Linking community dynamics and ecosystem function in mixed conifer broad -leaved forests

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, and by directly manipulating resource availability to separate the individual effects of particular resources. The experiments demonstrated that canopy-seedling feedbacks played a major role in regeneration dynamics in both coniferous and broad-leaved stands. Hemlock seedlings were favored in hemlock stands, as a result of their tolerance of lower light availability and more acidic soils, while a suite of mid-successional species were favored in red oak stands.;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. Only survival of the most late successional species (hemlock) was increased by nitrogen addition in the understory, while faster growing species showed greater nitrogen-induced growth enhancement in a simulated gap. With increased nitrogen loading in the future, successional dynamics in mixed temperate forests are likely to become more exaggerated. On balance, however, nitrogen deposition will interact with other human-induced disturbances to decrease the late successional character of forests and promote faster growing species.;Future changes in forest composition might then influence whole-ecosystem productivity. Using two scaling approaches (leaf-level aggregation, whole-tree sap flow), I examined how the dominant coniferous and broad-leaved species in mixed temperate forests differed in their contributions to canopy-level photosynthesis. A low leaf area index (for a conifer) and cold winter temperatures meant that hemlock only gained limited benefits from maintaining an evergreen canopy, and overall the broad-leaved species (particularly red oak) fixed more carbon annually than hemlock. If broad-leaved species do increase in abundance at the expense of hemlock, as predicted, this change will increase forest productivity two- to four-fold.