Suppression of canopy tree recruitment beneath rapidly spreading thickets of Rhododendron maximum L. (Ericaceae) in southern Appalachian forests is an issue of major concern because of the potential impacts on forest productivity, hydrology and wildlife habitat. Many studies have investigated the causes of seedling inhibition beneath dense shrub understories, but few have uncovered specific mechanisms leading to seedling decline. In this study, I have examined the influence of the evergreen understory (R. maximum and Kalmia latifolia L.) on tree recruitment processes at multiple stages - seed rain, seed bank, and post-establishment seedling growth and survivorship. Effects of dense shrub cover on seed rain and seed bank density and composition were examined using a paired treatment design in which samples were collected beneath shrub-influenced and open understories. A second experiment investigated the influence of R. maximum and K. latifolia density on the growth and survivorship of Quercus seedlings, resource availability, and the rates / causes of seedling damage. I found that neither seed rain, nor seed bank density or species richness was inhibited by the presence of R. maximum or K. latifolia. Forest seed banks were dominated by sweet birch (Betula lenta L.), and were compositionally disparate from the overstory. Analysis of resource competition between shrubs and seedlings indicated that seedling performance and survivorship was a negative function of R. maximum density. Open-canopy light availability, nitrogen content in the organic horizon (litter and humus), and soil nutrient availability were potential resource-related mechanisms. Further, I found that the rates of insect herbivory on Quercus seedlings were positively correlated with R. maximum density. Kalmia latifolia had little influence on resource availability, seedling performance or herbivory rates, and does not appear to have a suppressive effect on tree seedlings. Overall, this research indicates that resource competition is the primary mechanism by which seedling suppression occurs beneath R. maximum, and that increased herbivory on seedlings may be an additional mechanism that demands further study. / Master of Science

<p>I examined recruitment patterns of multiple tree species in a western Amazonian floodplain forest at three ontogenetic stages: seed fall, seedling establishment, and sapling recruitment.</p><p>From analyzing a long-term seed rain dataset collected using a high-density array of seed traps, I confirmed that seed fall decreases sharply with increasing distance from fruiting trees, with disproportionately large contributions from a very small fraction of all trees. Patterns of seed fall, although idiosyncratic for individual species, tended to relate to dispersal syndrome. Intact seeds were found at significantly greater distances away from fruiting adults than ripe fruit and almost exclusively comprised the tail of the seed shadow for most species.</p><p>Saplings of all species examined recruited in areas of very low predicted seed density at significantly higher abundances than expected under a null hypothesis of "all seeds are equal". The value of a seed in terms of its potential to produce a sapling recruit - measured as sapling/seed ratio - initially increased greatly with increasing distance from reproductive conspecific adults and leveled off at farther distances, in almost all species.</p><p>A parallel experimental study employed >1000 individual seedlings of common tree species situated near and far from conspecific adults. Overall survival for all species pooled and for eight out of 11 individual species was significantly higher at sites located far from versus close to conspecific adults, with the study design controlling for seedling density at sites. Survival analysis based on multiple censuses revealed that a "distance effect" persisted and intensified over time, although the timing of onset of distance-related differential mortality differed amongst species. The role of host-specific invertebrate herbivores and microbial pathogens in causing seedling mortality near conspecific adults was confirmed by the use of mesh exclosures. </p><p>Overall, my results provide community-level support for the influence of distance-dependent processes on recruitment patterns. Seed dispersal appears critical for successful recruitment and undispersed seeds make a minimal contribution. When de-coupled from distance-dependence, effects of competition-based density-dependent processes on recruitment were weak or undetectable. I conclude that community-level tree recruitment processes and patterns in western Amazonian lowland rainforests that harbor intact floral and faunal assemblages conform closely to predictions of the Janzen-Connell hypothesis of tropical tree recruitment.</p> / Dissertation

No description available.

Tropical rainforests contain exceptionally high biodiversity and account for >30% of the world's carbon fixed by photosynthesis. Consequently, there are compelling reasons to deepen our understanding of the mechanisms that maintain these highly diverse forests and of the potential long-term threats to their preservation. An important process shaping tropical plant communities is negative density dependence (NDD). NDD occurs when plant performance is negatively impacted by increased neighbourhood density. Reduced performance at high neighbourhood density is thought to arise through ecological interactions between plants and their natural enemies. Thus in a healthy ecosystem, trophic interactions play vital roles as mechanisms driving NDD and are important as dispersers facilitating escape from NDD mortality. However, interruption to ecological processes caused by human activities, such as hunting, can perturb NDD interactions and cause cascading effects throughout an ecosystem. In my thesis I investigate the role of dispersal and mortality in NDD dynamics of tropical tree communities, as well as investigating local and global impacts of removing ecological interactions in tropical rainforests. In my thesis, I begin by addressing the presence and variation in strength of NDD among tree species and ontogenetic stages, the mechanisms driving NDD, and the role of trophic interactions in this process. The Janzen-Connell hypothesis predicts that host-specific natural enemies drive NDD by selectively reducing conspecific density, and increase diversity by suppressing competitive exclusion, thus allowing heterospecifics to persist. In chapters 2 and 3 of this thesis, I show that mortality driven by conspecific NDD is prevalent at the early life stages, and this effect is considerably stronger during the year after germination. Furthermore, this process is driven exclusively by host-specific fungal pathogens, which cause mortality selectively among conspecifics and drive diversity. As seedlings age beyond their first year, NDD interactions become less impacted by conspecifics but are impacted by closely related neighbours or by general neighbourhood density, representing changes in the mechanism driving NDD as seedlings age, and a decline in host-specificity of natural enemies. Equally, relative growth rates (RGR) are reduced under high neighbourhood density irrespective of species identity. Results suggest insect herbivores are the strongest driver of reduced RGR but not mortality under increased neighbourhood density. As a consequence of stronger inter than intra-specific NDD effects on RGR, insects had no impact on seedling diversity in the short term. This study supports assertions that regionally rare species experience stronger NDD than common species, accounting for the high variability in species relative abundance in the tropics. In the second part of my thesis, I address the role of large vertebrate dispersers in shaping tropical tree communities and the consequences of defaunation for tree assemblage and carbon storage. Dispersal allows seeds to escape NDD and persist to reproductive maturity and is therefore vital for the maintenance of diversity. Vertebrates disperse the seeds of more than 70% of neo-tropical tree species. However, many large vertebrates are becoming scarce due to widespread hunting. The decline of large vertebrates and their role as dispersers is predicted to alter tree community composition. Additionally, large vertebrates are responsible for the dispersal of large-seeded species, which are linked to species with high wood density. With wood density positively associated with carbon storage, there is a potential cascading influence of defaunation on global carbon storage. We investigate the consequences of declining large vertebrate mortality agents in chapter 3, and the consequences of declining large vertebrate dispersers in chapters 4 and 5. Although community composition is altered in a defaunated forest, species dispersed by extirpated fauna do not appear to drive this. In fact we find that many species thought to be heavily reliant on extirpated fauna manage to persist. Although it is thought that the simultaneous loss of seed predation from large terrestrial vertebrates may create compensatory effects, we found little support for this, with an absence of large terrestrial vertebrates driving only temporary changes to species diversity. Neither a loss of large frugivores or large-seeded species lead to declines in species with high wood density, but we detect a worrying decline in large stemmed species, which has negative implications for carbon storage. Overall, my thesis highlights the importance of NDD and trophic interactions, particularly fungal pathogens, at the early life stages in shaping tropical tree communities and in maintaining diversity. I provide evidence that the removal of trophic interactions among larger natural enemies and dispersers does not impact community assemblage in the directional manner found in previous studies. I provide evidence for the variability in response to trophic interactions among species and ontogenetic stages. I show disproportionate relative importance among natural enemies and dispersers in the maintenance of tropical tree assemblage, with implications for conservation and for assessing the consequences for tree diversity under the influence of degradation.