Academic journal article The American Midland Naturalist

Spatial Asymmetry in Tree-Shrub Clusters in a Subtropical Savanna

Academic journal article The American Midland Naturalist

Spatial Asymmetry in Tree-Shrub Clusters in a Subtropical Savanna

Article excerpt

ABSTRACT.-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.

INTRODUCTION

Woody plant clusters or clumps are common features of the vegetation in a number of and and semiarid ecosystems worldwide (Yeaton, 1978; Fuentes et al., 1984; Hacker, 1984; Kellman and Kading, 1992; Haase et al., 1996; Verdu and Garcia-Fayos, 1996; Callaway and Da-is, 1998) and are especially prevalent in the subtropical savannas and dry woodlands of Texas (Rykiel and Cook, 1986; Archer et al., 1988; McPherson et al., 1988). On the Edwards Plateau of central Texas, woody plant clusters often consist of a central Plateau live oak (Quercus virginiana P. Miller var. fusiformis (J.K. Small) C. Sargent; syn = Q. fusiformis Small) tree or trees and a mixed species understory of various evergreen and deciduous shrubs and small trees (Fonteyn et al., 1988; Fowler, 1988). These oak clusters (locally referred to as "mottes") occur as discrete woody clumps embedded within C4 dominated grasslands and may cover up to 50% of the landscapes in parts of this region (Knight et al., 1984). Both pattern analysis (Fowler, 1988) and experimental studies (Anderson et al., 2001) indicate that live oaks serve as nurse trees in these woody plant clusters by facilitating the establishment of understory shrubs. However, little is known of the nature and duration of this facilitation or the factors that contribute to the development and maintenance of these woody clumps.

Mechanisms of facilitation vary, but generally involve direct effects on species by alterations in habitat (e.g., modification of soils or microclimate) or indirect effects involving other species (e.g., protection from herbivores or enhanced seed dispersal) (Callaway, 1995). For example, in savanna ecosystems isolated trees attenuate solar radiation and moderate temperature extremes in their understories (Vetaas, 1992; Scholes and Archer, 1997). In some cases these microclimate alterations appear necessary for the germination and establishment of species that infrequently colonize open herbaceous zones (Lohstroh and van Auken, 1987; Fulbright et al., 1995; Franco-Pizana et al., 1996; Weltzin and McPherson, 1999). In addition, trees modify the physical and chemical properties of soils in their immediate surroundings such that soil fertility is greater beneath woody plants than in the grassy interspaces (McPherson, 1997; Hibbard et al. …

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