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

Abstract

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.

Global declines in human‐driven mangrove loss

“…Understanding the causes of mangrove loss is important for establishing opportunities for blue carbon projects (Macreadie et al., 2019). In particular, quantifying the reasons for mangrove loss is critical towards estimation of carbon emissions (López‐Angarita, Tilley, Hawkins, Pedraza, & Roberts, 2018; Sasmito et al., 2019), and the opportunities for enhancing blue carbon through management (López‐Angarita et al., 2018). Using two‐high resolution datasets on mangrove extent (Giri et al., 2011) and global deforestation (Hansen et al., 2013), mangrove loss in Southeast Asia was primarily associated with anthropogenic land conversion to agriculture and aquaculture (Richards & Friess, 2016).…”

Mangrove blue carbon stocks and dynamics are controlled by hydrogeomorphic settings and land‐use change

“…In response, the number of mangrove blue carbon assessments has increased rapidly over the past decade (Adame et al, 2013; Donato et al, 2011; Kauffman, Heider, Norfolk, & Payton, 2014; Nam, Sasmito, Murdiyarso, Purbopuspito, & MacKenzie, 2016; Stringer, Trettin, Zarnoch, & Tang, 2015; among many others). However, the majority of mangrove carbon studies have been conducted in natural or relatively undisturbed systems, making it difficult to generate estimates of carbon stock loss or recovery as a consequence of land‐use change and restoration efforts (Sasmito, Taillardat, et al, 2019). Estimates of carbon stock loss are further complicated by the fact that biomass and soil carbon vary substantially across climatic gradients (Simard et al, 2019) and geomorphological settings (Rovai et al, 2018; Twilley, Rovai, & Riul, 2018).…”

Future carbon emissions from global mangrove forest loss

“…The emission factor is the fraction of carbon that is emitted given conversion to a specific land-use. We selected an emission factor for each province and activity from a recent global systematic review (Sasmito et al, 2019). Each emission factor was given a level of confidence (Table S2) from low to high, with Level 1 (lowest confidence) given to emission factors obtained from a global average, specific to that proximate driver; Level 2 to those obtained from a similar region, specific to that proximate driver; and Level 3 (highest confidence), from a similar region with the same geomorphic setting, specific to that proximate driver (Dürr et al, 2011).…”

“…To overcome current limitations in global estimations, we developed a spatial model that projects emissions caused by mangrove loss. Our model synthesised information from multiple newly available global datasets, including carbon stocks (Kauffman et al, 2020;Sanderman et al, 2018;Simard et al, 2019), mangrove distribution (Bunting et al, 2018), deforestation rates (Hamilton & Casey, 2016), drivers of land-use change (Goldberg et al, 2020) and emissions factors (Sasmito et al, 2019). We provide predictions of future CO2 emissions from mangrove loss, accounting for the effect of proximate drivers of land-use change including: a) conversion to commodities, such as agriculture or aquaculture, b) coastal erosion, c) clearing, d) extreme climatic events, and e) conversion to human settlements (Goldberg et al, 2020).…”