Light-dependent changes in biomass allocation and their importance for growth of rain forest tree species

1. Sapling growth of six rain forest tree species was compared to evaluate whether species respond in a similar way to a natural light gradient. Sapli ngs were measured non-destructively; production and loss of leaves, stem an d branches were analysed in detail. 2. Sapling height growth was positively related to light environment and le af area. No single descriptor of light environment explained sapling growth best. Direct or diffuse Light could explain plant growth, depending on spe cies. 3, Seventeen percent of the saplings had negative relative biomass growth r ates, although they occurred in fairly bright conditions. Negative growth r ates were caused by leaf shedding and stem breakage. Sapling relative growt h rate increased with irradiance, mainly because of an increase in net assi milation rate. 4. On a shoot basis, shaded plants had a smaller leaf mass fraction (LMF) a nd a larger specific leaf area, resulting in similar leaf area ratios (LAR) to those of sun plants. This contrasts with the results of seedling studie s under controlled conditions, where LMF and LAR increased with shade. 5. Biomass partitioning to leaf growth decreased with irradiance and relati ve growth rate of the sapling. This leaf partitioning ratio was better corr elated with RGR than with irradiance. 6. Species differed in the effect of light-dependent changes in specific le af area (SLA) on growth. This underscores the importance of SLA in explaini ng differences in species performance in a forest environment. Nevertheless , the effect of SLA was not related to the shade tolerance of the species.