Interactive effects of season and light environment on growth and leaf dynamics of evergreen tree seedlings in the humid subtropics

Interactive effects of season and light environment on tree seedling growth were studied in four evergreen species in the humid subtropics of China. These species were the needle-leaved pioneer Pinus massoniana and the broad-leaved shade-tolerant Castanopsis fargesii, Sloanea leptocarpa, and Elaeocarpus japonicus. The experimental outdoor light environments, which broadly simulated those in forest gaps and clearings, were 100, 55, 33, and 18% of incident light quantity. Mean relative growth rates for both total leaf area and stem height revealed interaction of season and light environment in Pinus, Castanopsis, and Elaeocarpus. In these species, mean relative growth rates were higher in summer in more shaded environments and in autumn in more exposed environments, but this shift was not as clear in Castanopsis. Winter growth was virtually nil in all species, although Pinus still achieved some winter stem height change. The hypothesis that late-successional rather than early-successional species would demonstrate interactions of season and light environment on growth and development was not sufficiently supported by the data. Variation in mean relative growth rates for total leaf area was due to changes in whole-plant average leaf size as well as leaf numbers. In Sloanea and Elaeocarpus seedlings in high light environments, reduced leaf size and enhanced leaf abscission rates gave rise to negative mean relative growth rates for total leaf area. The more sun-tolerant Castanopsis seedlings revealed no such leaf size response and shed few, if any, leaves in any of the light environments. These results indicate that seasonal variability in the growth response of tree seedlings to the light environment is an important factor to be taken into account in studies on gap dynamics of subtropical forests