Abstract: In this paper, the seedling’s early growth and establishment of Pometia tomentosa, a canopy tree species in Xishuangbanna tropical seasonal rainforest, were investigated under different illumination gradients (gap center, gap edge, and under canopy). The seedling height, basal stem diameter, total dry mass, leaf area per seedling, and relative growth rate at early stage peaked in gap center, with the values of 24.45 cm, 3.17 mm, 0.79 g, 122.45 cm², and 14.78×10^-3 g·d^-1, respectively. The seedlings had a higher root-shoot ratio under canopy (0.87) than in gap center (0.20) due to the influence of both illumination and water. The brighter illumination in gap center facilitated the growth of established seedlings, with the peaks of seedling height (31.48 cm), basal stem diameter (3.80 mm), total dry mass (2.22 g), leaf area per seedling (174.52 cm²), relative growth rate (2.29×10^-3 g·d^-1), and net assimilation rate (2.54×10^-5 g·cm^-2·d^-1) attained by the end of the investigation. The death of established seedlings might be closely related to water stress. Under canopy, the death rate caused by water stress was the highest (26.88%) but that caused by the predation of vertebrates was the lowest (2.93%), which in final, induced the highest survival rate (70.19%) of the seedlings. Illumination was the important factor affecting the morphological regulation of P. tomentosa eedlings. In gap center, the specific leaf area of the seedlings at their different growth stages was the smallest, but their relative growth rate and net assimilation rate were the greatest. The root-shoot ratio of established seedlings peaked at gap edge (0.33), suggesting that water stress and illumination were the important factors affecting the biomass allocation of established seedlings.

Key words: stem configuration, leaf biomass, twig biomass, canopy, allometry