Seasonal patterns of the CO2 exchange characteristics of understory plants from a deciduous forest

The seasonal patterns of CO 2 exchange characteristics were determined for six deciduous forest understory species that differed substantially in time of photosynthetic activity with regard to the time of overstory canopy development. Species that were photosynthetically active only before overstory canopy development were characterized by high maximum light-saturated net photosynthetic rates and high leaf and mesophyll conductances. These early species were, however, active for only a relatively brief period of time. Species that were present during overstory canopy closure initially had moderate light-saturated net CO 2 uptake rates that dropped markedly in response to the reduced light intensities after overstory canopy closure. All species present during the summer under the strongly reduced light intensities had low rates of light-saturated net CO 2 uptake and low leaf and mesophyll conductances. These species, however, possessed low dark respiration rates which resulted in higher rates of net CO 2 uptake at low light intensities than those of the early species. Light-saturated photosynthetic rates were correlated with ribulose-1,5-diphosphate (RuDP) carboxylase activities in mature but not in young expanding leaves.