Photosynthetic characteristics of dipterocarp seedlings in three tropical rain forest light environments: A basis for niche partitioning?

In three tropical rain forest light environments in Sabah, Malaysia, we compared photosynthesis in seedlings of ten climax tree species with putatively differing shade tolerances. The objectives of the study were (a) to characterise the range of photosynthetic responses in ten species of the Dipterocarpaceae and (b) to elucidate those photosynthetic characteristics that might provide a basis for niche partitioning. Seedlings were acclimated (c. 7 months) in three light environments; understorey, partial shade and a gap (140 m²). The light environments represented a gradation in median diurnal (0630-1830 hours) photon flux density (PFD) ranging from understorey (4.7 μmol m^-2 s^-1) through partial shade (21.2 μmol m^-2 s^-1) to gap (113.7 μmol m^-2 s^-1). Integrated diurnal PFD were in the sequence gap > partial shade > understorey (15.2, 4.7, 1.3 mol m^-2 day^-1, respectively). In gap-acclimated plants, species differed in the photosynthetic light-response variables apparent quantum yield, dark respiration rate, light compensation point, net saturated leaf assimilation rate (A(sat)), and in stomatal conductance (g(s sat)) when assimilation rate (A) was saturated. A light-demanding pioneer species (Macaranga hypoleuca) and a shade-demanding understorey species (Begonia sp.) had, respectively, higher and lower A(sat) and g(s sat) than the dipterocarp species. In high-light conditions A(sat) and g(s sat) were strongly positively correlated in dipterocarp species. Differing photosynthetic characteristics of gap acclimated plants suggest that, in these dipterocarp species, different rates of carbon fixation may be an important factor contributing towards niche partitioning. Mean integrated diurnal A (A(diurnal)) in the gap, partial shade and understory were, respectively, 122.9, 52.7, 20.5 mmol m^-2 day^-1. Differences occurred in A(diurnal) of dipterocarp species between light environments. When Macaranga was included, differences in A(diurnal) were evident in the gap and partial shade, and in both cases were attributed to the pioneer. For the variable A(diurnal), there was of a shift in the rank position of Macaranga among light environments, but a shift did not occur among the dipterocarp species. Results from this study are consistent with the idea that rates of carbon fixation per unit leaf area may contribute towards niche differentiation between the climax and single pioneer species, but not within the group of climax species. Other physiological and/or carbon allocation factors may be involved in any niche partitioning; dipterocarp species often have inherently different growth rates and susceptibility to herbivory. As an alternative to niche partitioning, dipterocarp species may co-exist in natural light environments as a result of habitat disequilibrium or purely stochastic processes.