Light gains and physiological capacity of understorey woody plants during phenological avoidance of canopy shade

1. Carbon gain during phenological avoidance of canopy shade by an understorey plant depends on the extent of avoidance, the leaf stage during avoidance, and whether young and old leaves can exploit greater light availability in spring and autumn. 2. For Asimina triloba (L.) Dunal., Aesculus glabra Willd., Acer saccharum Marsh., Lindera benzoin (L.) Blume and Carpinus caroliniana Walt. in a deciduous forest in Illinois, USA, spring avoidance with leaves at full size ranged from 0 days for Asimina to 24 days for Aesculus, and brought 36-98% of estimated total annual irradiance. Autumn avoidance was non-existent to minimal in all species. 3. Total chlorophyll reached maximum concentrations at the middle of leaf life span, and declined well before senescence. Leaf nitrogen concentrations and net photosynthetic capacity both peaked in youngest leaves during spring avoidance, and were low in old leaves during autumn avoidance. 4. Aesculus had especially high photosynthetic capacity during precanopy closure, while Asimina had relatively low capacity in its later developing leaves. 5. Young leaves of species with phenological avoidance can enhance C gain, while old leaves in autumn do not. Thus phenological avoidance in spring may enhance the persistence of understorey woody individuals of some species.