Abstract

The environment is constantly changing. A change in one environmental factor will cause changes in many other environmental factors either simultaneously or sequentially. For example, an increase in available light, due to canopy opening by natural disturbances, may increase soil temperature and moisture on the forest floor. Therefore, a changing environment has interactive effects on the trees. In forest ecological studies, the effects of some single environmental factors on tree growth are known, but there is a lack of knowledge of the interactions between multiple factors. An understanding of the interactive effects of abiotic and biotic factors on trees is critical for an understanding of the growth and survival of understory saplings in a complex changing environment. The aboveground and belowground systems of a tree respond differently to the same abiotic factors, such as light, since these systems grow in differing environments. This thesis will focus on the interactive effects among available light (including canopy gap size), tree size, artificial shading, liming and plant competition on the aboveground and belowground systems of yellow birch and sugar maple growing in the understory in two field experiments and the interactive effects of light, elevated CO 2 and mycorrhizae on seedlings of both species in a phytotron experiment. The three investigations will address several key questions concerning the growth of these two species and the development of their mycorrhizae in a complex changing environment in the present and future.