Photosynthetically Active Radiation Environment, Patch Dynamics and Architecture in a Tropical Rainforest in Sumatra

Abstract

The photosynthetically active radiation (PAR) environments of three sites within a tropical rainforest in Sumatra were characterised. The sites and sensor locations were chosen according to a previous study of forest dynamics based on forest mosaic maps and an analysis of forest architecture by means of detailed profile diagrams. Two sites were located in mature forest patches: one showed a layered architecture while the other had a continuous vertical distribution of foliage. A third site represented a 320 m� treefall gap. Vertical gradients of PAR from above canopy to ground level were studied in the mature forest zones, while a horizontal gradient of PAR at ground level was studied across a gap-mature forest boundary. Daily total PAR at ground level in the mature forest sites was 0.78% and 0.57% of incident radiation above the forest. The decrease of radiation from above canopy downwards was strikingly different between the sites: the layered site showed a 'PAR inversion' between successive layers, the amount of radiation under a given layer being less than at the top of the next lower layer. The non-layered site did not show such an inversion. The treefall gap radiation received was approximately 20% of the radiation recorded above the canopy. The spatial distribution of PAR in the forest is highly variable but can be usefully analysed against a framework of forest dynamics and architecture. Temporal distribution of PAR is also highly variable, and both distributions need to be analysed to understand photosynthetic responses and growth rates of rainforest species.