Flux and concentration footprint modelling: state of the art.

Since early 1990s, the development of footprint models has been rapid with presently four different approaches being available: (i) analytical models, (ii) Lagrangian stochastic particle dispersion models, (iii) large-eddy simulations, and (iv) closure models. Parameterizations of some of these approaches have been developed, simplifying the original algorithms for use in practical applications. The paper provides a review of the footprint modelling. It also discusses our present understanding of the theoretical background, the most successful modelling approaches, as well as the usage and benefits of the footprint concept as it relates to flux measurements. There has recently been a trend emerging in modelling the behavior of the footprint functions using a less idealized, more realistic description of inhomogeneities, vegetation structure and topography, ultimately for reactive compounds. The estimation of footprints for application in the real world, complete with a multitude of interesting gaseous and particulate substances, remains a complex problem.

Respiration as the main determinant of carbon balance in European forests.

Carbon exchange between the terrestrial biosphere and the atmosphere is one of the key processes that need to be assessed in the context of the Kyoto Protocol. Several studies suggest that the terrestrial biosphere is gaining carbon, but these estimates are obtained primarily by indirect methods, and the factors that control terrestrial carbon exchange, its magnitude and primary locations, are under debate. Here we present data of net ecosystem carbon exchange, collected between 1996 and 1998 from 15 European forests, which confirm that many European forest ecosystems act as carbon sinks. The annual carbon balances range from an uptake of 6.6 tonnes of carbon per hectare per year to a release of nearly 1 t C ha(-1) yr(-1), with a large variability between forests. The data show a significant increase of carbon uptake with decreasing latitude, whereas the gross primary production seems to be largely independent of latitude. Our observations indicate that, in general, ecosystem respiration determines net ecosystem carbon exchange. Also, for an accurate assessment of the carbon balance in a particular forest ecosystem, remote sensing of the normalized difference vegetation index or estimates based on forest inventories may not be sufficient.