Molybdenum limitation of asymbiotic nitrogen fixation in forests of Pacific Northwest America

We describe an approach to investigating and understanding the interactions betweenvegetation structure and ecosystem processes that uses simulation models as a frameworkfor comparison and synthesis across ecosystems arrayed along environmental gradients. Themodels are individual-based vegetation simulators and compartment models of nutrientcycling and soil water relations. Applications focus on interactions and feedbacks betweenvegetation structure (species composition, size structure) and ecosystem processes (waterbalance, nutrient cycling), and how these relationships vary across environmental gradients.Preliminary results indicate that life-history traits of plants have a profound influence onsystem-level behaviors, and that differences between grasslands and forests can be at-tributed largely to contrasting traits of grasses and trees. Experiments with linked vegeta-tion-ecosystem process models diverge from simulations with either model run indepen-dently, suggesting the importance of feedbacks between details of vegetation pattern andecosystem processes. The development of a fully coupled vegetation-ecosystem processmodel that is sufficiently general to simulate systems dominated by multiple (deformspresents several conceptual, logistical, and scaling challenges, but also provides for newopportunities in ecosystem theory.