INFLUENCE OF SEED SIZE AND SEEDLING ECOLOGICAL ATTRIBUTES ON SHADE-TOLERANCE OF RAIN-FOREST TREE SPECIES IN NORTHERN QUEENSLAND

1 Growth, leaf anatomy and stomatal responses were examined in seedlin gs of 12 rainforest tree species in northern Queensland. Plants were k ept in a shade house for a period of 15 months at 37%, 10% and 2.5% ph otosynthetically active radiation (PAR), roughly corresponding to ligh t environments in large gaps, small gaps, and forest understorey, resp ectively. The species were chosen to represent a wide array of taxa, e cological, and morphological characteristics, and were assigned regene ration niche (RN) index values based on observations of life history a nd spatial distribution of their seedlings and saplings in various for est microsites. 2 Growth, biomass allocation patterns, leaf anatomy an d stomatal responses varied appreciably among species, as well as in r esponse to shading. All species showed reduced growth with decreasing light intensity. Plants grown at 37% and 10% PAR exhibited morphologic al and physiological responses typical of plants found at forest edges and in large gaps: i.e. low leaf area ratio (LAR), high specific leaf weight (SLW), high root-shoot ratio (RSR), high relative growth rate (RGR), high net assimilation rate (NAR), high leaf conductance, and hi gh chlorophyll a/b ratio. Those grown at 2.5% PAR showed the opposite trends. Comparing species within each light level, NAR and LAR showed positive correlation with RGR, but pooled data for all light levels su ggests that the contribution of LAR and NAR to RGR are in opposite dir ections, depending on light conditions. 3 Sensitivity of seedling grow th to shading was significantly correlated with mean seed reserve weig ht of the species: small-seeded species were more affected than large- seeded ones. Relationship between seed reserve weight and species RN i ndex value, though linear, was weak and nonsignificant. RGR was signif icantly correlated with seed reserve weight at 37% and 10% PAR but not at 2.5% PAR, while LAR showed the opposite trend. 4 Plasticity of mos t parameters examined appeared highest in species that regenerate most ly in large gaps (> 200 m2) and at forest edges, and least in species that regenerate mostly in forest understorey and small gaps (< 50 m2). However, ordination of the study species, using seed size and various shade tolerance criteria for the seedlings, yielded a continuum rathe r than discrete groups. The first axis, which explained most of the va riation (65.6%), was influenced by NAR, RGR, stomatal conductance and total biomass, and was strongly correlated with species regeneration s tatus.