Changes in mangroves at their southernmost African distribution limit

Adams, Janine B.; Rajkaran, Anusha

doi:10.1016/j.ecss.2020.107158

Cited by 20 publications

(25 citation statements)

References 74 publications

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“…Comparing the projected GMSL with elevation changes within the mangroves, Vanga paints a picture of imminent threat, but the threats can be mitigated with informed interventions and adaptations. This threat is also reported for similar habitats, including the projected loss of ~50% of Mngazana, South Africa estuary mangroves with a 3.7 mm yr -1 SLR rate (Yang et al, 2014;Adams and Rajkaran, 2021), the reported loss of >15,000 ha of mangroves due to SLR in Sundarbans (Rahman et al, 2011;Bomer et al, 2020). A regional assessment in Australia, Singapore, Indonesia, Philippines, Malaysia, and Vietnam using a network of comparable SET elevation measurements indicated a ~6 mm yr -1 elevation deficit with respect to current and predicted SLR rates (Alongi, 2008;Woodroffe et al, 2016).…”

Section: Discussionsupporting

confidence: 53%

Surface elevation changes in an estuarine mangrove forest in Vanga, Kenya: Implications for management and mitigation of sea-level rise

et al. 2022

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Mangrove ecosystems are often called “makers of land” due to their ability to promote deposition, trap, and augment sediments. Accurate location- and region-specific elevation information is required to assess and mitigate threats to mangroves caused by their vulnerability to sea-level rise. The provision of land building services by mangroves is primarily sediment-dependent. It is therefore influenced by local factors, including sediment availability and supply. In the present study from Kenya, we measured and examined the variations in surface elevation in mangroves at variable distances from the creek channel using a combination of surface-elevation tables and horizon markers for three years. Elevation changes varied with distance from the creek channel (p < 0.05), with both surface loss and gains recorded. Elevation changes varied between -80 mm (most significant subsidence) and 42 mm (highest accretion) in stations closer to the creek, while farther from the creek (~200 m away), elevation changes ranged between -68 mm (most significant subsidence) and 29 mm (highest accretion). However, net surface elevation changes over the three years showed that shallow subsidence occurred in both stations closer to the creek (-45 ± 7.2 mm) and those farthest from the creek (-20 ± 7.1 mm). At the same time, an average of 18 mm of sediments were accreted above the horizon markers translating to ~9 mm yr-1 of accretion, a rate larger than both the current global rates of sea-level rise (~3.1 mm yr-1) and local measured rates of sea-level rise (3.8 mm yr-1) in Mombasa, a tide-gauge station nearest (~100 km) to the study site. Cumulatively, sediment elevation changes in Vanga indicate that they are outpacing the current rates of sea-level rise. However, they could be vulnerable to predicted and accelerated rates. It, therefore, calls for more holistic management and monitoring of the dynamics within the mangrove forests and adjacent terrestrial hinterlands.

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Section: Discussionsupporting

confidence: 53%

Surface elevation changes in an estuarine mangrove forest in Vanga, Kenya: Implications for management and mitigation of sea-level rise

et al. 2022

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“…Using models trained with historical drought indicators, we have demonstrated the relative importance of microclimate drivers in tropical (East Africa) and subtropical coastlines (southern Mozambique and SW Madagascar) to mangrove well‐being. In drier coastal reaches that span rainfall gradients, rainfall, and freshwater availability determine the abundance of coastal wetland plants (Vilas et al., 2019), while in subtropical coastlines, winter air temperatures strongly determine the abundance and limit of mangrove forests relative to salt marshes (Adams & Rajkaran, 2021).…”

Section: Discussionmentioning

confidence: 99%

Identifying global and local drivers of change in mangrove cover and the implications for management

Maina

Bosire

Kairo

et al. 2021

Global Ecol Biogeogr

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Background Climate change and human activities continue to drive a widespread decline in global mangrove coverage, undermining their capacity to provide ecosystem benefits. While global and local scale drivers of change on mangroves are widely acknowledged, the relative importance and the exposure of mangroves to climatic, geomorphological, and direct human threats vary spatially. Understanding the role and relative importance of the multiscale and multiple threats to mangroves and how these vary spatially is fundamental for formulating a spatially adaptive approach to their management and conservation. Aim Our study investigated the role of multiple threats on mangroves and the relative exposure. Location Western Indian Ocean (WIO) region. Time period Recent past (2002–2019 and future (2050–2060). Major taxa studied Mangrove. Methods Using satellite‐derived indicators of mangrove condition aggregated over 19 years (2002 to 2019) and 14 proxies of climate, human activity, and geomorphology, we applied machine learning methods to determine the role and relative importance of the change drivers. Using outputs from this deductive statistical process, we applied inductive methods to map mangrove exposure spatially. Results Model results highlight the importance of catchment erosion, human pressure, sea level, and macroclimate as the main drivers of the present‐day ecological condition of mangroves in the WIO. Normalized difference vegetation index (NDVI) was more sensitive to the identified drivers than the vegetation condition index (VCI), with the relative importance of variables varying across the two vegetation indicators. Main conclusions In anticipation of a stronger manifestation of climate change impacts, the resilience of mangroves in the WIO could be improved through adaptive management over time and space. Testing the efficacy of the essential biodiversity variables (EBV) is critical for understanding the mechanisms of ecosystem change and managing biodiversity change.

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“…and Bruguiera gymnorrhiza (L.) Lam., each of which occur in 27 estuaries (i.e. mostly co-occurring but some estuaries have records for only one of these species), while Adams & Rajkaran, 2021). The MaxEnt SDMs are therefore not species specific, but rather characterise environmental conditions of extant mangrove habitat in this region-although this generally is dominated by the pioneer species, A. marina.…”

Section: Mangrove Datamentioning

confidence: 99%

“…At the scale of individual estuaries, king tides and overtopping by wave events can transport propagules into zones within an estuary that would not be accessible during typical conditions. This is evident in the mangrove population structure within some estuaries that indicate a single successful recruitment event along a flood line (Adams & Rajkaran, 2021). Local estuarine geomorphic settings, such as intertidal gradients and surface elevation, can also influence successful propagule establishment (Oh et al, 2017).…”

Section: Predicting Mangrove Range Expansion At the Southern African ...mentioning

confidence: 99%

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Dispersal and coastal geomorphology limit potential for mangrove range expansion under climate change

et al. 2022

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1. Latitudinal range limits for mangroves on high-energy, wave-dominated coasts are controlled by geomorphological features and estuarine dynamics. Mangroves reach a southern global range limit along the South African coastline, but the distribution is patchy, with stands occurring in only 16% of the estuaries in the region. Yet, the persistence of forests planted >50 years ago beyond the natural distribution limit suggests that additional estuaries could support mangroves.Understanding regional drivers is necessary to inform global-scale estimates for how this important ecosystem is predicted to respond to climate change.2. Here, we combine species distribution modelling (MaxEnt), Lagrangian particle tracking using an eddy-and tide-resolving numerical ocean model, and connectivity matrices, to identify suitable mangrove habitats along the South African coastline at present, as well as under the IPCC RCP4.5 and RCP8.5 climate scenarios.3. Within the current South African distribution range (±900 km), eight more estuaries were identified to be suitable under contemporary conditions. When considering potential range extension (±110 km), an additional 14 suitable estuaries were identified. Connectivity matrices suggest limited long-distance dispersal, stranding mostly at or near the release location, and a decreased probability of connectivity towards the range limit. Under both future climate scenarios, 30% of estuaries currently supporting mangroves are predicted to become unsuitable, while an additional six estuaries beyond the current distribution are predicted to become suitable. However, there is limited connectivity between these new sites and established forests. 4.Synthesis. This study shows that dispersal substantially limits mangrove distribution at the southern African range limit and highlights the importance of including this process in species distribution models. Ultimately, our results provide

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Changes in mangroves at their southernmost African distribution limit

Cited by 20 publications

References 74 publications

Surface elevation changes in an estuarine mangrove forest in Vanga, Kenya: Implications for management and mitigation of sea-level rise

Surface elevation changes in an estuarine mangrove forest in Vanga, Kenya: Implications for management and mitigation of sea-level rise

Identifying global and local drivers of change in mangrove cover and the implications for management

Dispersal and coastal geomorphology limit potential for mangrove range expansion under climate change

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