Date of Award
Doctor of Philosophy (PhD)
William J. Platt
Temperate mixed-species hardwood forests of northern Florida, USA, contain a high diversity of understory tree species, some of which have been hypothesized to require disturbance (e.g., hurricanes and/or treefall gaps). Taxus floridana is a rare understory conifer endemic to midslopes of ravines of the Apalachicola River Bluffs, and is found primarily on north-facing slopes. Vegetation sampling in the southern portion of T. floridana's range revealed the presence of an upslope oak-hickory community of suggested recent origin, and higher densities of American beech (Fagus grandifolia ), southern magnolia (Magnolia grandiflora), and evergreen understory trees on north-facing midslopes with T. floridana than on randomly chosen north-facing midslopes. Thus, T. floridana presently exhibits high habitat specificity to restricted areas of north-facing midslopes. The status of T. floridana was investigated by acquiring demographic data on stem recruitment, survival, and growth, in three different populations (Rock Creek, Long Branch, and Beaverdam Creek) over four years. Seedling recruitment was temporally variable; seedling recruitment was also characterized by an episode in 1996 that was approximately ten-fold higher than in the proceeding years. Fates of seedlings varied both spatially and temporally, while fates of non-seedling stems with small diameters varied spatially. Growth of individual stems was similar in all populations sampled. Periodic matrix models incorporating such demographic data predicted declining growth rates for all sampled populations of T. floridana . This is consistent with recent anthropogenic changes in T. floridana's habitat, and/or its prolonged existence in low quality habitats. Annual episodic regeneration essentially did not influence population growth rates. Increasing the survival of juvenile stems may help promote net positive population growth in such rare species. Finally, recruitment of Ostrya virginiana and Carpinus caroliniana, two more common understory species was examined following Hurricane Kate in 1985 using long-term data from Woodyard Hammock in north Florida. Post-hurricane recruitment was equally high in gaps of different ages (i.e., different histories), and was higher than in areas under closed canopy. The presence of a juvenile sapling bank before and at the time of Kate may be responsible for their present dominance in the understory stratum in Woodyard Hammock.
Kwit, Charles, "Habitat and Demography of Understory Trees in Mixed Species Hardwood Forests in Northern Florida, United States of America." (2000). LSU Historical Dissertations and Theses. 7275.