Linking Community Dynamics and Ecosystem Function at Harvard Forest 1996-2000

Human activities are effecting profound changes in the structure and function of natural ecosystems. A comprehensive understanding of current ecosystem dynamics and future responses to global change requires an integrated investigation of ecological processes at many levels of organization. My thesis research addressed this goal by examining interactions between community- and ecosystem-level dynamics in mixed conifer broad-leaved forests in eastern North America. I addressed the nature of canopy-seedling feedbacks in mixed forests by relating seedling regeneration patterns in contrasting stand types to understory conditions (EXPERIMENT 1), and by directly manipulating resource availability to separate the individual effects of particular resources (EXPERIMENT 2). To investigate how nitrogen deposition will influence future forest composition, I examined the impact of increased nitrogen availability on regeneration of both coniferous and broad-leaved tree species under both closed canopy (EXPERIMENT 3) and simulated gap (EXPERIMENT 4) conditions. Future changes in forest composition might then influence whole-ecosystem productivity. I used two scaling approaches (leaf-level aggregation, EXPERIMENT 5; whole-tree sap flow, EXPERIMENT 6), I examined how the dominant coniferous and broad-leaved species in mixed temperate forests differed in their contributions to canopy-level photosynthesis.