Hardwood-Redcedar Clusters in the Post Oak Savanna of Texas

Abstract

Hardwood-redcedar clusters occur when several redcedars (Juniperus virginiana) germinate and grow beneath the canopy of a single live hardwood tree. Of 187 redcedars observed in 12 clusters, 92% occurred within four m of the cluster center. Redcedar numbers ranged from 6 to 28 per cluster (16 ± 7 (x ± SD)). When the 12 clusters were divided into two types based on the presence (n = 7) or absence (n = 5) of a live post oak center tree, clusters without a live center tree had redcedar with greater height (6.28 ± 2.13 vs 3.87 ± 1.59 m) and greater redcedar dispersion (average distance of redcedars from cluster center: 2.57 ± 1.50 vs 1.95 ± 1.02 m). Clusters are common in the post oak savanna region, and appear to occur in high density patches following the co-occurrence of hardwoods and redcedars. The vegetation dynamics of clusters is unknown. Eastern redcedar and other juniper species often germinate in large number and grow beneath the canopy of hardwoods (Phillips, 1910; Parker, 1951) and pines (Phillips, 1910). This pattern has most often been attributed to seed deposition by birds (Phillips, 1910; Parker, 1951; Bard, 1952; Livingston, 1972). However, the formation of a cluster of redcedar beneath single hardwoods has been little studied and there are no published descriptions of the vegetation structure of such clusters. Ormsbee et al. (1976) described eastern redcedar as an early successional species adapted for growth in open areas. They concluded from photosynthetic measurements that redcedar is excluded from mature forests because it is shade-intolerant. However, Lassoie et al. (1983) reported that redcedar persisted as an understory component of oak-hickory forest in Missouri due to physiological adaptation to shaded conditions. Their measurements indicated that photosynthetic rates were highest in spring and autumn. They suggested that the persistence of redcedar could be explained by favorable growth conditions occurring when hardwoods were defoliated. Therefore, shading per se may not exclude redcedar as a vegetation component. The purpose of this report is to document the existence of hardwoodredcedar clusters and to describe the physical structure of clusters located in the post oak savanna region of Texas. STUDY AREA-The study site of about 16 ha is located within the 485 ha Native Plant and Animal Conservancy in College Station owned by Texas A&M University. The vegetation of the study site is typical of the post oak savanna (or claypan) region in east-central Texas (Gould, 1975). The dominant hardwood is post oak (Quercus stellata), and the landscape is a mosaic of small grassland areas, post oak-dominated woodland patches, and post oak-yaupon (Ilex vomitoria) thickets. METHODS-Clusters were defined by the canopy of a single hardwood tree, or by an isolated grouping of redcedars. In the 12 clusters measured, 7 had a live post oak center tree. The basal diameter (BD, cm), diameter at breast height (DBH, cm), height (HGT, m), and crown diameter E SOUTHWEST RN NATUR LIST 31(1):73-78 MARCH 31, 1986 This content downloaded from 157.55.39.120 on Mon, 05 Sep 2016 03:59:49 UTC All use subject to http://about.jstor.org/terms The Southwestern Naturalist TABI.E 1-Comparison of live post oak center trees in redcedar clusters. Measurements include post oak basal diameter (BD), diameter at breast height (DBH), height (HGT), and crown diameter (CD). Clusler BD ((m) DBH (cm) HGT (m) CD (m) 1 65.6 56.7 11.8 16.8