High variance in community structure and ecosystem carbon stocks of Fijian mangroves driven by differences in geomorphology and climate.

Mangrove ecosystems are particularly important for small island developing states of the Pacific, such as Fiji, which are at the forefront of the impacts of climate change. This is because of the ability of mangroves to mitigate storm surges and floods as well as their high carbon sequestration and storage capacity. However, there are few detailed studies on the spatial variation in mangrove structure and carbon stocks in Fiji, and this information is essential to support decision making by government and communities, enabling the development of effective mitigation and adaptation responses. We assessed mangrove forest structure in contrasting regions around Fiji's largest island, Viti Levu, within sites managed by indigenous (iTaukei) Fijians. Mangroves of the Ba, Nadroga-Navosa, and Rewa and Tailevu regions showed high variance in both structural complexity and ecosystem carbon stocks. Levels of variation were similar to that observed globally due to variable geomorphological and biophysical settings related to orographic rainfall, freshwater influx, tidal amplitude and cyclonic disturbances. High biomass, structurally complex forests occur on the wetter south-east coast (e.g. the Rewa Delta), while structurally uniform scrub mangroves dominate large areas of mangroves along the north-west (e.g. the Ba Delta) and west coast (e.g. the Tuva Delta). Mangroves of the Ba region displayed considerable damage from tropical cyclones, particularly in taller vegetation. All mangrove sites assessed were important reservoirs of carbon, with results when scaled to the spatial extent of mangroves in Fiji revealing that ecosystem carbon storage is disproportionate to area and equates to 73.3% of the carbon held within terrestrial rainforests, despite occupying just 7.3% of the total area. This underscores the importance of mangroves as valuable carbon sinks in Fiji and the need to develop incentives for improved conservation and restoration.

Effect of land-use and land-cover change on mangrove blue carbon: A systematic review.

Mangroves shift from carbon sinks to sources when affected by anthropogenic land-use and land-cover change (LULCC). Yet, the magnitude and temporal scale of these impacts are largely unknown. We undertook a systematic review to examine the influence of LULCC on mangrove carbon stocks and soil greenhouse gas (GHG) effluxes. A search of 478 data points from the peer-reviewed literature revealed a substantial reduction of biomass (82% ± 35%) and soil (54% ± 13%) carbon stocks due to LULCC. The relative loss depended on LULCC type, time since LULCC and geographical and climatic conditions of sites. We also observed that the loss of soil carbon stocks was linked to the decreased soil carbon content and increased soil bulk density over the first 100 cm depth. We found no significant effect of LULCC on soil GHG effluxes. Regeneration efforts (i.e. restoration, rehabilitation and afforestation) led to biomass recovery after ~40 years. However, we found no clear patterns of mangrove soil carbon stock re-establishment following biomass recovery. Our findings suggest that regeneration may help restore carbon stocks back to pre-disturbed levels over decadal to century time scales only, with a faster rate for biomass recovery than for soil carbon stocks. Therefore, improved mangrove ecosystem management by preventing further LULCC and promoting rehabilitation is fundamental for effective climate change mitigation policy.

Community structure dynamics and carbon stock change of rehabilitated mangrove forests in Sulawesi, Indonesia.

To date, discourse associated with the potential application of "blue carbon" within real-world carbon markets has focused on blue carbon as a mitigation strategy in the context of avoided deforestation (e.g., REDD+). Here, we report structural dynamics and carbon storage gains from mangrove sites that have undergone rehabilitation to ascertain whether reforestation can complement conservation activities and warrant project investment. Replicated sites at two locations with contrasting geomorphic conditions were selected, Tiwoho and Tanakeke on the island of Sulawesi, Indonesia. These locations are representative of high (Tiwoho, deep muds and silty substrates) and low (Tanakeke, shallow, coralline sands) productivity mangrove ecosystems. They share a similar management history of clearing and conversion for aquaculture before restorative activities were undertaken using the practice of Ecological Mangrove Rehabilitation (EMR). Species diversity and mean biomass carbon storage gains after 10 yr of regrowth from the high productivity sites of Tiwoho (49.2 ± 9.1 Mg C·ha-1 ·yr-1 ) are already almost of one-third of mean biomass stocks exhibited by mature forests (167.8 ± 30.3 Mg C·ha-1 ·yr-1 ). Tiwoho's EMR sites, on average, will have offset all biomass C that was initially lost through conversion within the next 11 yr, a finding in marked contrast to the minimal carbon gains observed on the low productivity, low diversity, coral atoll EMR sites of Tanakeke (1.1 ± 0.4 Mg C·ha-1 ·yr-1 ). These findings highlight the importance of geomorphic and biophysical site selection if the primary purpose of EMR is intended to maximize carbon sequestration gains.

Policy challenges and approaches for the conservation of mangrove forests in Southeast Asia.

Many drivers of mangrove forest loss operate over large scales and are most effectively addressed by policy interventions. However, conflicting or unclear policy objectives exist at multiple tiers of government, resulting in contradictory management decisions. To address this, we considered four approaches that are being used increasingly or could be deployed in Southeast Asia to ensure sustainable livelihoods and biodiversity conservation. First, a stronger incorporation of mangroves into marine protected areas (that currently focus largely on reefs and fisheries) could resolve some policy conflicts and ensure that mangroves do not fall through a policy gap. Second, examples of community and government comanagement exist, but achieving comanagement at scale will be important in reconciling stakeholders and addressing conflicting policy objectives. Third, private-sector initiatives could protect mangroves through existing and novel mechanisms in degraded areas and areas under future threat. Finally, payments for ecosystem services (PES) hold great promise for mangrove conservation, with carbon PES schemes (known as blue carbon) attracting attention. Although barriers remain to the implementation of PES, the potential to implement them at multiple scales exists. Closing the gap between mangrove conservation policies and action is crucial to the improved protection and management of this imperiled coastal ecosystem and to the livelihoods that depend on them.

Impact of an extreme monsoon on CO2 and CH4 fluxes from mangrove soils of the Ayeyarwady Delta, Myanmar.

Mangrove ecosystems can be both significant sources and sinks of greenhouse gases (GHGs). Understanding variability in flux and the key factors controlling emissions in these ecosystems are therefore important in the context of accounting for GHG emissions. The current study is the first to quantify GHG emissions using static chamber measurements from soils in disused aquaculture ponds, planted mangroves, and mature mangroves from the Ayeyarwady Delta, Myanmar. Soil properties, biomass and estimated net primary productivity were also assessed. Field assessments were conducted at the same sites during the middle of the dry season in February and end of the wet season in October 2019. Rates of soil CO2 efflux were among the highest yet recorded from mangrove ecosystems, with CO2 efflux from the 8 year old site reaching 86.8 ± 17 Mg CO2 ha-1 yr-1 during February, an average of 862% more than all other sites assessed during this period. In October, all sites had significant rates of soil CO2 efflux, with rates ranging from 31.9 ± 4.4 Mg CO2 ha-1 yr-1 in a disused pond to 118.9 ± 24.3 Mg CO2 ha-1 yr-1 in the 8 year old site. High soil CO2 efflux from the 8 year old site in February is most likely attributable to high rates of primary production and belowground carbon allocation. Elevated CO2 efflux from all sites during October was likely associated with the extreme 2019 South Asian monsoon season which lowered soil pore salinity and deposited new alluvium, stimulating both autotrophic and heterotrophic activity. Methane efflux increased significantly (50-400%) during the wet season from all sites with mangrove cover, although was a small overall component of soil GHG effluxes during both measurement periods. Our results highlight the critical importance of assessing GHG flux in-situ in order to quantify variability in carbon dynamics over time.

Estimating the full greenhouse gas emissions offset potential and profile between rehabilitating and established mangroves.

Mangrove forests are extremely productive, with rates of growth rivaling some terrestrial tropical rainforests. However, our understanding of the full suite of processes underpinning carbon exchange with the atmosphere and near shore-waters, the allocation of carbon in mangroves, and fluxes of non-CO2 greenhouse gases (GHGs) are limited to a handful of studies. This constrains the scientific basis from which to advocate for greater support for and investment in mangrove restoration and conservation. Improving understanding is urgently needed given the on-going landuse pressures mangrove forests face, particularly throughout much of Southeast Asia. The current study reduces uncertainties by providing a holistic synthesis of the net potential GHG mitigation benefits resulting from rehabilitating mangroves and established forests. Rehabilitating sites from two contrasting locations representative of high (Tiwoho) and low (Tanakeke) productivity systems on the island of Sulawesi (Indonesia) were used as case studies to compare against established mangroves. A carbon budget, allocation and pathways model was developed to account for inputs (carbon sequestration) and outputs (GHG emissions of CO2, N2O and CH4) to estimate Net Ecosystem Production (NEP) and Net Ecosystem Carbon Balance (NECB). Our results indicate that while Tiwoho's rehabilitating sites and established mangroves represent a significant carbon sink (-10.6 ±â€¯0.9 Mg CO2e ha-1 y-1 and 16.1 Mg CO2e ha-1 y-1 respectively), the low productivity of Tanakeke has resulted in minimal reductions to date (0.7 ±â€¯0.3 Mg CO2e ha-1 y-1). Including NEP from mangrove-allied primary producer communities (e.g. benthic algae) and the portion of dissolved inorganic carbon exported from mangroves (EXDIC) that remains within the water column may drive overall removals considerably upwards in established forests to -37.2 Mg CO2e ha-1 y-1. These values are higher than terrestrial forests and strengthen the evidence base needed to underpin the use of forest carbon financing mechanisms for mangrove restoration.

Hydroperiod, soil moisture and bioturbation are critical drivers of greenhouse gas fluxes and vary as a function of landuse change in mangroves of Sulawesi, Indonesia.

The loss and degradation of mangroves can result in potentially significant sources of atmospheric greenhouse gas (GHG) emissions. For mangrove rehabilitation carbon projects, quantifying GHG emissions as forests regenerate is a key accounting requirement. The current study is one of the first attempts to systematically quantify emissions of carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4) from: 1) aquaculture ponds, 2) rehabilitating mangroves, and 3) intact mangrove sites and frame GHG flux within the context of landuse change. In-situ static chamber measurements were made at three contrasting locations in Sulawesi, Indonesia. The influence of key biophysical variables known to affect GHG flux was also assessed. Peak GHG flux was observed at rehabilitating (32.8 ±â€¯2.1 Mg CO2e ha-1 y-1) and intact, mature reference sites (43.8 ±â€¯4.5 Mg CO2e ha-1 y-1) and a dry, exposed disused aquaculture pond (30.6 ±â€¯1.9 Mg CO2e ha-1 y-1). Emissions were negligible at low productivity rehabilitating sites with high hydroperiod (mean 1.0 ±â€¯0.1 Mg CO2e ha-1 y-1) and an impounded, operational aquaculture pond (1.1 ±â€¯0.2 Mg CO2e ha-1 y-1). Heterogeneity in biophysical conditions and geomorphic position exerted a strong influence on GHG flux, with the longer hydroperiod and higher soil moisture content of seaward fringing mangroves correlated with decreased fluxes. A greater abundance of Mud lobster mounds and root structures in landward mangroves correlated to higher flux. When viewed across a landuse change continuum, our results suggest that the initial conversion of mangroves to aquaculture ponds releases extremely high rates of GHGs. Furthermore, the re-institution of hydrological regimes in dry, disused aquaculture ponds to facilitate tidal flushing is instrumental in rapidly mediating GHG flux, leading to a significant reduction in baseline emissions. This is an important consideration for forest carbon project proponents seeking to maximise creditable GHG emissions reductions and removals.