Light Environments of Tropical Forests

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Abstract

Measurements of photosynthetically active radiation (PAR) in tropical forests are reviewed and discussed. Many studies of the light environment in tropical forests have emphasized the importance of sunflecks to the total light energy reaching the forest floor. Much of the spatial variation in total daily PAR in understory environments has been attributed to localized sunfleck activity, but levels of diffuse radiation were also found to vary spatially. Values of percent transmission differs greatly among forests throughout the world, ranging from 0.4–3.8, depending on forest structure and weather conditions. Total daily photosynthetic photon flux density (PPFD) in understory habitats is often below 1.0 mol m-2 d-1, and measurements as low as 0.15 mol m-2 d-1 have been recorded.

Light environments in four rain forest habitats were compared over a 13-day period during wet and dry seasons in a lowland rainforest in Costa Rica. Daily total PPFD in the understory, 200 m2 gap, and 400 m2 gap were 1–2%, 9% and 20–35%, respectively, of PPFD in a 5000 m2 clearing. Daily total PPFD in the 400 m2 gap was 2–3 times greater than in the 200 m2 gap and 15–35% greater than in the understory. Seasonal differences in light availability occurred only in the clearing habitat, where decreased cloudiness during the dry season resulted in higher PPFDs.

In the understory, more than 70% of the daily 10-min averages of PPFD were below 10 μmol m-2 s-1; on clear days, sunflecks contributed 55–77% of the total quantum flux. The 200 m2 gap center received little direct radiation; total daily PPFD ranged from 1.52–3.07 mol m-2 d-1. Total daily PPFDs in the 400 m2 gap center ranged from 3.86–13.6 mol m-2 d-1 and 11% of the 10-min averages were above 1000 μmol m-2 s-1 on the day with maximum PPFD. In the clearing, daily total PPFDs ranged from 13.7–33.9 mol m-2 d-1, and over 40% of the 10-min averages were above 1000 μmol m-2 s-1 on the maximum day.

Measurements of the spectral distribution of radiation from 300–1100 nm were made in 2 gaps, a clearing, during sunflecks, and in deep shade. The median red:far-red ratio in gaps and sunflecks was 0.86 and 0.99, respectively, as compared to 0.42 in the shade and 1.23 in the clearing. The median % of total energy in the photosynthetic waveband was 53% in the clearing, 39% in sunflecks and gaps, and 18% in the shade.