Light distribution varies vertically and also horizontally depending on openness of forest, season of the year, and size of canopy openings or gaps. The upper tree layer of a forest receives 25-100% of relative illuminance depending on the angle of the sun, which in turn depends on latitude and time of day. This is called the euphotic layer (Longman and Jenik 1987). Lower down, below the middle tree layer there is an oligophotic phase with 1-3% and even less illuminance, where seedlings and saplings grow, along with ferns and mosses. Cloud conditions also influence light distribution in the forest. More PAR reaches the understory of a rain forest on an overcast than on a sunny day because the diffuse light of the cloudy day enters the canopy at many different angles, and thus has a greater probability of penetrating the canopy.
Openings in the forest canopy dramatically affect the light environment. Forest canopy openings can be small (selective logging, branch or tree fall) or extensive (landslide, hurricane damage, or land clearing) (Chazdon et al. 1996). Chazdon and Fetcher (1984) compared the light environment in a small (200-m2) gap, in another gap of twice that size (400 m2), in a large clearing, and in the understory of the rain forest at La Selva Biological Sta tion in Costa Rica. Total PPFD of the 400-m2 gap was 20-35% of that in the clearing, and in the 200-m2 gap, it was 10% that of the clearing. Total PPFD in the center of the 400-m2 gap was 25 times greater than in the understory, and two to three times greater than in the 200-m2 gap. The decrease in PPFD in gaps of different sizes was not proportional to size of the gaps.
The influence of a gap may extend far beyond the immediate gap area, as shown by elevated PPFD in understory locations up to 20 m from a gap edge, although this effect has not always been observed (Chazdon et al. 1996). Gap light regimes exhibit substantial yearly variation due to changes in both solar angle and weather conditions. Differences in canopy opening sizes within rain forests promote differences in regeneration survival and growth of canopy tree species (Ashton et al. 1997 a). In Sri Lanka, studies have demonstrated this variability in survival and growth of late-successional canopy species of Shorea trees in relation to light, soil moisture, and nutrient status (Ashton and Berlyn 1992; Ashton 1995). Specific management guidelines have thus to be followed to ensure the growth of these species in plantations, enrichment plantings, or other forest management or restoration projects.
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