Leaf morphology and photosynthetic adjustments among deciduous broad-leaved trees within the vertical canopy profile

Photosynthetic acclimation of deciduous broadleaved tree species was studie d along a vertical gradient within the canopy of a multi-species deciduous forest in northern Japan. We investigated variations in (1) local light reg ime and CO2 concentration ([CO2]), and (2) morphological (area, thickness a nd area per mass), biochemical (nitrogen and chlorophyll concentrations) an d physiological (light-saturated photosynthetic rate) attributes of leaves of seven major species on three occasions (June, August and October). We st udied early successional species, alder (Alnus hirsuta (Spach) Rupr.) and b irch (Betula platyphylla var. japonica (Miq.) Hara); gap phase species, wal nut (Juglans ailanthifolia Carriere) and ash (Fraxinus mandshurica var. jap onica Rupr.); mid-successional species, basswood (Tilia japonica (Miq.) Sim onk.) and elm (Ulmus davidiana var. japonica (Rehd.) Nakai); and the late-s uccessional species, maple (Acer mono Bunge). All but maple initiated leaf unfolding from the lower part of the crown. The [CO2] within the vertical p rofile ranged from 320-350 ppm in the upper canopy to 405-560 ppm near the ground. The lowest and highest ambient [CO2] occured during the day and dur ing the night, respectively. This trend was observed consistently during th e summer, but not when trees were leafless. Chlorophyll concentration was p ositively related to maximum photosynthetic rate within, but not among, spe cies. Leaf senescence started from the inner part of the crown in alder and birch, but started either in the outer or top portion of the canopy of ash , basswood and maple. Chlorophyll (Chl) to nitrogen ratio in leaves increas ed with decreasing photon flux density. However, Chl b concentration in all species remained stable until the beginning of leaf senescence. Maximum ph otosynthetic rates observed in sun leaves of early successional species, ga p phase or mid-successional species, and late successional species were 12. 5-14.8 mu mol m-(2) s(-1), 4.1-7.8 mu mol m(-2) s(-1) and 3.1 mu mol m(-2) s(-1), respectively.