Scale and Contingency in Plant Demography: Quantitative Approaches and Inference

Ecologists have long recognized that patterns measured in nature often depend upon the context in which they are observed and the scale at which they are observed. When studying plant populations, the role of scale and contingency becomes crucial. Thinking about a plant community as a system is essential as populations of plants are centered within a network that influences their dynamics in direct and indirect ways. Plant populations are inherently scale-dependent because they have properties as a group that can be independent of their properties as individual stems. Although the challenge of interpreting population patterns in the face of contingency and scale has been addressed conceptually, there has been less success in applying those concepts to observational and experimental studies. This dissertation addresses the challenges of modeling the demographic dynamics of a forest understory herb, Eurybia chlorolepis (Asteraceae) or mountain aster. The study population consisted of twenty patches containing between 20 and 70 individual stems in each patch. These patches spanned three sites within the Indian Camp Creek watershed in the Cosby Ranger district of Great Smoky Mountains National Park. Plants in the forest understory in this dense old-growth forest are influenced by a myriad of biotic and abiotic components of the community: light, soil characteristics, other plant species, herbivores, pollinators, seed predators, and the feet of bears. This dissertation shows that the mechanisms that influence sexual reproduction of this plant are structured almost entirely on the stem-to-stem scale, indicating little coarse-scale influence of the environment over sexual reproduction. The use of a Bayesian learning network showed that the environmental influences (soil in particular) operated most importantly in the transition from juvenile stage to adult stage. Taken together, these analyses indicate that the coarse-environtment (such as gaps, soil profiles, soil moisture, and the presence of other plants) dictates where E. chlorolepis becomes reproductive, while the success of that reproduction is dictated by mechanisms operating between individual stems.