Scaling isoprene fluxes from leaves to canopies: Test cases over a boreal aspen and a mixed species temperate forest

The rate at which isoprene is emitted by a forest depends on an array of en vironmental variables, the forest's biomass, and its species composition. A t present it is unclear whether errors in canopy-scale and process-level is oprene emission models are due to inadequacies in leaf-to-canopy integratio n theory or the imperfect assessment of the isoprene-emitting biomass in th e flux footprint. To address this issue, an isoprene emission model (CANVEG ) was tested over a uniform aspen stand and a mixed-species, broad-leaved f orest. The isoprene emission model consists of coupled micrometeorological and phy siological modules. The micrometeorological module computes leaf and soil e nergy exchange, turbulent diffusion, scalar concentration profiles, and rad iative transfer through the canopy. Environmental variables that are comput ed by the micrometeorological module, in turn, drive physiological modules that calculate leaf photosynthesis, stomatal conductance, transpiration and leaf, bole and soil/root respiration, and rates of isoprene emission. The isoprene emission model accurately predicted the diurnal variation of i soprene emission rates over the boreal aspen stand, as compared with microm eteorological flux measurements. The model's ability to simulate isoprene e mission rates over the mixed temperate forest, on the other hand, depended strongly upon the amount of isoprene-emitting biomass, which, in a mixed-sp ecies forest, is a function of the wind direction and the horizontal dimens ions of the flux footprint. When information on the spatial distribution of biomass and the flux footprint probability distribution function were incl uded, the CANVEG model produced values of isoprene emission that compared w ell with micrometeorological measurements. The authors conclude that a mass and energy exchange model, which couples flows of carbon, water, and nutri ents, can be a reliable tool for integrating leaf-scale, isoprene emission algorithms to the canopy dimension over dissimilar vegetation types as long as the vegetation is characterized appropriately.