Solar UV‐B and visible radiation in tropical forest gaps: measurements partitioning direct and diffuse radiation

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Abstract
Solar ultraviolet‐B (UV‐B) radiation penetrates plant canopies to a different degree than photosynthetically active radiation (PAR) because UV‐B is diffused to a greater degree by the atmosphere. We measured both global (total) and diffuse solar radiation in canopy gaps of a semideciduous tropical forest in Panama. Measurements were simultaneously made in the UV‐B and PAR wavebands. Compared to unobstructed measurements taken outside the forest, the sunlit portions of gaps were depleted in the proportion of UV‐B relative to PAR, especially at midday. Shaded areas, in contrast, were always richer in UV‐B relative to PAR, but the magnitude of the change varied greatly. Presumably this variation was due to the differences in the directional nature of diffuse solar UV‐B radiation as compared to diffuse PAR. Measurements in the gaps showed substantial reductions in the proportion of radiation in the diffuse components of both the UV‐B and PAR wavebands. However, because of the greater proportion of UV‐B which is diffuse, it tended to predominate in shaded areas. Similar patterns were seen in measurements taken at temperate latitudes. Response of shade‐ and gap‐dwelling plants to these high UV‐B:PAR ratios has received little attention.