Restoration and ecosystem functions in Kenyan mangroves

Mangrove forests are found in tropical and sub-tropical intertidal zones of sheltered tropical shores between the latitudes of 32^o N and 38^o S, with a global cover of around 180,000-200,000 km2. Mangrove trees have special adaptations to survive in saline environment (salt secreting leaves, salt-excluding roots, arial roots, distributed aerenchima and viviparous seeds). They provide economic services and products to the coastal populations of tropical regions, i.e, chemicals (e.g., tannins), construction, firewood and charcoal. They also are important fishing areas for local communities.

Mangroves also provide ecological services, such as protection against floods and severe storms, e.g., Tsunamis (Dahdouh-Guebas et al., 2005), control of shoreline and riverbank erosion, water purification, maintenance of biodiversity, contributions to nearshore productivity and genetic resources, sinks for carbon dioxide as well as feeding and protection grounds for numerous fish and crustaceans.

Mangrove ecosystems are increasingly threatened by human activities, with annual global rates of loss of around 2% or higher (Primavera, 2005). This raises concerns since mangroves are believed to provide a number of ecosystem services. Projections of sea level rise caused by climate change suggest that, in order to adjust, mangroves need to generate surface elevation at a pace matching the increase in the tidal level (Furukawa et al., 1997).

In addition to global change, mangroves are threatened by overexploitation to produce firewood for local consumption, conversion to shrimp and prawn farming as well as salt production.

This project, carried out at Gazi Bay (Southern Kenya, Location 4⁰ 25′S and 39⁰ 50′E) aims, in conjunction with the local communities, to a) replant mangroves in degraded areas due to overexploitation and b) monitor the return of ecosystem functions as well as c) avail the trees to the communities in future.

The specific objectives are to 1) test how the diversity of mangrove species in replanted stands affects a range of ecosystem functions; 2) use controlled experiments to test the effects of replanted mangroves on sediment dynamics, and 3) explore the potential for replanted mangrove stands to act as carbon sinks.

The study approach and the methodology is given in Figure 1 below.

Experimental design to test for the effect of mangrove species richness on recovery of ecosystem functions. The plots have been established at the border between the intact mangrove forest and the inland agro-forestry communities in areas previously occupied by mangroves and then deforested. Each shade of grey represents a different combination of species A, B, C, A+B, A+C or A+B+C. The experimental plantings have been carried out over two separate blocks with different tidal timinngs.

Crab exclosure plots were also established since crabs are known to be ‘weeders’ of the mangrove litter fall. However, they also affect tree regeneration through seed and propagule predation. Two crab exclosure plots were established using plastic 5mm mesh fencing, 70cm high and 70cm below surface). Crabs were cleared before planting and regularly cleared after planting (Figure 2).

Experimental crab exlusion plots

The study was very much inspired by the kenyan 2004 Nobel Peace Prize winner, Prof. Wangari Maathai (Fig.3), from whom we take this famous quote: "When you start working with the environment seriously, the whole arena comes: human rights, women's rights, environmental rights, children's rights, you know, everybody's rights. Once you start making these linkages, you can no longer do just tree-planting.”