Composition of the woody plant understory of plateau live oak (Quercus virginiana var fusiformis) clusters in a central Texas savanna

Phillips, Patricia L.
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Plateau live oak is thought to act as a nurse plant to other woody species on the Edwards Plateau of central Texas; however, little is known of the nature, extent and duration of this facilitation. In this study, a comprehensive assessment of the composition and abundance of understory woody plants in oak clusters was undertaken to test the hypotheses that, 1) asymmetry exists in the woody plant understory composition and abundance around the live oak, which correspond to asymmetries in microclimate beneath oaks and, 2) Ashe juniper acts to competitively exclude other co-occurring woody plants from the cluster such that the abundance of Ashe juniper is inversely related to the abundance of other understory species. Woody species composition and abundance in the understory of twenty live oak clusters in five distinct landscapes (pastures) at the SWT Freeman Ranch was determined in each of 4 quarters (NE, NW, SE, and SW) beneath the live oak nurse tree. The central live oaks in these clusters ranged in size from 37.4-93.7 cm dbh, 10.417.9 min canopy diameter, 8.2-14.0 m2 canopy area and 6.1-12.5 m maximum height. The number of woody species in the understory of these clusters ranged from 5 to 11 and the most dominant species included Juniperus ashei, Celtis laevigata, Diospyros texana, and Ulmus crassifolia. The number of individual plants ranged from 46-267 individuals/cluster. In general, there were significantly (P < 0.05) more woody plants in the northern half of the clusters than southern halves and densities were greatest in the northwest quarter. Plants in the north sides of clusters were also, generally, larger in size than those in the southern half of the clusters. The abundance of individual species, however, varied about the oak cluster. In particular, J. ashei was found to be most abundant and individuals were larger in the north side of clusters while C. laevigata was most abundant in the southern half of clusters. Despite their abundance and size, no inverse relationship was found between J. ashei abundance and that of other woody species suggesting that junipers were not competitively excluding other woody plants from these clusters. These results indicate that asymmetry in woody plant composition does exist around these live oak nurse plants though it is more subtle than the asymmetry documented for nurse-plant associations in other, drier ecosystems. This asymmetry in: the woody plant understory may be due to asymmetry in microclimate effects, but woody plant competition, animal use, or fire may also play important roles.
Central Texas, Quercus virginiana, Vegetation dynamics, Live oak, Texas savanna
Phillips, P. L. (1999). <i>Composition of the woody plant understory of plateau live oak (Quercus virginiana var fusiformis) clusters in a central Texas savanna</i> (Unpublished thesis). Southwest Texas State University, San Marcos, Texas.