Search Results

This chapter focuses on the distinction between ecosystem function and ecosystem dynamics. Ecosystem function refers to the manner in which the ecosystem of interest works, and interactions among its component parts and fluxes, including biotic and abiotic compartments. Meanwhile, ecosystem dynamics refers to variation in ecosystem function through time in response to perturbations that are continuous or stochastic in nature, or in relation to changes in ecosystem components. Therefore, the study of ecosystem dynamics derives from an understanding of ecosystem function, and this, in turn, depends critically on successful identification of the important drivers within the ecosystem. Inevitably, a discussion of ecosystem function and dynamics boils down to the factors that influence and contribute to variation in net ecosystem production—the result of net primary productivity and ecosystem respiration.

There are several atmospheric changes that may affect physiological and biogeochemical processes in old-growth tropical forests. Elevated CO₂ is likely to directly influence numerous leaf-level physiological processes. To assess potential ecosystem-level responses for a Central Amazon forest, an individual-tree-based carbon cycling model was used to carry out a model experiment constituting experimentally-observed tree growth rate increases linked to the known and expected increase in atmospheric CO₂. The results suggested a maximum carbon sequestration rate of only 0.05 Mg C ha^-1 yr^-1 for an interval centred on 1980-2020. This low sequestration rate results from slow-growing trees and the long residence time of carbon in woody tissues. In contrast, changes in disturbance frequency, precipitation, and other factors can cause marked and relatively rapid losses and gains in ecosystem carbon storage. Observed changes in tropical forest inventory plots over the past few decades may be driven by changes in disturbance regimes and factors, rather than by a response to elevated CO₂. Whether observed changes in tropical forests are the beginning of long-term permanent shifts or a transient response is still uncertain.

The chapter first shows carbon dioxide variability over long geological timescales. The current stocks and fluxes of carbon are then given, for the whole planet and for the atmosphere, ocean and land separately. The main flows of carbon in the ocean, through the biological pump (via uptake through photosynthesis) and the physical pump (via involving chemical transformation uptake in water and production of carbonate), and on land, through photosynthesis (Gross Primary Production) and respiration leading to Net Primary Production, Net Ecosystem Production and Net Biome Production and through the storage of carbon in biomass, are described. Next, carbon interactions during the Paleocene–Eocene Thermal Maximum and glacial–interglacial transitions, thought to involve changes in ocean circulation and upwelling, are examined. The key changes from anthropogenic perturbation of the natural carbon cycle are shown to be due to fossil fuel burning and land-use change (deforestation). The effects of the carbon–climate feedback on temperature and carbon stocks are also shown.