Irradiance and spectral quality affect Asian tropical rain forest tree seedling development

Plant developmental responses to shade are the combination of reductions in photosynthetic photon flux density (PPFD) and changes in spectral quality (reductions in the quantum ratio of red to far-red band widths, R:FR). We studied the seedling development of six Asian tropical rain forest trees, Dryobalanops aromatica, Endospermum malaccense, Hopea wightiana, Parkia javanica, Shorea singkawang, and Sindora echinocalyx under varying PPFD and R:FR. Seedlings were grown in replicated shadehouse 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) branch to trunk internodes; (4) stem length/mass; (5) leaf area/stem length; (6) percent allocation to leaf, stem and root mass; (7) specific leaf mass; (8) mean leaf area; (9) leaf thickness; (10) petiole length; and (11) stomatal density. The simple factorial design of treatments 2-5 allowed a two-way ANOVA and the calculation of coefficients of determination of the treatment effects. The characters in most taxa were primarily influenced by light intensity, but spectral quality also influenced characters in many cases. The taxa that responded most strongly to the light treatments were the most shade-intolerant: E. malaccense and P. javanica; the former species responded strongly to R:FR, particularly 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 morphological responses to reduced PPFD and R:FR help explain how the shade tolerances of the seedlings of rain forest trees vary in a continuous manner. Recommendations concerning seedling shade tolerance for sylviculture or nursery practice may need revision if they were based on 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 spectral quality under canopy shade.