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Title
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Photosynthesis and the role of foliar nutrients in the lowland tropical rainforests of French Guiana
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Author
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Abstract
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| Tropical forest are among the most productive ecosystems on earth, accounting for more than one third of global terrestrial gross primary productivity (GPP), and thereby play an important role in the global carbon cycle. GPP of tropical forests is shown to be constrained by nutrients. The two nutrients most often limiting plant growth are nitrogen and phosphorus, and the old and severely weathered soils in tropical forests are particularly low in phosphorus availability. Moreover, increases in carbon and nitrogen availability due to rising atmospheric CO2 levels and human induced N inputs to ecosystems, respectively, are not paralleled by a similar increase in phosphorus inputs, leading to stoichiometric imbalances with especially low phosphorus concentrations. It is yet unclear how foliar nutrients, and especially foliar phosphorus, affect photosynthesis in tropical forests, but that insight is essential for our understanding of terrestrial carbon cycling and to improve global models that aim to project future climate. In this thesis, we assessed how to best measure photosynthesis in a tropical rainforest, which is challenging because of the very tall trees and strong light gradients within the dense canopies, among others. Branch excision prior to measuring photosynthesis is a commonly used method in tropical forest and we showed this method to be appropriate, and thus not affecting photosynthesis measurements. Additionally, we studied at which light level photosynthesis becomes light saturated, which is critical to avoid over- or underestimation of photosynthesis. Foliar nitrogen has been widely identified as the major driver of net photosynthetic capacity and although foliar phosphorus is deemed to be important in tropical forest growing on phosphorus-poor soils, it has not been well-understood how phosphorus regulates the photosynthetic capacity. We found that foliar phosphorus is of major importance for leaf-level photosynthesis at the spatial scale, but surprisingly not at the vertical scale. Lastly, fertilisation experiments have been carried out to identify to what extent nutrient availability affects tropical rainforests and foliar nutrient concentrations and leaf-level photosynthesis have been shown to increase in response to fertilisation. However, three years of nitrogen and phosphorus addition did not affect foliar nutrient concentrations nor the photosynthetic capacity in French Guiana. To conclude, we provide a framework for future measurements of leaf-level photosynthesis in tropical rainforests, guiding future tropical research, and we provide a comprehensive dataset of photosynthesis and potential drivers of carbon uptake in tropical rainforests to further improve model representations of photosynthesis. |
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Language
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English
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Publication
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Antwerpen
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Universiteit Antwerpen, Faculteit Wetenschappen, Departement Biologie
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2021
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Volume/pages
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xvi, 187 p.
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Note
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Janssens, Ivan [Supervisor]
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Vicca, Sara [Supervisor]
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Full text (open access)
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