 1B), to an open huisache savanna which lasts for 10-20 years (Fig. 1C). Baccharis (Baccharis Neglecta), an invader shrub, will occur during this stage for a few years. During this entire time Texas sugarberry is present in the community as a seedling or shrub, but apparently it is suppressed or grows very slowly. In communities that are about 25 years old (Fig. 1D), the sugarberry begins rapid growth and by 35 years after abandonment, it is the community dominant. At this point, huisache begins to disappear (Fig. 1E). The huisache is not present in the mature forest, which is dominated by the sugarberry (Fig. 1F). Other important species in the mature community are cedar elm (Ulmus crassfolia), bumelia (Bumelia lanuginosa) and pecan (Carya illinoinensis) (Bush and Van Auken, 1984a).
When relative density, relative basal area, or relative importance are examined for these communities the same trends occur (Figs. 2,3,4). That is, huisache increases to its peak of dominance in communities that are about 25 years old and then declines and disappears. Texas sugarberry, on the other hand, which is present in the community as a seedling or shrub from the earliest time, begins rapid growth in stands that are about 25 years old and then quickly becomes a dominant and remains a community dominant in the mature stands (Van Auken and Bush, 1984a).
Why does the huisache disappear from the mature communities? Apparently because the Texas sugarberry over-grows the huisache and shades it out. The huisache is a shade-intolerant species (Bush and Van Auken, 1984b). Competition for some other resource may occur, but no one knows for sure at this time. A second more difficult question is why the Texas sugarberry grows so slowly for 25 years? Texas sugarberry could be inhibited by the high light intensity present in the early communities. That is, it may require shade for proper early growth and development. A second possibility is that Texas sugarberry is suppressed by chemicals present in the environment, possibly chemicals secreted by the huisache or other early community dominant. A third possibility would be the lack of a critical nutrient. The last hypothesis was tested. Texas sugarberry would not grow in native soil after farmland abandonment or excessive disturbance, unless supplemented with nitrogen. The huisache, a legume and probable nitrogen fixer, did not need added ammonia or nitrate nitrogen, but grew fine without it.
The spread of huisache and other woody species in South Texas was probably caused by the activities of man. Nitrogen in the soil was used up by cattle grazing or cropping by farmers. Next, the nitrogen was transported out of the ecosystem giving huisache, a probable nitrogen fixer, a competitive edge over the grasses, which are not nitrogen fixers. When the nitrogen was added back to the soil by the nitrogen-fixing huisache, the huisache lost its competitive advantage to the Texas sugarberry. The sugarberry then became the community dominant and apparently caused the demise of the huisache. This succession sequence has probably occurred time and time again over the years. That is, forest or prairie was cleared, crops were raised, abandonment occurred, followed by forest regrowth. The overall cause of abandonment was soil nitrogen removal and export, which in turn caused soil nitrogen depletion and deficiency. The relationship to man seems obvious. When soil becomes nutrient depleted, productivity goes down. This may be followed by farm or ranch abandonment and emigration of the inhabitants.
MS. KALLISON: I have two questions. Could you please tell me the normal life span of mesquite and the normal life span of huisache?
DR. VAN AUKEN: It is hard to say because of differences in local conditions. The tree corings that we did showed the largest to be 55 years old.
The largest mesquite we cored was 125 years old. They are not real old trees.
MS. KALLISON: My next question concerns observations of my husband, who is going to be 80 years old in November. He remembers, as a boy, the oldtime ranchers telling the story that there were no huisache or mesquite this far north in Texas. Both species migrated from the south.
DR. VAN AUKEN: I think that the huisache and the mesquite have been present in central Texas here since before the arrival of the Spanish Explorers. However, the spread of the huisache and the mesquite into grasslands has occurred more recently. The reason for the spread of these woody species is directly related to the increase in the livestock industry in Texas.
Fuel for these prairie fires was reduced by grazing. Trees became established in the grasslands and were able to carry out their whole life cycle. Once trees are established, they are very difficult to kill with fire and consequently they remain, further reducing the possibility of fire.
Prairie fire kept the woody species out of many native grasslands and localized in creek or river bottoms.
MS. LIGHT: So many of the ranchers have been trying to control mesquite, so would your advice be to try to keep some of the mesquite or huisache?
DR. VAN AUKEN: Mesquite and huisache are very important species, and they appear to be nitrogen fixers. If you are grazing cattle, the cattle eat the grasses which use the nitrogen from the soil. If one then removes these cattle, sends them to market, the soil nitrogen is reduced. If the nitrogen is not put back into the soil the productivity of that land will decrease. To answer your question, yes, it is very important to keep some of those mesquite or huisache trees to replace soil nitrogen.
MS. FORD: As I remember growing up, the huisache would freeze and that was how we lost so many. We have a place near San José Mission and there was an area there called Huisache Bow. When a hard freeze occurred after the trees had bloomed, many of them would die.
DR. VAN AUKEN: That is true, the huisache is sensitive to cold weather and low temperatures reduce growth. Excessive periods of cold weather will kill them.
MS. FORD: Another thing that I have heard since I was a child is that the Indians had burned the prairie land, and that was one of the reasons why the brush was kept out.
Is that true, or is that just a tale?
DR. VAN AUKEN: I think it is probably very true. I think that the Indians, the early Americans and Native Americans knew that if the prairies were not burned that the brush would take over.
A lot of the early Spanish ranchers here, they would burn as well and the burning is very important. That practice has died out pretty much and now, because of the nature of fire, it seems that we are very afraid of it and do not use it. We do not use it very much at all.
MS. FORD: Well, we want the mesquite and the huisache.
DR. VAN AUKEN: The grasses and other plants need nitrogen and the huisache and the mesquite are very important nitrogen fixers.
MS. LIGHT: Why was 1983 such a bumper year for huisache flowers? Was it because we had a rather mild winter?
DR. VAN AUKEN: I think the mild winter was the key.
MR. ESCOBEDO: Yes. When you give the stand ages you are talking about a very stable climate and not an overabundance of precipitation nor drought; am I correct?
DR. VAN AUKEN: We are dealing with the weather as it has been over the last 50 years including wet and dry years. We are assuming that over the past 50 years the climate has not changed in a major extent. Over a longer period of time we would certainly expect some major changes.
Bush, J. K. and O. W. Van Auken. 1984a. Woody Species Composition of the Upper San Antonio River Gallery Forest. Texas Journal of Sciences (In Press).
______________ and ______________ 1984b. Light requirements of Acacia Smallii and Celtis Laevigatta Two successional species. Amer. Midl. Nat. (In Press.)
Correll, D.S. and M.C. Johnston. 1970. Manual of the Vascular Plants of Texas. Texas Res. Found., Renner, Texas.
Inglis, J.M. 1962. A history of Vegetation on the Rio Grande Plain. Bull. 45, Texas Parks and Wildlife Department, Austin.
Smith, R.L. 1977. Elements of Ecology and Field Biology. Harper and Row, Publishers, N.Y.
/van Auken, O.W. 1982. Landscape study of the San Antonio Missions: Botanical Aspects. Final Report submitted to the U.S. Dept. of Interior, National Park Service, San Antonio Missions Nat. Historical Park.
Van Auken, O.W. and J.K. Bush, 1984. Growth of Huisache (Acacia smallii: Leguminosae) in Early Successional Communities in South Texas. In: Tamaulipan Biotic Province, A Symposium Resources, Management, Conservation; Eds. D.H. Riskind and G.W. Blacklock (In Press).
Last Updated: 24-Apr-2011