Vulnerability Assessment Of The Coastal Mangrove Ecosystems In Guangxi, China To Sea-level Rise

Sea level rise caused by global climate change has significant impacts on coastal zone. The mangrove ecosystems occur at the intertidal zone in tropical and subtropical coasts and provide valuable ecosystem services, including habitat biodiversity, coastal protection, water purification and support of various material resources. Located in the transition zone between land and ocean, the mangrove ecosystems are particularly sensitive to sea-level rise. The vulnerability assessment of the impacts of sea-level rise on mangrove ecosystem could provide a scientific basis on formulating feasible and practical mitigation strategies for coastal mangrove, which is an important prerequisite for securing the coastal zone ecosystems.Taking the mangrove ecosystems in the coastal zone of Guangxi’ province, China as a case study, the potential impacts of sea-level rise were analyzed by adopting the SPRC (Source-Pathway-Receptor-Consequence) model and the SPRC model for vulnerability assessment under sea-level rise on the mangrove ecosystems in the Guangxi coast was established. Based on the SPRC model and the IPCC definition of vulnerability, i.e. the aspects of exposure, sensitivity and adaptation, an indicator system for vulnerability assessment and the process-based methodology for assessing vulnerability for the mangrove ecosystem in Guangxi coast undergoing sea-level rise was developed. A spatial quantitatively vulnerability assessment on the mangrove ecosystem in Guangxi coast was performed based on a GIS platform. The spatial distribution and area change of coastal mangroves under the impacts of sea-level rise were projected, the threat from sea-level rise to the coastal mangroves was evaluated quantitatively by applying Sea Level Affecting Mars Model (SLAMM). Finally, based on the results from this study, mitigation measures were proposed for the mangrove ecosystems in the Guangxi coast. The main results of this study were as follows:1. Analysis of the impacts of sea-level rise on mangrove ecosystems based on the SPRC modelThe SPRC model shows great logicality, it can reflect the interactions and processes between the sources and receptors and analyze causal relationships of the elements in the system. The SPRC model for vulnerability assessment of mangrove ecosystem was established and the main impacts of sea-level rise on the mangrove ecosystems were analyzed based on the SPRC model. Absolute sea-level rise (Source, S) caused by global climate change is the potentially affecting source for the mangrove ecosystem, which directly impacts on the coastal mangroves (receptors, R) according to the routes (pathways, P), including crust vertical movement and sedimentation. The main mangrove communities were divided into14kinds of receptors based on the key geomorphological characteristics of the mangrove habitats. The effects of absolute sea-level rise, combined with crust vertical movement and sedimentation, affect the relative sea-level rise. The relative sea-level rise will alter the elevation of coastal wetland habitats (elevation), which will impact mangrove ecosystem (C). If the relative sea-level rise exceeds the tolerance range of the habitat/ecosystem, it will affect the growth of mangroves, change the ecosystem structure and function, and finally result in habitat loss. The presence of a fixed landward boundary (seawalls) will prevent intertidal mangroves to migrate upward, resulting in mangroves loss with sea-level rise.2. The development of methodology for assessing vulnerabilityAccording to the SPRC model and the IPCC definition of vulnerability, i.e. the aspects of exposure, sensitivity and adaptation, an indicator system for vulnerability assessment on coastal mangrove ecosystems under sea level rise was worked out, in which the rate of sea level rise, subsidence/uplift rate, habitat elevation, daily mean inundation duration, intertidal slope and sedimentation rate were selected as the key indicator variables, taking into account of the characteristics of quantification, data accessibility, spatial and temporal heterogeneity. According to integrate the causal relationship between the indicators, the vulnerability index (Ⅵ) calculation process was established. The vulnerability grades were defined based on the difference of the mean daily inundation duration among the low, middle and high intertidal zones. A quantitatively spatial assessment method based on the GIS platform was established by quantifying each indicator, developing the vulnerability index calculation process and grading the vulnerability.3. Spatial vulnerability assessment for the mangrove ecosystems in the Guangxi coast under the sea-level riseIn the GIS platform, according to the VI calculation process, the VI values were calculated and then graded into vulnerability grades, the spatial vulnerability assessment based on the sea-level rise rates of the present trend (the rate of sea level rise in the past40years)(0.29cm/yr), the IPCC A1FI (0.59cm/yr) and IPCC R.CP8.5(0.98cm/yr) scenarios were performed for three sets of projections of short-term (2010-2030), mid-term (2010-2050) and long-term (2010-2100). The results showed the impacts of sea-level rise on mangrove ecosystems were negligible under the present trend,5.4%of the mangroves would be within areas of low vulnerability in2100. Under the IPCC A1FI and IPCC RCP8.5scenarios, the impacts of sea-level rise on mangrove ecosystems were serious. Under the A1FI scenario,13.4%of mangroves were within areas of moderate vulnerability in2100. Under the RCP8.5scenario,25.8%of mangroves were within areas of moderate vulnerability in2050, while37.3%of mangroves were within areas of high vulnerability in2100. The affected areas occur mainly in coasts of Yingluo Bay, Dandouhai, Tieshangang, Beihai, Maoweihai, Qinzhou bay, et al., where the rate of sedimentation and land uplift could not offset the rate of sea-level rise, and result in the inundation duration exceeds the tolerance range of the habitat.4. Projection of the impacts of sea-level rise on the coastal mangroves by applying SLAMM Model Taking the mangrove ecosystems in the coastal zone of Tieshangang Bay, southern China as a case study, the quantitatively projection based on the sea-level rise rates of the present trend (the rate of sea level rise in the past40years) and IPCC RCP8.5scenarios were performed for three sets of projections of short-term (2007-2025), mid-term (2007-2050) and long-term (2007-2100), using Sea Level Affecting Mars (SLAMM) model. The results showed that the sea-level rise rate of the present trend scenario would result in a negligible loss of mangrove habitat at the Tieshangang Bay in2025and2050, and a more significant impact on mangrove habitats in2100. While under the IPCC RCP8.5scenario, sea-level rise could result in a considerable loss of the coastal mangrove habitats, in2100the area of mangrove communities located on the middle and higher intertidal zones will decrease by49.4%and60.2%, respectively. The relatively low sedimentation and land uplift will give rise to a high probability of a significant decrease or even a complete loss of mangrove community.5. Formulating the mitigation measures to sea-level riseBased on the results from this study, mitigation measures for mangrove ecosystems in the Guangxi coast were proposed:(1) management of sedimentation: sustaining mangrove forest health, designing rationally coastal engineering, constructing properly silt promoting engineering, controlling sediment removal or dredging from the mangrove habitats, which reduce coastal erosion and increase sedimentation in mangrove habitat according to characteristics of ecosystem, engineering design and reduction of anthropogenic disturbances;(2) controlling reclamation:Controlling the large-scale reclamation strictly, keeping a dynamic balance between the conservation on coastal mangroves and the ongoing increase in sustainable social and economic development in the region, maintaining the integrity of structure and function of mangrove ecosystem;(3) rehabilitation of mangroves: Reasonably planning the suitable mangrove forest land, using dredging to develop mudflat for recreating pioneer mangrove vegetation to promote sedimentation, mitigating coastal erosion and the impacts of sea-level rise on the mangrove ecosystems.The SPRC model, the indicator system and the methodology for vulnerability assessment developed from this study can objectively and quantitatively assess the vulnerability of coastal mangrove ecosystems in the Guangxi coast under the impact of sea-level rise. SLAMM model can quantitatively evaluate the impacts of sea-level rise on coastal mangroves in the Guangxi coast. The results from this study could provide a scientific basis on formulating feasible and practical mitigation strategies for securing the coastal mangrove ecosystems.