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1 January 2003 Spatial Asymmetry in Tree-shrub Clusters in a Subtropical Savanna
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Plateau live oak (Quercus virginiana var. fusiformis) is thought to act as a nurse plant to other woody species in the upland savannas of the Edwards Plateau of central Texas; however, little is known of the nature, extent and duration of this facilitation. We tested the hypothesis that spatial asymmetry exists in the composition of the understory woody plant community associated with live oak trees that would indicate facilitative effects of the oak canopy on understory microclimate. The central live oaks in 20 discrete tree-shrub clusters sampled at a location in the eastern Edwards Plateau were relatively large and possessed a dense understory of 5 to 11 species of shrubs and small trees. Both adult and juvenile woody plants were nonuniformly distributed around the central live oak with significantly more individuals and species found in the northern than southern halves of clusters. No statistical differences were found between northern (NE vs. NW) or southern (SE vs. SW) quadrants or between eastern and western sides. Similar patterns were found for total cover (canopy and basal diameter) of adults and density and cover of Juniperus ashei, the dominant understory species. Neither the degree of this asymmetry or extent of understory development (i.e., total shrub density or species richness) was related to the size of the central live oak. Results are consistent with the hypothesis that microclimate modification by the live oak overstory is an important and perhaps persistent mechanism of facilitation in these woody clusters. These findings also suggest that processes structuring these live oak clusters differ from those of other tree-shrub clusters in subtropical savannas in this general region.

PATRICIA L. PHILLIPS and PAUL W. BARNES "Spatial Asymmetry in Tree-shrub Clusters in a Subtropical Savanna," The American Midland Naturalist 149(1), 59-70, (1 January 2003).[0059:SAITSC]2.0.CO;2
Accepted: 1 July 2002; Published: 1 January 2003

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