Vertical profiles in a Brunei rain forest: I. Microclimate associated with a canopy tree

The microclimate variables, carbon dioxide (CO₂) concentration, air temperature, relative humidity and photon flux density (PFD), were measured in a vertical profile of rain forest in Brunei associated with a Dtyobalanops lanceolala canopy tree. This descriptive study of vertical microclimate was conducted to answer the questions: what specific microclimate can leaves occupying different height positions be expected to experience diurnally, and what are the physiological implications of microclimate differences? Measurements were made at 1-m intervals between the understory (at 1 m) and the upper canopy (at 34 m), during the morning (0800-1100h), mid-day (1100-1400h), and afternoon (1500-1700h) during three non-consecutive days. Understory CO₂ concentrations were high, but only at 1 m and only during one morning; thus, the hypothesis that the availability of CO₂ for photosynthesis is greater in the understory than elsewhere in the profile was not supported in this study. Temperature and vapour pressure deficit data suggest an afternoon "inversion surface" beneath the main canopy. During mid-day and afternoon in the upper part of the canopy, steep gradients of PFD were detected, and PFD was relatively high in the mid-canopy. A case is argued for more extensive and intensive studies, in conjunction with investigations of above-ground spatial organisation of vegetation, to provide information on vertical resource-utilisation by plants.