Hamish McGill, Junior Projects Officer Intern  

Background: Hamish is a Junior Projects Officer Intern at the Plan Vivo Foundation. He has a particular interest in mangrove restoration and recently graduated from Edinburgh Napier University with a First Class Honours Degree in Animal and Conservation Biology. In this blog he reflects upon the research he undertook in Kenya as part of his undergraduate thesis. 

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With the UN declaring 2021-2030 as the 'Decade on Ecosystem Restoration' mangrove restoration has gained prominence as a nature-based solution for mitigating climate change. Mangrove forests are effective carbon sinks that can store up to five times more carbon hectare than terrestrial forests and are vital hotspots of terrestrial and marine biodiversity. Their tangled roots not only function as nursery habitat for fish, and thus support subsistence fishers, but also help to protect coastlines and coastal communities against flooding and erosion. Additionally, in terms of benefitting coastal communities, mangroves are a key source of food, firewood, and timber. Despite this, estimates suggest that we have lost between 20- 35% of global mangrove cover in the last 50 years, primarily due to human activities. This rate of loss has slowed in recent decades; however, degradation still threatens many of the world’s remaining mangrove forests and their wealth of ecosystem services. Although mangrove conservation and restoration efforts are increasing, many projects of this nature often fail. But why is this happening? And is there a way to reverse this trajectory? 

Historically, efforts to restore mangrove forests have usually involved mass planting alone. However, this approach is plagued with high rates of seedling mortality. One large-scale study estimates a 51% seedling survival rate. The main reasons: planting the wrong species in the wrong places and planting in sites which mangroves cannot tolerate. Amid widespread planting failures mangrove experts have advocated against mass planting as a first choice. Planting is often considered unnecessary when mangroves can recolonise an area, as natural regeneration can yield more resilient, functional, and biodiverse mangrove forests. However, degraded mangrove sites often can’t recover naturally, as the essential flow of sediment and freshwater which these ecosystems require has been disrupted. In these cases, interventions are required to restore an area’s altered hydrology, and thus promote tree growth and natural recovery. This approach, called ecological mangrove restoration, has been shown to work at restoring degraded habitats such as abandoned aquaculture ponds.

Ecological mangrove restoration with Vanga Blue Forest

One such example of ecological mangrove restoration is being undertaken by the Plan Vivo certified Vanga Blue Forest project based in southern Kenya. 30 years ago, nine hectares of mangrove forest were cleared and converted to salt pans near Vanga village. After the salt pans became unproductive the site was abandoned and has since experienced little to no natural regeneration. This situation is not uncommon because the dikes which prevent free tidal flow are often left intact when salt pans are abandoned. Not only can this prevent seedlings washing in from nearby forests and recolonising the site, but the soil conditions can become too harsh to support mangrove growth and survival. As such, the chances of mangrove planting succeeding in these types of sites is unlikely. That is why Vanga Blue Forest, coordinated by the Association for Coastal Ecosystem Services, have instead opte­d to strategically breach the dikes around the abandoned salt pans to restore tidal flow in the area.

 

Vanga Blue Forest mangrove restoration site. Credit: Hamish McGill

 

Establishing baseline conditions and ensuring successful project interventions

Natural recovery doesn't just happen overnight. It is important for an ecological restoration project to first establish the baseline conditions at a project site, after which the success of the restoration activities can then be measured. It was for this purpose that I conducted a 5-week field trip to Vanga to collect research for my undergraduate dissertation. During my time in Vanga I was able to document the extent and effects of degradation at the abandoned salt pans, highlighting three crucial factors. Firstly, mangrove stands were largely stunted and in poor health. Secondly, soil quality was severely reduced, with predominantly sandy, nutrient poor soils in which salinity and temperature levels exceeded the tolerance levels of all but the hardiest mangrove species. Thirdly, the density of mud crabs - whose feeding and burrowing activities significantly impact mangrove ecosystem functioning (from nutrient cycling to enhancing soil drainage and aeration) - was drastically diminished.

Following the interventions that Vanga Blue Forest have undertaken, we can expect the baseline conditions to change in the following ways. With free tidal flow comes the restoration of normal salinity levels, surface temperatures, and the increased availability of sediment and nutrients transported in the watershed. This will improve the growth of mangroves in the area which had been stunted by the previously harsh environmental conditions. This can increase the amount of leaf litter, which in turn may enable the mangrove forest to support a larger population of mud crabs. In short, we predict ecological restoration will restore the capacity of soil at the site to support both mangroves and their abundant fauna.

 

A mature mangrove forest directly adjacent to the Vanga Blue Forest restoration site. Credit: Hamish McGill 

 

Before attempting restoration, it is important to find out why former mangrove areas are showing little sign of natural recovery. Removing potential sources of stress, such as dikes, can correct the hydrological conditions which have prevented secondary succession from taking place. That said, mangrove planting cannot be entirely written off as a viable restoration strategy. If natural recovery is still lacking once the right conditions have been restored, planting can be considered to help supplement the shortfall in seedling recruitment.

By fostering the natural regeneration of mangrove forests through ecological restoration approaches, mangrove restoration projects can avoid falling into the common pitfalls of mass planting. This is a win-win investment that can deliver the climate, biodiversity, and livelihood benefits that are central to the Plan Vivo approach.

 

Hamish’s dissertation entitled “Understanding the Determinants of Natural Mangrove Regeneration in a Degraded Site in Kenya” is available to read here.