Vertical variation in photosynthetic parameters in two different tropical forest ecosystems

Abstract

Màster project submitted to obtain the degree of Master in Biology, specialisation Biodiversity: conservation and restoration. Universiteit Antwerpen. Faculty of Science. Department of Biology. Academic year 2015-2016

Forests contribute to the carbon balance as the largest vegetative sink for atmospheric carbon (CO2). Anthropogenic emissions are counteracted by carbon sequestration in trees, but nutrients could be limiting photosynthesis and the effect could possibly be not as large as believed. In tropical forests, phosphorus (P) is only available from weathered bedrock and is thereby in an imbalance with the rising levels of carbon and nitrogen in the atmosphere. If P is limiting carbon uptake in tropical forests, global carbon cycle models are likely overestimating uptake by forests. Another overestimation might be to only conduct photosynthesis measurements on sunlit leaves of the canopy and take this as an overall canopy average, whilst a vertical profile in photosynthesis is very likely. Our study was conducted on two sites of the Amazonian rain forest in French Guiana. Photosynthesis and dark respiration (Rd) was measured of 120 trees in 12 plots per site. The plots were situated along a geographical gradient (at top, slope and bottom) to cover a large variety in soil P concentration. We derived the photosynthetic parameters Vcmax and Jmax from the photosynthesis measurements using the Farquhar model (Farquhar et al., 1980). The measurements were performed at two different height levels in the canopy to investigate the vertical profile. In this study we aimed to relate the spatial and vertical variability to parameters such as leaf P concentration, leaf height, light availability, the specific leaf area and the chlorophyll content (SPAD). Soil P concentrations were correlated with the leaf P concentrations, which indicates P uptake from the soil is limited. There were significant vertical differences in the leaves in Vcmax, Jmax, Rd and leaf P concentrations. We conclude that P limits the photosynthetic capacity in our study areas and vertical profiles of photosynthesis should be taken into account when estimating carbon uptake by a tropical forest ecosystem.