Response to Disturbance and Habitat Characterization of Abronia macrocarpa (Nyctaginaceae)

Abstract

Population demography, community composition and similarity, and habitat requirements of Abronia macrocarpa, an endangered Texas endemic were examined to aid in monitoring changes in populations over time, assist in determining possible reintroduction sites and help in restoring populations subjected to disturbances.

I examined population density and structure, and community composition of existing populations. Density varied significantly (p<0.005, F=8.387, df=6) among seven populations. Sites with highest density of A. macrocarpa have greater than 50% bare ground. Population structure varied, with significant differences between number of seedlings and vegetative plants (p = 0.007, df = 2). Coefficient of Community Index indicates that communities supporting populations of A. macrocarpa are similar, with coefficient values ranging from 0.67 to 0.99. The majority of sites have a pH ranging from 5.3 to 6.6, with one strongly acidic at pH 4.8.

I also collected data to compare disturbed and undisturbed areas of one population that was disturbed in 1992 by construction of an oil well. Soil pH is 6.6 in the undisturbed portion and 7.2 in the disturbed area. Significant differences (p=0.01, df=38) in A. macrocarpa density were found. Density in the undisturbed area was 5.2 individuals per m , whereas density in the disturbed portion was 0.2 individuals per m2. There is a significant difference in the amount of bare ground and vegetative cover (p0.001, df=2,38, F=33.46 bare ground, F=37.113 vegetation). The undisturbed area, which has a higher density of A. macrocarpa, has 66.75% bare ground. Abronia macrocarpa has not recolonized more than 60% of the total disturbed area.

Recovery criteria require the existence of 20 populations. Obtaining the goal of recovery may entail creating populations by reintroduction. This study has revealed specific habitat requirements that should be considered when selecting potential sites for the reintroduction. Soil chemistry should fall within similar ranges of the known A. macrocarpa sites. The community should support commonly associated species with a community similarity index of at least 0.65. Because the sites with high A. macrocarpa densities have a higher percentage of bare ground, this should also be taken into consideration in selecting reintroduction sites.