Belowground carbon and hydrological dynamics of mangrove forests

Mangroves are among the most carbon-dense ecosystems in the world. However, it is unclear whether natural and restored mangroves will continue to sequester carbon in the face of global environmental change. I used both laboratory mesocosms and field experiments to investigate the effect of global changes (reforestation, warming and sea level change induced by climate change) on fine root production (chapters 2 and 3) and on soil organic matter (SOM) decomposition (chapter 4). In addition, I identify the controls on soil water regimes in mangrove soils (chapter 5). My research showed that warming from 27 to 31°C increased SOM decomposition by 21 %, though this impact was reduced in mangrove soils affected by rising sea levels. Drought conditions (simulated as a suppression of soil inundation) sharply increased SOM decomposition (+66 %) and acted with warming to exaggerate this effect. Rising sea levels (simulated as an increase of inundation duration from two to six hours per day), alone or combined with warming, did not affect SOM decomposition. Therefore, the persistence of mangrove to sea level rise by accumulating SOM is likely driven by mangrove root production, and not by reduced SOM decay as previously assumed. Root production might be altered by additional global change factors (chapter 3). I showed that following mangrove restoration the fine root production declines monotonically with mangrove stand ages. This age-related pattern has been observed in aboveground production, but has not previously been reported in belowground carbon. Forecasts of changes to mangrove carbon storage and the persistence of these ecosystems in the face of rising sea levels need to account for stand ages in reforested mangroves (chapter 3). Finally, I showed that clayey mangrove sediment does not drain during ebb tides, even in locations with high densities of animal burrows. Only mangroves with coarser sediments seem to be sensitive to animal burrows. A possible shift toward coarser sediment with rising sea levels may lead to changes in lateral water flows within mangrove soils and their associated carbon loss (chapter 5). Overall, my thesis findings have important implications for understanding both mangrove carbon dynamics and the persistence of mangroves and other coastal wetlands under current and future environmental conditions.