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

Ecology, March, 1996 by David W. Lee, Krishnapillay Baskaran, Marzalina Mansor, Haris Mohamad, Son Kheong Yap

INTRODUCTION

Light is the most important physical factor controlling the development of tree seedlings in tropical rain forests. Light environments in these forests are extremely heterogenous, and species are generally adapted to respond optimally to different light environments within forests (Whitmore 1995, Chazdon et al. 1996). The biological significance of seedling light responses is now being explored under the rubric of "gap phase dynamics" (Augspurger 1984, Brokaw 1985, Platt and Strong 1989), a concept originally developed to help explain the high tree species diversity of tropical forests.

Tree seedling shade responses have long been the subject of intensive research, the goal being to predict the light responses of plants under natural regeneration or in sylvicultural treatments. Tropical foresters have qualitatively assessed shade tolerances of seedlings to develop management systems (Whitmore 1984). Attempts to classify forest tree species into guilds, based on successional status and shade tolerance (cf. Whitmore 1989) have generated considerable discussion, with a growing consensus that there may be a continuum of shade responses among seedlings of different trees. Furthermore, some species may not partition light gaps (Welden et al. 1991).

Research on light responses in Asian tropical rain forest trees is largely limited to germination (Raich and Gong 1990) and seedling demography in natural or disturbed forest (Liew and Wong 1973, Brown and Whitmore 1992, Turner et al. 1992). Nicholson (1960) compared light requirements of five dipterocarp seedlings under partial shade, but light levels were too high to be of ecological significance. Sasaki and Mori (1981) employed a combination of field observations and experiments to assess the shade tolerances of Shorea talura, S. ovalis, Hopea helferei, and Vatica odorata. Seedling growth in all taxa was partially inhibited by light levels higher than 50% shade. Ashton and De Zoysa (1990) also documented the partial suppression of seedling growth under full sunlight in Shorea trapezifolia from Sri Lanka, and Ashton (1995) and Ashton and Berlyn (1992) demonstrated shading effects on seedling growth, leaf anatomy, and gas exchange characteristics in five Sri Lankan Shorea species with varying shade tolerances. On the other hand, seedlings of Intsia palembanica grew most rapidly in direct sunlight (Sasaki and Ng 1981). In general, little research has been published on seedling shade tolerances of Asian tropical rain forest trees, none on the taxa selected for this study (Whitmore 1995).

Natural light climates. - Light environments under vegetation canopies vary in quantity and quality. The intensity of radiation is altered by passage through foliage, surface reflectance, and penumbral effects due to small holes in the canopy. In tropical rain forests the mean intensity of radiation on the forest floor ranges from 5 to 25 [[micro]mol][center dot][m.sup.-2][center dot][s.sup.-1] of photons 400-700 nm (photosynthetic photon flux density, PPFD), or 1-3% of sunlight above the canopy (Chazdon et al. 1996). Median values are much lower, since 10-85% of the photons are supplied during brief flecks of sunlight, above 50 [[micro]mol][center dot][m.sup.-2][center dot][s.sup.-1]. Thus, daily totals of PPFD in forest understory are 0.3-1.0 mol[center dot][m.sup.-2][center dot][d.sup.-1] (Chazdon 1986, Oberbauer et al. 1988, Raich 1989, Ashton 1992), compared to values above the canopy of [approximately equal to]30 mol[center dot][m.sup.-2][center dot][d.sup.-1]. Openings, or gaps, in the canopy allow more sunlight to penetrate, the total depending upon the size of the gap and the structure of the surrounding canopy. For instance, Raich (1989) measured PPFD of 2.6 mol[center dot][m.sup.-2][center dot][d.sup.-1] in a small gap in a Malaysian dipterocarp forest.

Passage of sunlight through the canopy also alters its quality, or spectral distribution. Leaves absorb 80-90% of the visible quanta, but few of the quanta above 700 nm (Gates et al. 1965, Lee and Graham 1986). Foliage thus functions as a selective filter, particularly altering the ratio of red to far-red wavelengths. Quantum ratios of these wavelengths determine phytochrome equilibria in tissues, and may profoundly influence plant development. Smith (1982, 1994) and colleagues have pioneered research on the ecological importance of foliage shade to plant development, defining the significant band widths as a ratio of quanta centering on 660 nm to that at 730 nm (with a half peak band width of 10 nm), or R:FR. R:FR is dramatically reduced under canopy shade, from a high of 1.05-1.35 in direct sunlight (Lee and Downum 1991) to 0.20 in dense shade (Stoutjestijk 1972, Tasker and Smith 1976, Lee 1987, Turnbull and Yates 1992; [ILLUSTRATION FOR FIGURE 1 OMITTED]).

Seedling shade responses. - Shading affects plant development and functional ecology. Acclimation to shade occurs at all levels of organization, from general architecture to biochemistry (Grime 1979, Bjorkman 1981, Givnish 1988, Woodward 1990, Turnbull 1991, Chazdon et al. 1996). Shading alters: (1) internode distance, branch length and plant height; (2) axillary bud initiation and branching; (3) photosynthate allocation to stems, leaves, and roots; (4) leaf area, specific mass and anatomy; (5) photosynthesis and transpiration; (6) chloroplast ultrastructure; and (7) the stoichiometry of components of both the light and dark reactions of photosynthesis. Ultimately, shade tolerance means the seedling's capability of growth and/or survival in these low light conditions.