| Mid-latitude forests of the northern hemisphere may serve as the “missing sink” in an otherwise unbalanced global carbon budget. Long-term flux networks (e.g., AmeriFlux) measuring surface-atmosphere CO2 exchanges have been established, in part to address this issue. However, significant spatial and temporal variability of terrestrial ecosystem structure and function, and measurement uncertainty, mean that C exchange estimates are unconstrained by reliable benchmark values. Before tower-based net C flux data can be scaled-up to quantify regional C sink strengths, direct and independent validation is necessary. To this end, investigation of net C fluxes across forest stand chronosequences is also necessary: stand age has a large influence on carbon cycling, and areas sampled by towers are usually part of a larger mosaic consisting of differently-aged forest stands. Finally, to assess the impact of a changed climate on carbon cycling in forests, component carbon fluxes (e.g., photosynthesis, heterotrophic respiration) must be quantified, as these have different relationships to climatic conditions.; My research employs an integrated regime of ground-based biometric and ecophysiological sampling to quantify major C pools and fluxes in south-central Indiana forests. First, inter-annual variability of net C fluxes estimated from ground-based methods, and consistency with tower-based estimates is considered. Second, carbon stocks in forest ecosystem compartments are examined respecting stand age; temporal patterns are compared to those reported for other U.S. deciduous forests. Net C fluxes are derived from changes in C stocks through time, and these are compared to estimates for the tower site. Third, inter- and intra-annual variability of one component flux, litterfall, is examined in detail in relation to stand age and (tree species) composition. As a whole, this research represents a considerable contribution to the validation of tower-based eddy-correlation as a method for examining C fluxes and, more generally, towards understanding C cycling in forests ecosystems. |
