Phenological and morphological adaptations to the light environment in two woody and two herbaceous plant species

1. Leaf emergence, leaf longevity, light regime and photosynthetic rates were investigated in order to clarify the morphological and phenological adaptations to light conditions by avoiding self-shading in two herbaceous (Polygonatum odoratum and Polygonum sachalinensis) and two woody (Fagus crenata and Alnus sieboldiana) plants in artificially created open habitats in Japan. 2. Fagus and Polygonatum open their leaves simultaneously as a flush during a short period at the start of the growing season (simultaneous foliar phenology), while Alnus and Polygonum open their leaves one by one successively for a longer period (successive foliar phenology). 3. Light conditions at the level of single leaves, defined as photon flux density relative to that above the plant, were essentially the same throughout a growing season for simultaneous species. On the other hand, light conditions for the two successive species degraded with time. There were gradients of light from the distal to basal part of a shoot only in successive species. 4. Reflecting the abrupt decrease in irradiance with time, photosynthetic rates of individual leaves decreased quickly with time in successive species (Alnus and Polygonum), while those of simultaneous species (Fagus and Polygonatum) decreased slowly. Photosynthetic rates of the canopy as a whole, however, were maintained constant over the season in successive species by replacing leaves frequently. Canopy-level photosynthesis decreased slowly with time in the two simultaneous species. 5. Successive leaf emergence is adaptive in open habitats such as flood plains and canopy gaps, while simultaneous leafing is adaptive in light-limited habitats such as the forest understorey.