FUNCTIONAL LEAF PHENOTYPES FOR SHADED AND OPEN ENVIRONMENTS OF A DOMINANT DWARF BAMBOO (SASA-SENANENSIS) IN NORTHERN JAPAN

Dense patches of evergreen dwarf bamboo species (1-3 m), such as Sasa senanensis, often prevent forest tree regeneration, and they suppress competing grassland species in Japan. We hypothesized that its evergre en habit contributes to the dominance of S. senanensis through carbon gain during the ''phenological gaps'' of spring and late fall under th e improved irradiance and moisture conditions in a deciduous forest. W e monitored the seasonal patterns of gas exchange in S. senanensis gro wing in the forest understory and in the open. The timing of greatest carbon gain in shade leaves was during early spring and late fall unde r a leafless forest canopy compared with that of late spring and early summer for open-grown leaves. Compared to understory S. senanensis, t hose grown in the open attained high photosynthesis and chlorophyll (C hl) a/b but lower area-based total chlorophyll, specific leaf area, an d leaf nitrogen. In understory plants, these leaf characters and light fleck responses indicated that they retain shade-acclimated characters throughout the year. Vulnerability to leaf water stress appeared to b e the cause for the seasonal decline in carbon gain and rapid senescen ce of open S. senanensis leaves. The extended (>2 yr) leaf longevity o f understory S. senanensis agrees with cost-benefit expectations and i s an important contributing factor to the dominance of this clonal, mo nocarpic plant in the Japanese forest.