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Papyrus Ecosystem C dynamicsEmergent macrophytes in swamps, marshes and floodplains form some of the most productive plant communities. For example, natural stands of the grass Echinochloa polystachya in the central Amazon region have yielded 39.8 t C ha-1 yr-1 and the sedge Cyperus papyrus L. (papyrus) on lake Naivasha, Kenya had an above-ground yield of 20.6 t C ha-1 yr-1 . Although emergent macrophytes cover only about 1% of the continental land surface they have been estimated to produce as much as 5% of the total annual primary production . Papyrus forms the dominent emergent vegetation in most permanently flooded wetlands of tropical Africa (Hughes and Hughes, 1992). No accurate records of the area covered by papyrus swamps exist but one estimate puts it at about 4000 km2 in Africa (Thompson and Hamilton, 1983). Papyrus is amongst the largest of herbaceous species, with culms growing to a height of 5 m and an above-ground standing-biomass often in excess of 12.0 t C ha-1 . The culms are topped by the characteristic large reproductive umbel which is also the main photosynthetic surface. This is typically about 50 cm in diameter and consists of several hundred cylindrical rays, each of which extends into three to five flattened (leaf-like) bracteoles, the outer points of which describe a spherical form . Papyrus, like other productive tropical emergent wetland species such as E. polystachya and Paspalum repens, uses the C4 photosynthetic pathway. The papyrus vegetation of African wetlands often overlies several metres of peat. This peat forms in the anaerobic environment below the floating rhizome mat as detritus accumulates from the dead and decomposing papyrus vegetation. Few estimates have been made of the amount of peat below papyrus but it is likely that the African swamps could form a significant sink for carbon (Jones and Muthuri, 1997). Conversely, when the detritus is exposed to the atmosphere by a hydrological drawdown or drainage then these systems may become a net carbon source for the atmosphere. Papyrus swamps are a relatively simple ecosystem with a single primary producer, no major herbivores and they show no marked seasonality in primary productivity (Muthuri et al., 1989). Further, monotypic stands are formed at continuously wet and nutrient rich sites in which neither water stress nor nutrient shortage are likely to limit photosynthesis. The relatively homogenous stands of papyrus often extend over large areas and therefore represent a very practical system for ecosystem-level simulation and measurement. However, the height of the papyrus vegetation and the floating nature of the root and rhizome mats makes stand enclosure for the measurement of canopy photosynthetic rates impractical. Consequently, process-based models provide a useful method to bridge the gap between small scale leaf and large scale ecosystem measurements. Advances in both instrumentation and theory have made it possible to measure, non-invasively and over long periods, the net ecosystem flux of carbon from vegetated areas using the eddy covariance technique. This technique has recently been applied to measurements of forested areas, and in the past to crops for short term micrometeorology studies . However, very few measurements have been made of wetland ecosystems. The work reported here is, as far as we are aware, a first attempt to characterize the carbon fluxes of tropical wetland vegetation. Our objective was to characterise the papyrus ecosystem carbon budget at scales from the leaf to the whole canopy using measurements made with conventional small scale techniques, such as leaf photosynthesis and respiration of plant components and detritus, alongside eddy covariance measurements. The flux measurements obtained in this way were then compared with predictions from WIMOVAC (Windows Intuitive Model of Vegetation response to Atmosphere and Climate Change) which is a process-based model of leaf physiology and canopy structure (Humphries and Long, 1995) to validate leaf to ecosystem scaling. The ultimate aim of this work is to determine the contribution that papyrus swamps make to carbon cycling in the African tropical wetlands and to examine the hypothesis that papyrus swamps are potentially a major sink for carbon in the tropics. |
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