Toward the end of the growing season in deciduous forests, leaf senescence heralds a number of changes in forest-atmosphere interaction. The canopy resistance to water vapor transport increases, the net uptake rate of CO2 rapidly diminishes, and the emission rates of hydrocarbon byproducts of photosynthesis, such as isoprene, drop precipitously. At the same time, a decrease in the PAR extinction coefficient through the canopy and an increase in the PAR albedo are observed, well before the LAI starts dropping. These developments, associated with leaf senescence, are the result of measurable changes in the leaf reflectance and transmittance spectra in the visible and near infrared.
In this paper, the changes in the light interception characteristics during leaf senescence are related to fluxes of CO2, H2O, heat, and isoprene, measured at Borden, Ontario (44.3N,80.9W) during the fall of 1995. These results are compared to similar data from a more southerly site, Harvard Forest, Petersham, MA (42.9N,72.3W). The timing of senescence and potential factors (local climate, species) are discussed, and the relative role of respiration in net CO2 exchange during this period is examined.
The dangers of parametrizing hydrocarbon emission rates only as a function of irradiance, temperature and LAI, as is currently done in commonly used biogenic emission inventory systems, are also discussed and improvements suggested.