Two decades of vegetation change on terraces of a south Texas river

Abstract

J. K. Bush, F. A. Richter (Department of Earth and Environmental Sciences, The University of Texas at San Antonio, San Antonio, Texas 78249), and O. W. Van Auken (Department of Biology, The University of Texas at San Antonio, San Antonio, Texas 78249). Two decades of vegetation change on terraces of a south Texas river. J. Torrey Bot. Soc. 133(2): 280–288. 2006.—A chronosequence of stands on floodplain terraces of the San Antonio River in south Texas was sampled in 1983 and resampled in 2003. The youngest resampled stand, an early successional stage originally 19 yrs of age, was an open Acacia farnesiana (huisache) woodland. After 20 yrs this stand was a mid- successional A. farnesiana—Celtis laevigata (Texas sugarberry) woodland, and demonstrated a decrease in A. farnesiana density from 427 plants ha−1 to 167 plants ha−1 and an increase of C. laevigata density from zero plants ha−1 to 860 plants ha−1. The 25 yr old stands were originally mid-successional A. farnesiana woodlands. After 20 yrs, A. farnesiana density decreased by 462 plants ha−1 and C. laevigata density increased by 1031 plants ha−1. Acacia farnesiana basal area decreased by 9.4 m2 ha−1, while C. laevigata basal area increased by 5.8 m2 ha−1. The 27 and 29 yr old stands were originally A. farnesiana woodlands, but after 20 yrs were Celtis woodlands. The greatest decrease in A. farnesiana basal area (12.0 m2 ha−1) and the greatest increase in C. laevigata basal area (15.9 m2 ha−1) occurred in the now 47 yr old stands. The 32 yr old mid-successional stand was originally an A. farnesiana—C. laevigata woodland, and at 52 yrs was a C. laevigata woodland. Both density and basal area of A. farnesiana decreased in the successional sequence, while C. laevigata density first increased and then decreased, while basal area increased. Significant changes have occurred over the last two decades, however it is predicted that further changes will occur before the communities become mature. A major co-dominant in the mature stands, Ulmus crassifolia, was not found in the 52 yr old stand. In addition, several other mature community species including U. americana, Crataegus sp., and Ilex decidua were not found in the 52 yr old stand.