Light environments within montane tropical rainforest, Mt Bellenden Ker, North Queensland

AbstractIrradiance and photosynthetic photon flux density (PPFD) at five sites within a montane tropical rainforest were compared to test for differences among characteristic environments within the same forest type. The five habitats investigated were a large clearing (800 m2). a small gap (150 m2), the lower canopy, and two shaded understorey sites. Daily average irradiance ranged from 32.3 W m‐2 in the heavily shaded understorey to over 400 W m‐2 in the large clearing. Similarly, daily average PPFD ranged from 10.2 to over 800 μmol m‐2 s‐1 at corresponding sites, respectively. Daily total irradiation in the understorey, small gap, and lower canopy were 7.9–8.8%, 11.8% and 11.5%, respectively, of irradiation in the large clearing. For daily total PPFD, relative values for the same sites were 1.3–3.8%, 6.0% and 10.3%, respectively. Comparisons of frequency distributions for irradiance showed that the only significant difference was between the large clearing and the four forest sites. Comparisons of frequency distributions for PPFD showed significant differences among all five sites. Median PPFDs were always above the range of known light compensation points for rainforest plants even in the heavily shaded understorey, and usually above the known light saturation levels of canopy members. These results indicate that the reduction in PPFD by cloud cover does not limit the rates of photosynthesis.