Inter-related and spatially variable climate change factors including sea level rise, increased storminess, altered precipitation regime and increasing temperature are impacting mangroves at regional scales. This review highlights extreme regional variation in climate change threats and impacts, and how these factors impact the structure of mangrove communities, their biodiversity and geomorphological setting. All these factors interplay to determine spatially variable resiliency to climate change impacts, and because mangroves are varied in type and geographical location, these systems are good models for understanding such interactions at different scales. Sea level rise is likely to influence mangroves in all regions although local impacts are likely to be more varied. Changes in the frequency and intensity of storminess are likely to have a greater impact on N and Central America, Asia, Australia, and East Africa than West Africa and S. America. This review also highlights the numerous geographical knowledge gaps of climate change impacts, with some regions particularly understudied (e.g., Africa and the Middle East). While there has been a recent drive to address these knowledge gaps especially in South America and Asia, further research is required to allow researchers to tease apart the processes that influence both vulnerability and resilience to climate change. A more globally representative view of mangroves would allow us to better understand the importance of mangrove type and landscape setting in determining system resiliency to future climate change.

/pdf/impacts-of-climate-change-on-mangrove-ecosystems-a-region-by-11ywehwtkf.pdf

Impacts of climate change on mangrove ecosystems: a region by region overview

With the growing recognition that effective action on climate change will require a combination of emissions reductions and carbon sequestration, protecting, enhancing and restoring natural carbon sinks have become political priorities. Mangrove forests are considered some of the most carbon-dense ecosystems in the world with most of the carbon stored in the soil. In order for mangrove forests to be included in climate mitigation efforts, knowledge of the spatial distribution of mangrove soil carbon stocks are critical. Current global estimates do not capture enough of the finer scale variability that would be required to inform local decisions on siting protection and restoration projects. To close this knowledge gap, we have compiled a large georeferenced database of mangrove soil carbon measurements and developed a novel machine-learning based statistical model of the distribution of carbon density using spatially comprehensive data at a 30 m resolution. This model, which included a prior estimate of soil carbon from the global SoilGrids 250 m model, was able to capture 63% of the vertical and horizontal variability in soil organic carbon density (RMSE of 10.9 kg m−3). Of the local variables, total suspended sediment load and Landsat imagery were the most important variable explaining soil carbon density. Projecting this model across the global mangrove forest distribution for the year 2000 yielded an estimate of 6.4 Pg C for the top meter of soil with an 86–729 Mg C ha−1 range across all pixels. By utilizing remotely-sensed mangrove forest cover change data, loss of soil carbon due to mangrove habitat loss between 2000 and 2015 was 30–122 Tg C with >75% of this loss attributable to Indonesia, Malaysia and Myanmar. The resulting map products from this work are intended to serve nations seeking to include mangrove habitats in payment-for- ecosystem services projects and in designing effective mangrove conservation strategies.

/pdf/a-global-map-of-mangrove-forest-soil-carbon-at-30-m-spatial-5bipjh54eo.pdf

A global map of mangrove forest soil carbon at 30 m spatial resolution

Mangrove organic carbon is primarily stored in soils, which contain more than two-thirds of total mangrove ecosystem carbon stocks. Despite increasing recognition of the critical role of mangrove ecosystems for climate change mitigation, there is limited understanding of soil organic carbon sequestration mechanisms in undisturbed low-latitude mangroves, specifically on organic carbon burial rates and sources. This study assessed soil organic carbon burial rates, sources and stocks across an undisturbed coastal mudflat and mangrove hydrogeomorphological catena (fringe mangrove and interior mangrove) in Bintuni Bay, West Papua Province, Indonesia. 210Pb radionuclide sediment dating, and mixing model of natural stable isotope signatures (δ 13C and δ15N) and C/N ratio were used to estimate organic carbon burial rates and to quantify proportions of allochthonous (i.e., upland terrestrial forest) and autochthonous (i.e., on-site mangrove forest) organic carbon in the top 50 cm of the soil. Burial rates were in the range of 0.21–1.19 Mg C ha−1 yr−1. Compared to the fringe mangroves, organic carbon burial rates in interior mangroves were almost twice as high. Primary productivity of C3 upland forest vegetation and mangroves induced soil organic carbon burial in interior mangroves and this was consistent with the formation of the largest organic carbon stocks (179 ± 82 Mg C ha−1). By contrast, organic carbon stored in the fringe mangrove (68 ± 11 Mg C ha−1) and mudflat (62 ± 10 Mg C ha−1) soils mainly originated from upland forests (allochthonous origin). These findings clearly indicate that carbon sequestered and cycling in mangrove and terrestrial forest ecosystems are closely linked, and at least a part of carbon losses (e.g., erosion) from terrestrial forests is buried in mangrove ecosystems.

Organic carbon burial and sources in soils of coastal mudflat and mangrove ecosystems

https://www.fs.fed.us/psw/publications/mackenzie/psw_2020_mackenzie002_sharma.pdf

The impacts of degradation, deforestation and restoration on mangrove ecosystem carbon stocks across Cambodia

Carbon storage in a restored mangrove forest in Can Gio Mangrove Forest Park, Mekong Delta, Vietnam

Increased sea level is the climate change effect expected to have the greatest impact on mangrove forest survival. Mangroves have survived extreme fluctuations in sea level in the past through sedimentation and belowground carbon (C) accumulation, yet it is unclear what factors may influence these two parameters. We measured sedimentation, vertical accretion, and belowground C accumulation rates in mangrove forests from the Republic of Palau and Vietnam to examine how diversity (high-Vietnam vs. low-Palau), land use, and location (fringe vs. interior) might influence these parameters. Land use in this study was identified as disturbance and restoration for all mangrove forests sampled in Palau and Vietnam, respectively. Vertical accretion rates were significantly greater in Vietnam (2.44 ± 1.38 cm/year) than Palau mangrove forests (0.47 ± 0.08 cm/year; p < 0.001, F1,17 = 24.96). Vertical accretion rates were positively correlated to diversity (R = 0.43, p < 0.05). However, stronger correlations of accretion to bulk density (R = 0.64, p < 0.01) and significantly higher bulk densities in Vietnamese (0.67 ± 0.04 g/cm3) than Palau mangroves (0.30 ± 0.03 g/cm3; p < 0.001, F1,17 = 54.4) suggests that suspended sediments played a greater role in mangrove forest floor maintenance relative to sea level rise. Average vertical accretion rates were similar between naturally colonized (1.01 ± 0.10 cm/year) and outplanted sites (1.06 ± 0.05 cm/year) and between fringe (1.06 ± 0.12 cm/year) and interior mangrove (0.99 ± 0.09 cm/year) in Vietnam. In Palau, vertical accretion rates did not differ between disturbed (0.42 ± 0.11 cm/year) and undisturbed (0.51 ± 0.13 cm/year) mangrove forests and were higher in fringe (0.61 ± 0.15 cm/year) than interior sites (0.33 ± 0.09 cm/year; p = 0.1, F1,7 = 3.45). Belowground C accumulation rates did not differ between any factors examined. C accumulation rates (69–602 gC/m2/year) were similar to those reported elsewhere in the literature and suggest that intact coastal ecosystems play an important role in the global C cycle, sequestering C at rates that are 10–20× greater than upland forests. Assuming vertical accretion rates measured using 210Pb are an effective proxy for surface elevation, the Vietnamese and Palauan mangroves appear to be keeping up with current rates of sea level rise.

https://www.fs.fed.us/psw/publications/mackenzie/psw_2016_mackenzie001.pdf

Kimberly Weckerly

Papers

Sedimentation and belowground carbon accumulation rates in mangrove forests that differ in diversity and land use: a tale of two mangroves