Mangrove ecosystems experienced significant deforestation over the last five decades, with as much as half global coverage lost. Recent awareness of mangroves’ roles in storm defence and carbon cycling has led to increased political will to conserve and protect these ecosystems, with declines in global deforestation rates as a consequence. Climate change is now regarded as the biggest threat to mangroves. Yet, it remains generally unclear how mangroves will respond under predicted climate change. This thesis investigates how understanding of mangrove functioning in an extreme environmental setting can refine global present-day estimates of mangrove productivity and carbon stocks and inform predictions of future mangrove productivity and carbon stocks.

Mangrove research has centred on tropical humid settings, leaving their counterparts at the extremities of their climatic distribution under-represented. It is unclear how forest productivity and carbon stocks of arid and hot environment mangroves compare to their humid tropical counterparts. Chapters 2 and 4 of this thesis quantify mangrove primary productivity and carbon stocks in three mangrove stands in the hot and arid country of Qatar, the Arabian/Persian Gulf (‘the Gulf’). Here, data collected in situ in Qatari mangroves reveal that mangroves in the country are less productive and store less carbon than many humid tropical mangroves. The study also finds productivity and biomass dynamics of Qatar mangroves differ from those of humid regions, likely as a result of differing nutrient dynamics in arid settings. For instance, biomass and productivity of Qatari mangroves had an inverse relationship with tidal elevation, which is the opposite to many humid tropical locations. Chapters 3 and 5 incorporate the empirically derived biomass, carbon stock and productivity data into numerical models to estimate present and future global mangrove productivity and carbon stocks. Similarly to previous global modelling work, the thesis finds the greatest mangrove productivity and carbon stocks to occur in Southeast Asia. Future predictions suggest a likely increase in global carbon stocks, although productivity is unlikely to change significantly. Models show the most mangrove-rich countries might have a >10% increase in carbon stocks. However, under the more extreme climate change scenarios, the rising temperatures will elevate the probability that carbon stocks will decrease across countries. This finding is because models used in this dissertation, through the inclusion of new empirical data from Qatar, are trained on data that allocates greater representation to mangroves in hot arid climates than done in previous modelling.

Inclusion of empirical data from the climatic extremes of global mangrove distribution, as done here, generates a more realistic prediction of what may occur to mangrove carbon stocks and productivity under predicted climate change, particularly through effects of heightened temperature. Mangroves are expanding their global distribution into higher latitudes, with forests on the edge of their climatic distribution likely to become more widespread. It is hoped that the increased understanding of mangrove functioning under extreme climatic conditions and predicted climate change delivered by this thesis will be instructive to the effective future management and conservation of the global mangrove ecosystem.


Chapters 2 (Mangrove primary productivity in an extreme environment) and 3 (Past, present and future global mangrove primary productivity) have not been published. However, chapter 4 (Mangrove carbon stocks and biomass partitioning in an extreme environment) was published in 2020 in Estuarine, Coastal and Shelf Science, and chapter 5 (Future mangrove carbon storage under climate change and deforestation) was published in 2022 in Frontiers in Marine Science.