Partitioning Carbon Fluxes Within Forest Stand Beneath Flux Tower, Methodology And Application

Emissions of carbon dioxide (CO₂) from fossil fuel combustion and total anthropogenic emissions have been increasing by around 6.3 and 8.0 Gt C yr-1, respectively (IPCC 1996), of which 3 Gt C year-1 has been explicitly linked to ‘uptake by Northern Hemisphere forest regrowth’ and to additional terrestrial sinks resulting from the combination of CO₂ fertilization, nitrogen deposition, and increasing temperature indirect effects. However, little scientific evidence for the ‘additional terrestrial sinks’ has yet been shown to confirm this, except for relatively short-term results from CO₂ enrichment experiments. The Kyoto Protocol was adopted in 1997, requesting that developed countries reduce their total CO₂ emissions by 2008-2012 to 92-95% of the level in 1990, taking the CO₂ balance of their forest ecosystems into consideration. This has lead to greatly increased attempts to measure directly the fluxes of CO₂ between the atmosphere and forest ecosystems. The main approach has been to use eddy covariance from towers above forest canopies to quantitatively evaluate the net ecosystem exchange of carbon dioxide over short periods (NEE). During the last five years many CO₂ flux towers have been built worldwide (ref. FluxNet: AmeriFlux, CarboEuro, AsiaFlux, etc.). Based on the tower data collected by the CarboEuro program, Valentini et al. (2000) suggested that most of the 30 forest stands monitored to date function as sinks for atmospheric CO₂, with the rate of increase rising from northern (boreal) to southern (warm-temperate) forests. Malhi et al. (1999) indicated that the magnitude of net carbon balance for tropical forest stands depends strongly on their biomass or net primary productivity. However, it is still not clear what suite of mechanisms in forest ecosystems collectively function to create a sink for carbon or where in any particular forest this occurs. Young forest stands generally function as carbon sinks, due to their positive increment in tree and litter biomass, and in some cases soil carbon. In this case, CO₂ fertilization, nitrogen deposition, or human fertilization and other cultivation generally greatly increases the magnitude of the sink. Saigusa et al. (2002) and Goulden et al. (1996) found that variation in weather conditions (precipitation, temperature and radiation) could account for much of the annual fluctuation in annual NEE (or net ecosystem production, NEP), based on long-term tower data over cool-temperate forests in central Japan and the northeastern U.S., respectively. However, few such long-term studies exist in the world. One approach to resolution is to measure simultaneously the component carbon fluxes within ecosystems (e.g., CO₂ fluxes of soil, root or stem respiration and photosynthesis), because the balance of CO₂ fluxes at the tower is the net result of these fluxes (i.e., the balance between net CO₂ assimilation by vegetation and the mineralization of carbon from soil organic matter). For example, Jarvis et al. (1997) attempted to do this beneath a tower at a boreal forest in Canada, although they found that the data could not be scaled up to the stand level.