干旱区生态环境知识资源中心

Governed by a series of biotic and abiotic processes, the size of carbon storage in wetlands is important on a global scale. However, the various feedback mechanisms and the effect that climatic fluctuations have on them are not well documented. In this study, we introduce a mechanistic model connecting hydrology, vegetation and carbon to investigate how climate variability affects wetland ecosystems and the resulting carbon budget. Our particular interest is devoted to the semi-arid wetlands found in south-west Western Australia, as they are expected to be highly sensitive to projected climatic changes due to their highly seasonal rainfall and often water limited nature. In the first part of the study, we couple a model for carbon cycling to an ecohydrological model able to resolve the dynamic partitioning of wetland hydrological environments. We then present preliminary simulations that investigate wetland carbon dynamics given different hydro-climatological conditions ranging from dry to wet within the regional range. Under the driest scenario, vegetation biomass was 182% greater than in the wettest scenario due to a shift from predominately hydrophytic to predominantly mesophytic vegetation. However, the total carbon fixation increased just 4%, as the ratio of leaf area index to biomass is lower for terrestrial (woody) species than for the aquatic plants that were more abundant in wet conditions. Additionally, the total wetland respiration increased by approximately 60% due to a greater fraction of the soil becoming unsaturated, thereby promoting aerobic oxidation of organic matter stored in the soil profile. As a result, the net wetland metabolism was found to be enhanced under drier climates and the system was able to store 17% less carbon under the lowest rainfall condition when compared to the highest rainfall scenario. These preliminary results indicate that the coevolution of functional vegetation types with a dynamic hydrological environment can mediate the response of vegetation biomass and wetland carbon stores to climate shifts. Long term vegetation responses may be counter-intuitive and potentially not a good indicator for overall changes in wetland metabolism due to the dominance of the soil carbon store in wetland carbon budgets. The model presented here provides a first step to explain how changing patterns of rainfall, temperature and evapo-transpiration can significantly change the carbon retention characteristics and efficiency of wetlands, and suggests that terrestrialization of wetlands in semi-arid regions may result in a net loss of stored carbon despite potential increases in woody vegetation.