The influence of energetic properties on plant success: Comparative studies of species from various ecosystem types

Anthropogenic activities are projected to cause numerous environmental changes, which are likely to significantly impact plant communities and associated ecosystem processes. A central issue in plant ecology is how plant communities will respond to these environmental changes, particularly those changes that affect the availability of various resources required for plant growth. Community-level responses to such changes in resource availability will be influenced by the differential responses of constituent species. Thus, understanding the mechanistic basis underlying species-specific properties that influence the relative success of plant species and how these properties will be impacted by altered resource availability could provide insight into potential community-level changes. Because plant growth entails an energetic expense, energy has basic and universal applicability to interspecific comparisons. It has been suggested previously that the most successful plant species in a given community are those that maximize their energetic gains while minimizing their energetic costs. As the process by which solar energy is used in the formation of carbohydrate molecules, photosynthetic activity mediates a plant's energy supply. Biomass construction cost is a quantifiable measurement of a plant's energy demand for biomass production, or growth, and biomass maintenance cost is a measure of a plant's energy demand for biomass maintenance processes. Consequently, we have hypothesized that the most successful plant species in a given community are those with high photosynthetic rates relative to their photosynthetic biomass construction and/or maintenance costs. We have hypothesized further that changes in plant species' success induced by changes in resource availability will be accompanied by resource-induced changes in energetic gains and/or costs. This thesis is a compilation of original research designed to test these hypotheses. Studies conducted in intact communities of three ecosystems—pond banks, forest and desert—generally supported our first hypothesis, suggesting that energetic properties were influential to species' community success in a variety of ecosystem types. Our second hypothesis was supported by research in an intact desert ecosystem, which found that atmospheric carbon dioxide-induced changes in productivity of constituent species were associated with altered energetic properties. In contrast, a study conducted in the laboratory without direct interspecific resource competition found only partial support for our hypothesis. We suggest that continued research investigating energetic properties in co-occurring plant species and the response of these properties to altered resource-availability in intact ecosystems may help us to elucidate how energetic properties could influence plant communities now and in the future.