Jose Alan A. Castillo

Mangroves’ ability to store carbon (C) has long been recognized, but little is known about whether planted mangroves can store C as efficiently as naturally established (i.e., intact) stands and in which time frame. Through Bayesian logistic models compiled from 40 years of data and built from 684 planted mangrove stands worldwide, we found that biomass C stock culminated at 71 to 73% to that of intact stands ~20 years after planting. Furthermore, prioritizing mixed-species planting including Rhizophora spp. would maximize C accumulation within the biomass compared to monospecific planting. Despite a 25% increase in the first 5 years following planting, no notable change was observed in the soil C stocks thereafter, which remains at a constant value of 75% to that of intact soil C stock, suggesting that planting effectively prevents further C losses due to land use change. These results have strong implications for mangrove restoration planning and serve as a baseline for future C buildup assessments.

Over the past 30 years, 30–40% of the world’s mangroves have been lost and continue to be lost to deforestation. In recent years, mangrove rehabilitation projects have increased, replacing once natural and diverse mangrove forests with monotypic stands (e.g., Rhizophora mucronata). However, the vulnerability to sea level rise of these artificial stands and their effectiveness at providing ecosystem services is unclear. To address these gaps, we compared the rate of sediment surface elevation change and contemporary soil C sequestration across three differently aged mangrove plantations (5, 10, 25 years) in relation to a natural mangrove forest in Quezon province, Philippines using rod surface elevation tables (rSETs) and soil C analyses. Over a 3-year period, we found that the elevation of the 5-year-old plantation had the highest rate (6.12 mm year⁻¹), followed by 10-year-old (1.70 mm year⁻¹) and 25-year-old (0.16 mm year⁻¹) plantations, while the natural stand had a decreasing rate (−1.9 mm year⁻¹). In addition, the mean soil carbon contemporary sequestration rate was 226, 123, and 8.9 g C m⁻² year⁻¹ for the 5-, 10-, and 25-year-old plantations. Old mangroves have bigger carbon stocks, whereas young mangrove plantations are more resilient to SLR and have higher contemporary carbon sequestration rates. Except for biomass and basal area, we found no correlation of the sediment elevation change and soil C sequestration rates with other stand variables. Apart from the 5-year-old plantations, the rates of sediment elevation change obtained are less than the global (3.2 mm year⁻¹) and local (2–4 mm year⁻¹) rates of sea level rise (SLR). These suggest that most of the forested coastal wetlands studied are not keeping pace with SLR and may potentially drown if there is no available land behind them to retreat. This also indicates that pro-active solutions such as strict protection must be made.

Although vegetation phenology thresholds have been developed for a wide range of mapping applications, their use for assessing the distribution of natural bamboo and the related carbon stocks is still limited, especially in Southeast Asia. Here, we used Google Earth Engine (GEE) to collect time-series of Landsat 8 Operational Land Imager (OLI) and Sentinel-2 images and employed a phenology-based threshold classification method (PBTC) to map the natural bamboo distribution and estimate carbon stocks in Siem Reap Province, Cambodia. We processed 337 collections of Landsat 8 OLI for phenological assessment and generated 121 phenological profiles of the average vegetation index for three vegetation land cover categories from 2015 to 2018. After determining the minimum and maximum threshold values for bamboo during the leaf-shedding phenology stage, the PBTC method was applied to produce a seasonal composite enhanced vegetation index (EVI) for Landsat collections and assess the bamboo distributions in 2015 and 2018. Bamboo distributions in 2019 were then mapped by applying the EVI phenological threshold values for 10 m resolution Sentinel-2 satellite imagery by accessing 442 tiles. The overall Landsat 8 OLI bamboo maps for 2015 and 2018 had user’s accuracies (UAs) of 86.6% and 87.9% and producer’s accuracies (PAs) of 95.7% and 97.8%, respectively, and a UA of 86.5% and PA of 91.7% were obtained from Sentinel-2 imagery for 2019. Accordingly, carbon stocks of natural bamboo by district in Siem Reap at the province level were estimated. Emission reductions from the protection of natural bamboo can be used to offset 6% of the carbon emissions from tourists who visit this tourism-destination province. It is concluded that a combination of GEE and PBTC and the increasing availability of remote sensing data make it possible to map the natural distribution of bamboo and carbon stocks.