Monitoring the Sediment Surface Elevation Change across a Chronosequence of Restored Stands of Tropical Mangroves and Their Contemporary Carbon Sequestration in Soil Pool

Abstract

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, replac-ing 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 provid-ing 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 yr−1), followed by 10‐year-old (1.70 mm yr−1) and 25‐year‐old (0.16 mm yr−1) plantations, while the natural stand had a decreas-ing rate (−1.9 mm yr−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.