| Modern industrial society is effecting fundamental changes to the global cycles of carbon, nutrients and water. These changes are predicted to alter the Earth's climate and will have direct and indirect effects on net primary production. Small increases to plant production due to increased resource availability may offset CO{dollar}sb2{dollar} emissions from fossil fuel combustion, thereby acting as a negative feedback to anthropogenic climate change. The response of forest ecosystems to the changing environment is critical to our ability to predict the extent of the changes wrought by human activities. Of the processes regulating the flow of carbon and nutrients through forest ecosystems, belowground dynamics associated with roots are some of the most important, but least understood.; It is the purpose of this dissertation to shed light on several critical, but poorly studied aspects of root biology with the goal of improving our ability to formulate complete carbon and nutrient budgets for two important forest ecosystems of the United States: loblolly and ponderosa pine. Emphasis is placed on root growth, tissue quality, and decomposition as a function of changing the environmental factors of atmospheric CO{dollar}sb2,{dollar} nitrogen availability, water availability and temperature to levels projected from recent general circulation models or studies of trends in production (i.e. of reactive nitrogen). I achieved my goal by working on several collaborative projects supported by grants to investigate the effects of predicted global change on forest ecosystems. Both greenhouse and field studies were combined in an effort to determine the applicability of scaling results from highly controlled environments to actual field conditions.; I found that under growth conditions simulating those predicted for the future, both loblolly and ponderosa pine substantially increased biomass production resulting in larger root systems, hence potentially more carbon storage. Elevated CO{dollar}sb2{dollar} did not induce shifts in biomass partitioning as hypothesized by some authors, however shifts did occur as a result of temperature, nutrition, and water availability. The direction of the shifts was partially controlled by the ontogenetic stage of development of the plants and if they occur in nature, will probably be small in magnitude. Root tissue quality (concentrations of non-structural carbohydrates and macro-nutrients) did not change as a result of growth under elevated CO{dollar}sb2,{dollar} suggesting rates of belowground herbivory and decomposition will not be greatly altered in the future. A field experiment on the decomposition of roots under irrigation and fertilization showed little effect of either factor on mass loss rates, further supporting the view that belowground decomposition processes will be little affected by predicted changes to the environment. |
