Photosynthetic Gas Exchange Responses of Australian Tropical Forest Trees in Canopy, Gap and Understory Micro-Environments

Abstract

The photosynthetic responses of three tropical tree species were investigated in the contrasting micro-environments of the canopy, a gap and the understory of a tropical forest in north-eastern Australia. Measurements were made with a small transportable field gas exchange apparatus on the evergreen species Argyrodendron peralatum (F.M. Bailey) Edlin ex I.H. Boas and Castanospermum apstrale (Cunn.) Frazer and Cook, which regenerate in the understory and the deciduous gap species Toona australis (F. Muell.) Harms. Acclimation to the prevailing light environment was evident in all species, with the canopy leaves having higher light saturated photosynthetic rates, dark respiration rates and stomatal conductances than the understory leaves. For A. peralatum, leaves in the gap were generally intermediate between those in the canopy and understory. There was no evidence for acclimation to the differences in temperature or leaf-air vapour pressure deficits between the canopy and understory leaves. Determinations of the daily courses of CO2 uptake showed that although the daily photon flux density (PFD) in the understory was only 3% of that at the canopy top, leaf carbon gain was nearly 10% of that measured for the canopy leaf. In the understory, photosynthesis during sunflecks accounted for about one third of the daily carbon gain. There was no evidence that temperatures or leaf-air vapour pressure deficits significantly limited CO2 uptake in the canopy. Key-words: Photosynthesis, stomatal conductance, acclimation, light environment, tropical forest, understory, canopy, Argyrodendron peralatum, Toona australis, Castanospermum australe * Current address: Professor R.W. Pearcy, Department of Botany, University of California, Davis, California 95616, USA. Introduction Tropical forests consist of a mosaic of contrasting micro-environments including those of the canopy, the understory and gaps created by tree falls. These micro-environments have been characterized in several forests (Akoi, Yabuki & Koyama, 1975; Odum, Drewry & Kline, 1970; Allen, Lemon & Muller, 1972) but to date relatively little attention has been given the physiological responses to them. Gas exchange responses have been reported for saplings and herbs in understories (Bjorkman, Ludlow & Morrow, 1972; Pearcy & Calkin, 1983) and for shrubs in gaps vs understory habitats (Lebron, 1979; Walters & Field, 1987) but there have been no measurements reported for the canopy. Tree species that germinate in the understory but ultimately reach the' canopy are typically exposed to this wide range of environments and thus the capacity to acclimate photosynthetic responses to these different conditions may be an important characteristic. Laboratory studies have shown that most, but apparently not all, tropical forest tree species can acclimate their photosynthetic apparatus to highand lowlight environments (Langenheim et al., 1984; Oberbauer & Strain, 1984; Fetcher et al., 1986; Pearcy & Francheschi, 1986). However, there have been few field studies in contrasting tropical forest micro-environments where other environmental stresses may influence the extent of acclimation to light. In temperate forests, large spatial and temporal variations in photosynthetic capacity have been reported (Fuchs, Schulze & Fuchs, 1977; Schulze, Fuchs & Fuchs, 1977ab). In this study, the photosynthetic gas exchange of tree species in an Australian tropical forest were investigated in canopy, understory and gap microenvironments in order to examine the contrasts in the physiological and environmental controls of photosynthetic CO2 exchange in these environments. Three species were selected for study. Argyrodendron peralatum (F.M. Bailey) Edlin ex I.H. Boas is a canopy-emergent evergreen having sclerophyllous trifoliate leaves with leaflets 4-6 cm wide and 15-25 cm long. Seedlings of this This content downloaded from 207.46.13.122 on Thu, 19 May 2016 04:52:06 UTC All use subject to http://about.jstor.org/terms