Comparative ecology of two closely associated scrub-dominating oaks in South Florida: Quercus myrtifolia and Q. geminata (Fagaceae)

Abstract

Recent authors have developed a detailed perspective on the parallel sympatric radiation into similar habitats throughout North America of oak species (Quercus, Fagaceae) representing different sections, including prominently Florida. Quercus myrtifolia and Q. geminata are a localized example of that radiation: two species in different sections codominating scrub habitats in the coastal Southeast Florida study area. Within that context the two are usually encountered at shrubby sizes almost without spatially related habitat differentiation, raising the question of how they coexist in essentially the same niche. The present project addresses this pronounced niche overlap with two different approaches: first, field exploration to discern local exceptions to the apparent near-inseparability of the two oaks, and second, to identify environmentally related differential growth responses. Exploring several scrub sites in Palm Beach and Martin counties, Florida, the study found three deviations from the usual intimate shrubby-site association having Q. myrtifolia proportionally dominant. Those deviations are: 1. Shrubby-sized Q. geminata tends proportionally to dominate dune bases vs. Q. myrtifolia dominating higher dune slopes. 2. Tree-sized Q. myrtifolia proportionally dominates sheltered scrub dune localities. 3. Tree-sized Q. geminata proportionally dominates exposed scrub dunes. During site maturation the two species diverge in relative abundance depending on sheltered shade vs. open exposure until fire or hurricanes restart succession from their intermixed subterranean portions. When young and shrubby, in exposed conditions as opposed to in partial shade, Q. geminata tends toward reduced leaf blade areas and increased marginal curling parallel to the long axis. By contrast, Q. myrtifolia in exposed conditions tends toward broad diversity in three-dimensional foliar conformations. Different foliar morphotypes characterize different Q. myrtifolia clumps.