Impacts of harvesting intensity on carbon allocation to species, size classes and pools in mangrove forests, and the relationships with stand structural attributes

Abstract

Mangroves are vital ecosystems that help mitigate climate change and natural hazards, despite representing only 0.5% of the world's coastlines. Recent studies have provided empirical evidence of the ongoing over-exploitation of mangrove forests in West Africa. Understanding the impact of such harvesting on mangroves’ carbon potential is essential to inform decision-making for management and carbon economy policies. This study investigated the impacts of harvesting intensity on (i) carbon allocation to species, size class, soil depth and pool (standing live trees, standing dead trees, litter and soil); and (ii) carbon stock in the different carbon pools, and its relationships with stand structural attributes, using the case study of Benin coastal line. Data were collected in 600 plots of 0.015 ha across 20 mangrove sites in high and low-harvesting-intensity areas. Sixty litter quadrats were also established to sample litter, while 160 soil samples were collected for soil carbon content analysis.Regardless of the harvesting intensity, two mangrove species, Rhizophora racemosa and Avicennia germinans, contributed more than 98% of the tree carbon. Small (1–10 cm) and medium (10–20 cm) size classes dominated tree carbon in high and low harvesting sites, respectively. Soil carbon up to 1 m depth had the greatest share (55%-70%) of the total carbon stock, followed by standing live trees (26%-40%) and litter, and was not influenced by harvesting intensity. Harvesting intensity influenced the carbon stocks in standing live and dead trees, with greater values in low and high harvesting sites. The total carbon stock was ∼ 1.46-fold higher in low harvesting sites (308.54 ± 32.74 MgC.ha−1) than in high harvesting sites (211.40 ± 14.91 MgC.ha−1). Mixed effect models showed that stand density and proportion of R. racemosa related positively with tree carbon stock, especially on low harvesting sites. Multiple factorial analyses revealed that carbon stocks in dead trees, litter and soil are high in low harvesting areas where stand densities are also high. However, in high harvesting sites, carbon stock in litter was positively related to relative density of A. germinans. This study expands our understanding of the carbon stock potential in a West African mangrove subject to different levels of disturbance and its contribution to mitigating greenhouse gas emissions. The implications for management were further discussed.