Light Variation and Carbon Gain in Rain Forest Understorey Palms

Median daily total photosynthetic photon flux density (PPFD) in gap-edge habitats was 1-1.5 mol m-2, compared with 0.26-0.33 mol m-2 in adjacent understorey sites. Median daily PPFD in the gap centre increased with gap size. In the understorey, >80% of averages calculated for 5-min periods were <10 mu mol m-2 s-1, whereas most readings in the centre of a 150-m2 gap were between 50-100 mu mol m-2 s-1 and along the gap edge 20-50 mu mol m-2 s-1. For Asterogyne martiana and Geonoma cuneata, 2 dwarf species, median daily total PPFD did not increase from ground level to 1.5 m. Some 50% of all PPFD measurements <1.5 m were 0.18-0.39 mol m-2. At crown heights >1.5 m, median daily total PPFD increased to 0.51 mol m-2. Within clones of G. congesta, daily total PPFD increased significantly with crown height; median PPFD incident on crowns 3.5-4.0 m tall was 5 times greater than PPFD at heights <1.4 m. On sunny days, the proportion of daily total PPFD contributed by sunflecks ranged from 10-78%. The relative proportion of daily PPFD contributed by sunflecks increased as the PPFD of background diffuse radiation decreased. Photosynthesis of seedlings grown in full shade did not differ significantly among the 3 species. -from Author