CORRELATIONS BETWEEN BIOMASS ALLOCATION, RELATIVE GROWTH-RATE AND LIGHT ENVIRONMENT IN TROPICAL FOREST SAPLINGS

The growth rates and patterns of above-ground biomass allocation were determined for saplings of nine species of a wet tropical forest in Costa Rica. Relative growth rate and allocation were calculated by remeasuring the dimensions of forest-grown saplings and using biomass-dimension relationships derived from harvested saplings to convert linear dimensions to biomass estimates. The light environment of each sapling was characterized in terms of direct and indirect site factors calculated from hemispherical 'fisheye' photographs of the canopy. Relative growth rate (RGR) was significantly correlated with both direct and indirect site factors. A significant negative correlation was found between ln(% leaf allocation) and RGR for six species, but no such correlation occurred for the three species most highly associated with canopy gaps. Thus, shade-tolerant saplings shift most of their above-ground growth to leaf replacement when heavily shaded, while gap-associated species allocate about 2/3 of above-ground growth to the stem and branches under all measured conditions. This differential plasticity in leaf vs wood allocation may be an important factor in determining the shade tolerance and successional status of woody species.