The study assessed conditions of mangrove forests co-managed with the community versus those solely managed by the state agency. The assessed forest attributes included forest structure in terms of stand density, basal area, biomass and natural regeneration.
The stocking rate was significantly higher in community managed central block compared to the state managed western and eastern blocks (Table 5). Central forest is particularly stocked with pau-boriti sized poles (diameter range: 2.5-13.0 cm) that are nearly depleted in the state managed stands. Pau – boriti are the most merchantable pole class and often harvested for building and construction industry (Bosire et al. 2012). Enhanced community scout patrols and surveillance has led to improved management of mangrove in the area as witnessed by higher complexity indices in the central block compared to other blocks (Table 5).
The government signed a forest management agreement with Gazi community to manage the mangroves in the central block of the Bay in 2013. The agreement defines dos and don’ts inside the co-managed forest area. For instance, harvesting of poles inside the co-managed area is strictly prohibited. However, non-consumptive utilization of the forest products and services such as honey collection, fishing and ecotourism activities are encouraged (GoK 2017). In the case of Gazi, the community utilizes the co-managed area for a carbon-offsetting scheme – Mikoko Pamoja. Sales of carbon credits from the community forest have served as a strong incentive of promoting mangrove rehabilitation, surveillance and monitoring. The outcome of this has been the enhanced stocking density, natural regeneration, biomass, forest quality and complexity index observed in the central block compared to state management blocks (Table 5). A similar study in Philippines observed an enhanced forest structure and regeneration in mangrove forests co-managed with the community (Walters 2005).
Prolonged logging of the pau-boriti sized poles (dbh: 2.5 – 13.0 cm) in state managed western and eastern blocks of Gazi mangroves has led to near depletion of the desired product from the forests. Similar conclusions were made in Mida creek (Kairo et al. 2002a) and Kiunga, Lamu (Kairo et al. 2002b) in Kenya where selective logging converted the forest from superior stands with desired products to inferior forest devoid of pau, mazio and boriti sized poles. In Malaysia, India and Indonesia where commercial logging of mangrove forests has been practiced over many decades, prolonged logging has contributed to loss of forest productivity (Wilkie 2007).
FAO has proposed juvenile density of 2500 mangrove saplings ha-1 randomly distributed as adequate to support natural regeneration of the forest (FAO 1994; Kairo et al. 2002a). Based on FAO’s recommendation, the three forest blocks in Gazi had adequate natural regeneration. The density of established regeneration (RCII and RCIII) of 12,313 juveniles ha-1 in co-managed block was high as compared to values of 8,377 and 6,606 juveniles ha-1 recorded in state managed eastern and western blocks respectively. A high concentration of established juveniles in co-managed forest signifies higher transition rate from juveniles to mature trees in co-managed forests compared to state managed forest. When mangrove forests are protected from overexploitation and degradation, the substrate become stable allowing saplings and propagules to establish making natural regeneration possible (Kairo et al. 2001). In co-managed block of Gazi, a high regeneration can be attributed to silvicultural activities like replanting and pruning and improved surveillance provided by community user group in collaboration with KFS (Huff and Tonui 2017). The observed ratio of RCI: RCII: RCIII of 2:2:1, 1:2:1 and 8:4:1 for central, eastern and western block respectively is within the sustainable stocking ratio of 6:3:1 proposed by FAO.
Straightness of trees is a relative measure of a quality forest. Class 1 and 2 poles are highly preferred in the market than the crooked class 3 poles. Significantly higher densities of class 1 poles in co-managed forests (1,274 trees ha-1) than in state managed western (265 trees ha-1) and eastern (397 stems ha-1) blocks is an indication of effective protection against illegal logging provided by the community. Carbon financing has enabled the Gazi community to hire scouts for surveillance of the forest against intruders. Improved forest conditions emanating from community conservation efforts have been reported also in Bolivia (Wright et al. 2016), Philippines (Camacho et al. 2011), and Tanzania (Blomley et al. 2008). In Bolivia for instance, communities are involved in monitoring and enforcement of forest rules, guided by a formal agreement between the state and local community (Wright et al. 2016).
Above ground biomass
The biomass of tree with butt diameter ≤ 9.0cm was significantly higher (p < 0.05) in co-managed central block (biomass 3.8 t ha-1) than in the Eastern (1.4 t ha-1) and Western (1.3 t ha-1) blocks. In the case of central block, the community has used carbon financing to promote recovery of the forest, as indicated by high regeneration densities, and biomass increment. The community group enjoys autonomy in developing bylaws and decision making relating to their silvicultural activities and allocation of income to social improvement projects of their choice in the village (Huff and Tonui 2017). According Hayes and Persha (2010), ownership rights, autonomy in decision making relating to the forest management and benefits from forest income motivates the community in controlling forest extraction allowing the forest to grow and accumulate carbon.
The old growth trees that existed before the signing of forest management agreement in Gazi in 2013, influenced the overall stand biomass in central (170.3 t ha-1) as well as in eastern (190.7 t ha-1) and western (130.8 t ha-1) blocks (Table 4). Large trees with butt diameter > 20cm are usually not preferred by the mangrove markets in Kenya and are thus avoided by cutters. Biomass values reported in Gazi compares well with the above ground mangrove biomass reported in Kiunga Marine Park, Kenya (199.6 t ha-1) (Cohen et al. 2013). However, the values are significantly higher (p<0.05) than the standing biomass of mangroves in North Sulawesi, Indonesia (61.4 t ha-1), and Sarawak, Malaysia (116.8 t ha-1) (Murdiyarso et al. 2009; Chandra et al. 2011). Old growth mangroves forests in Bahile, Philippines, and Cameroon recorded standing biomass of 561.2 t ha-1 and 504.5 t ha-1 respectively (Abino et al. 2014; Ajonina et al. 2014). Like in tropical rainforests, high biomass values are reported for tropical mangroves with high influence of fresh water (Kathiresan and Bingham 2001; Tomlinson 2016).
When compared with the 2013 baseline data, standing biomass in the co-managed block increased from 42.9 t ha-1 to 170.3 t ha-1 from 2013 to 2017; translating to annual biomass increment of 31.9 t ha-1 y-1. This biomass increase as a result of community interventions is what is converted to biomass carbon and traded as carbon credits. Annual sales from co-managed mangroves in Gazi has been estimated at 2,255 t CO2-eq. earning the community a direct income of about US$15,000/annum (Wylie et al. 2016). The community uses part of the carbon financing to restore degraded mangrove areas in the Bay; while the balance is used to support priority community projects in water and sanitation, education and health (Wylie et al. 2016). Incentivizing forest conservation activities is a triple win to climate regulations, biodiversity conservation, and community benefits (Huff and Tonui 2017).
The nature of future forests in Gazi
The current stocking rates of mangroves in Gazi can be used to predict the nature of future forests. Based on De Liocourt’s predictive model, un-even aged forest produces a reverse j curve, when the ratio between successive diameter classes, q ratio, is constant. A constant q only occurs in a balanced forest stand, with normal diameter distributions capable of producing sustainable yields (Meyer 1952; Govedar et al. 2018). The q ratios obtained in the current study were not constant in all the three forest blocks. The exponential curves failed to precisely fit into the actual data obtained for dbh classes 6.1 – 9.0 cm (mazio) and 9.1 – 13.0 cm (boriti) in central and eastern blocks (Fig. 5). The depletion of stock densities in these dbh classes can be attributed to unsustainable logging of those sizes whose demand is high in local markets (Dahdouh-Guebas et al. 2000). However, the high density of trees with girth of ≤ 6 cm in co-managed block is an indication of the conservation efforts by community groups since establishment of co-management regime. If current participatory activities continue, the normal stem density shown by the exponential curve would be achieved since the small trees (dbh < 6 cm) will grow to fill up the deficits in 6.1 – 9.0 cm (mazio) and 9.1 – 13.0 cm (boriti) classes thus stabilizing the forest. Using harvesting scheme to control extraction would prevent excessive removal of mazio sized poles and eventually allow restoration of depleted diameter classes in state managed blocks. Bundotich et al. (2009) in the mangroves forest of Ngomeni and Mohamed et al. (2009) in Tudor creek mangroves made similar observations where some diameter classes had lower tree densities than normal. This was attributed to selective cutting and an extraction regime that lacked a harvesting plan, resulting in over exploitation of specific diameter classes. The near perfect stock densities observed in western block can be attributed to the presence of a large number of A. marina species, a species that is less preferred in local market and community for building and firewood.