Abstract
In nature, the intensity of light, or photon flux density (PFD), shows great variation, both temporally and spatially. For example, a leaf in the under-story can experience changes in the incident PFD up to 100-fold within a few seconds (Chazdon and Pearcy 1991). Large changes in PFD are also experienced by exposed leaves when intermittent clouds obscure the sun. In addition, the total daily integrated photon flux varies greatly among habitats as well as within the canopy of a given plant stand. Plants on the floor of a tropical rainforest (Bjorkman and Ludlow 1972) or redwood forest (Bjorkman and Powles 1981) may receive as little as 1% of the daily photon flux above the plant canopy.
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