Mangroves, the woody halophytes that grow along tropical and subtropical coasts, are key ecosystems that provide important ecosystem services including nursery grounds for fisheries species, habitat for fauna, provision of coastal protection and carbon storage. The distribution and development of mangrove forests reflect their physiological responses to variation in both their biophysical environment and to geomorphological conditions on the coasts. Mangrove forests are under high levels of pressure from human activities in the coastal zone, particularly due to aquaculture, due to changes in sediment delivery to coasts and also due to sea level rise and other factors associated with climate change. Mangrove responses to variation in sediment supply and sea level change as well as to human use of the coastal zone will determine the fate of forests and their capacity to provide ecosystem services.
This thesis explores the response of mangroves to human interventions on the coast of Indonesia, with a focus on the effects of altered geomorphological processes and mangrove conversion to aquaculture ponds and the study of restoration after aquaculture on mangrove adaptation to sea level rise and the provision of carbon sequestration services. In the first chapter, I present an assessment of the impact of excessive sediment supply to mangrove forests in Porong, East Java. The next chapter describes carbon emissions associated with conversion of mangrove to shrimp ponds in Perancak, Bali (published as Sidik and Lovelock 2013). The study of mangrove ecosystems after restoration is presented in chapters 4 and 5. I provide an estimation of adaptive response to sea level rise and provision of carbon sequestration services of restored mangrove forests and natural mangrove forests as the natural reference.
My research was undertaken in two different mangrove ecosystems in Java and Bali, Indonesia. Indonesia has the world’s largest mangrove coverage of the total global mangrove area, however Indonesian mangroves are facing a high degree of disturbances, for example high sedimentation and mangrove conversion to aquaculture ponds. In Chapter 2, my study revealed that extremely high sediment supply in Porong, due to sediment derived from the LUSI mud volcano, has influenced wetland formation, shown by a strong positive relationship between increase in surface elevation change, measured with rod surface elevation tables, and vertical accretion. Surface elevation gain far exceeded that of local sea level rise and enables mangrove forests to keep pace with sea level rise as well as extending the habitat seaward. On the other hand, high sediment supply had adverse effects on the growth of Avicennia sp. trees. However, field observations found that the mangroves were able to withstand sedimentation rates that exceed tolerances published in the literature. My study on the implication of clearing of mangrove forests for aquaculture (Chapter 3) showed that CO2 released from working shrimp ponds is comparable to other land use conversions in the this region. CO2 released from the wall of working shrimp ponds was higher than from the floors of ponds, demonstrating that the type of habitat modification influences CO2 emissions after disturbance of mangrove forests.
In response to extensive mangrove loss, there is new interest in restoring mangrove forests that aims at restoring their ecosystem functions. However, the capacity of restored forests in abandoned aquaculture ponds to rebound to their original state is poorly understood. A study of sediment surface elevation processes in restored and natural mangrove forests in Perancak, Bali (Chapter 4) indicated that surface elevation in restored forests increased at similar rate to that in natural forests. Restored and natural forests were keeping pace with sea level rise as their rates of surface elevation increase were higher than rates of current and projected sea level rise. Surface elevation in both restored and natural forests increased over the time with a strong seasonal pattern, indicating the importance of different processes in the wet and dry season in maintaining surface elevation trajectories. In addition, mangrove root production contributed to surface elevation change with root production tending to be higher in restored forests than in the natural forests, suggesting that restored mangroves undergo strong vegetative growth belowground. In Chapter 5, where I studied the restoration of productivity of carbon sequestration services, revealed the recovery in net primary productivity (NPP) and the carbon sequestration capacity in restored forests. NPP in restored forests was similar to adjacent natural forests and NPP in both sites was relatively high in comparison with values reported in the literature. The total ecosystem carbon stocks in Perancak were low compared to other values reported in the literature for mangrove forests in the tropics. Restored forests had lower carbon stocks but similar rates of carbon sequestration compared to natural forests. The source of carbon in mangrove sediments, as indicated by natural stable isotopes of carbon was dominated by mangrove material.
Overall, my study supports the view that mangrove forests are highly productive ecosystems that have a strong capacity to withstand and recover from disturbances and environmental change. Management actions that increase primary productivity of mangrove forests will contribute to the capacity of these forests to keep pace with sea level rise and will have a positive impact on provision of mangrove carbon sequestration. The results of my study provide valuable information to support valuation of ecosystem services provided by mangrove forests and to improve coastal restoration and management plans for sustaining mangrove forests in Indonesia in the future.