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1. Human population density and blue carbon stocks in mangroves soils

   https://doi.org/10.1088/1748-9326/ad13b6  

   Chien, Shih-Chieh; Knoble, Charles; Krumins, Jennifer Adams (February 2024, Environmental Research Letters)

   Abstract Mangrove soils provide many important ecosystem services such as carbon sequestration, yet they are vulnerable to the negative impacts brought on by anthropogenic activities. Research in recent decades has shown a progressive loss of blue carbon in mangrove forests as they are converted to aquaculture, agriculture, and urban development. We seek to study the relationship between human population density and soil carbon stocks in urban mangrove forests to quantify their role in the global carbon budget. To this end, we conducted a global analysis, collecting mangrove soil carbon data from previous studies and calculating population density for each study location utilizing a recent database from the European Commission. Results indicate population density has a negative association with mangrove soil carbon stocks. When human population density reaches 300 people km⁻², which is defined as ‘urban domains’ in the European Commission database, mangrove soil carbon is estimated to be lower than isolated mangrove forests by 37%. Nonetheless, after accounting for climatic factors in the model, we see the negative relationship between population density and soil carbon is reduced and is even non-significant in mixed effects models. This suggests population density is not a good measure for the direct effects of humans on mangrove ecosystems and further implies mangrove ecosystems in close proximity to very high population density can still possess valuable carbon stocks. Our work provides a better understanding of how soil carbon stocks in existing mangrove forests correlate with different levels of population density, underscores the importance of protecting existing mangroves and especially those in areas with high human population density, and calls for further studies on the association between human activities and mangrove forest carbon stocks. 

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2. Overestimation of Mangroves Deterioration From Sea Level Rise in Tropical Deltas

   https://doi.org/10.1029/2024GL109675  

   Dai, Zhijun; Long, Chuqi; Mei, Xuefei; Fagherazzi, Sergio; Xiong, Yuan (October 2024, Geophysical Research Letters)

   Abstract Mangrove forests are critical coastal ecosystems that provide great socio‐ecological services, which are also highly vulnerable to climate change, particularly to sea level rise (SLR). Here we assess changes in mangrove forests in four distinct river/tide/wave‐dominant large deltas along the Indo‐Pacific coast based on 1,336 remote sensing images by machine learning techniques. We find that mangroves are migrating seaward at a rate of 18% ± 12% m/yr, which can offset landward mangroves loss, 67% of which caused by land use conversion. The fact that mangroves are expanding seaward with accretion rates exceeding SLR suggests that climate change has not yet triggered substantial loss in deltaic mangrove forests. Assuming that present environmental conditions do not change and that sediment and organic deposition in the deltaic topsets match SLR rates, we project that 90% of deltaic mangrove forests may start to retreat after 132–194 years. Early inundation of mangroves will occur in wave‐dominated delta. 

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3. Climate drives coupled regime shifts across subtropical estuarine ecosystems

   https://doi.org/10.1073/pnas.2121654119  

   Hesterberg, Stephen G.; Jackson, Kendal; Bell, Susan S. (August 2022, Proceedings of the National Academy of Sciences)

   Ecological regime shifts are expected to increase this century as climate change propagates cascading effects across ecosystems with coupled elements. Here, we demonstrate that the climate-driven salt marsh–to–mangrove transition does not occur in isolation but is linked to lesser-known oyster reef–to–mangrove regime shifts through the provision of mangrove propagules. Using aerial imagery spanning 82 y, we found that 83% of oyster reefs without any initial mangrove cover fully converted to mangrove islands and that mean (± SD) time to conversion was 29.1 ± 9.6 y. In situ assessments of mangrove islands suggest substantial changes in ecosystem structure during conversion, while radiocarbon dates of underlying reef formation indicate that such transitions are abrupt relative to centuries-old reefs. Rapid transition occurred following release from freezes below the red mangrove ( Rhizophora mangle ) physiological tolerance limit (−7.3 °C) and after adjacent marsh-to-mangrove conversion. Additional nonclimate-mediated drivers of ecosystem change were also identified, including oyster reef exposure to wind-driven waves. Coupling of regime shifts arises from the growing supply of mangrove propagules from preceding and adjacent marsh-to-mangrove conversion. Climate projections near the mangrove range limit on the Gulf coast of Florida suggest that regime shifts will begin to transform subtropical estuaries by 2070 if propagule supply keeps pace with predicted warming. Although it will become increasingly difficult to maintain extant oyster habitat with tropicalization, restoring oyster reefs in high-exposure settings or active removal of mangrove seedlings could slow the coupled impacts of climate change shown here. 

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4. Uncovering mangrove range limits using very high resolution satellite imagery to detect fine‐scale mangrove and saltmarsh habitats in dynamic coastal ecotones

   https://doi.org/10.1002/rse2.394  

   Doughty, Cheryl L; Cavanaugh, Kyle C; Chapman, Samantha; Fatoyinbo, Lola (December 2024, Remote Sensing in Ecology and Conservation)

   Abstract Mangroves are important ecosystems for coastal biodiversity, resilience and carbon dynamics that are being threatened globally by human pressures and the impacts of climate change. Yet, at several geographic range limits in tropical–temperate transition zones, mangrove ecosystems are expanding poleward in response to changing macroclimatic drivers. Mangroves near range limits often grow to smaller statures and form dynamic, patchy distributions with other coastal habitats, which are difficult to map using moderate‐resolution (30‐m) satellite imagery. As a result, many of these mangrove areas are missing in global distribution maps. To better map small, scrub mangroves, we tested Landsat (30‐m) and Sentinel (10‐m) against very high resolution (VHR) Planet (3‐m) and WorldView (1.8‐m) imagery and assessed the accuracy of machine learning classification approaches in discerning current (2022) mangrove and saltmarsh from other coastal habitats in a rapidly changing ecotone along the east coast of Florida, USA. Our aim is to (1) quantify the mappable differences in landscape composition and complexity, class dominance and spatial properties of mangrove and saltmarsh patches due to image resolution; and (2) to resolve mapping uncertainties in the region. We found that the ability of Landsat to map mangrove distributions at the leading range edge was hampered by the size and extent of mangrove stands being too small for detection (50% accuracy). WorldView was the most successful in discerning mangroves from other wetland habitats (84% accuracy), closely followed by Planet (82%) and Sentinel (81%). With WorldView, we detected 800 ha of mangroves within the Florida range‐limit study area, 35% more mangroves than were detected with Planet, 114% more than Sentinel and 537% more than Landsat. Higher‐resolution imagery helped reveal additional variability in landscape metrics quantifying diversity, spatial configuration and connectedness among mangrove and saltmarsh habitats at the landscape, class and patch scales. Overall, VHR satellite imagery improved our ability to map mangroves at range limits and can help supplement moderate‐resolution global distributions and outdated regional maps. 

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5. Resilience to Hurricanes Is High in Mangrove Blue Carbon Forests

   https://doi.org/10.1111/gcb.70124  

   Reed, David; Chavez, Selena; Castañeda‐Moya, Edward; Oberbauer, Steven_F; Troxler, Tiffany; Malone, Sparkle (March 2025, Global Change Biology)

   ABSTRACT Mangrove forests are typically considered resilient to natural disturbances, likely caused by the evolutionary adaptation of species‐specific traits. These ecosystems play a vital role in the global carbon cycle and are responsible for an outsized contribution to carbon burial and enhanced sedimentation rates. Using eddy covariance data from two coastal mangrove forests in the Florida Coastal Everglades, we evaluated the impact hurricanes have on mangrove forest structure and function by measuring recovery to pre‐disturbance conditions following Hurricane Wilma in 2005 and Hurricane Irma in 2017. We determined the “recovery debt,” the deficit in ecosystem structure and function following a disturbance, using the leaf area index (LAI) and the net ecosystem exchange (NEE) of carbon dioxide (CO₂). Calculated as the cumulative deviation from pre‐disturbance conditions, the recovery debt incorporated the recapture of all the carbon lost due to the disturbance. In Everglades mangrove forests, LAI returned to pre‐disturbance levels within a year, and ecosystem respiration and maximum photosynthetic rates took much longer, resulting in an initial recovery debt of 178 g C m⁻²at the tall forest with limited impacts at the scrub forest. At the landscape scale, the initial recovery debt was 0.40 Mt C, and in most coastal mangrove forests, all lost carbon was recovered within just 4 years. While high‐intensity storms could have prolonged impacts on the structure of subtropical forests, fast canopy recovery suggests these ecosystems will remain strong carbon sinks. 

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