IRRADIANCE AND SPECTRAL QUALITY AFFECT ASIAN TROPICAL RAIN-FOREST TREE SEEDLING DEVELOPMENT

Plant developmental responses to shade are the combination of reductio ns in photosynthetic photon flux density (PPFD) and changes in spectra l quality (reductions in the quantum ratio of red to far-red band widt hs, R:FR). We studied the seedling development of six Asian tropical r ain forest trees, Dryobalanops aromatica, Endospermum malaccense, Hope a wightiana, Parkia javanica, Shorea singkawang, and Sindora echinocal yx under varying PPFD and R:FR. Seedlings were grown in replicated sha dehouse treatments: (1) 40% solar PPFD and 1.25 R:FR; (2) 11% PPFD and 1.25 R:FR; (3) 11% PPFD and 0.24 R:FR; (4) 3% PPFD and 1.25 R:FR; and (5) 3% PPFD and 0.23 R:FR. Species differed in the influence of light variables on seedling (I) total height; (2) internode distance; (3) b ranch to trunk internodes; (4) stem length/mass; (5) leaf area/stem le ngth; (6) percent allocation to leaf, stem and root mass; (7) specific leaf mass; (8) mean leaf area; (9) leaf thickness; (10) petiole lengt h; and (11) stomatal density. The simple factorial design of treatment s 2-5 allowed a two-way ANOVA and the calculation of coefficients of d etermination of the treatment effects. The characters in most taxa wer e primarily influenced by light intensity, but spectral quality also i nfluenced characters in many cases. The taxa that responded most stron gly to the light treatments were the most shade-intolerant: E. malacce nse and P. javanica; the former species responded strongly to R:FR, pa rticularly in stem mass allocation and leaf area/stem length. The four taxa with moderate-to-extreme shade tolerance varied considerably in responses of individual characters to R:FR and PPFD. The patterns of m orphological responses to reduced PPFD and R:FR help explain how the s hade tolerances of the seedlings of rain forest trees vary in a contin uous manner. Recommendations concerning seedling shade tolerance for s ylviculture or nursery practice may need revision if they were based o n shade trials using spectrally neutral shade fabrics or slat houses. Future research on the effects of shading on tree seedling development and ecology must consider the potential influence of changes in spect ral quality under canopy shade.