Phenlogy, Community Composition, and Soil Moisture in a Relict at Austin, Texas

Two plant communities, one dominated, by Andropogon scoparius Michx. and the other by shorter grasses and forbs, have changed considerably in floristic composition and areal coverage since the end of the 1950—56 drought. A phenological study at the end of the drought indicated a direct relationship between percentage of species vegetative and reproducing and percentage soil moisture. A second study after 4 years of above—normal precipitation showed no such relationship. The total number of species in an exclosure encompassing both communities was relatively unchanged after 4 years of above—normal precipitation, although the species turnover was 37%. Frequency data from 40 permanent plots, 20 in each community, showed a species turnover of 85.5% in the plots in the Andropogon community and 91.7% in those in the grass and forb community during the 5—year period from 1958—59 to 1963—64. The total number of species in the plots, however, remained relatively unchanged. The years of above normal precipitation resulted in the invasion of the grass and forb community by A. scoparius and the replacement of the dominant grass, Aristada longiseta Steud., in the remainder of the community by Trisetum interrumptum Buckl. and Sporobolus vaginiflorus (Torr.) Wood. The position of the ecotone between the two communities at the end of the drought and the path of A. scoparius invasion appear to be a function of soil moisture as affected by soil depth. The soil in the Andropogon community to a depth of 6 dm is more moist at almost all times of the year than that in the grass and forb community. At the end of the drought A. scoparius occupied soil 9 dm or more deep. With succeeding years of above—normal precipitation it invaded the grass and forb community along a gradient of decreasing soil depth.