Edinburgh Napier University

  Mangrove forests are considered one of the most efficient natural carbon sinks and their preservation is thus important in climate change mitigation. However, they are declining at higher rates than terrestrial forests, due to human activities; with Kenyan mangroves being no of exception. One of the main drivers of mangrove decline in Kenya is over-exploitation for wood products. The present study aimed to assess (a) the effects of tree removal on the fluxes of greenhouse gases, surface elevation and other ecosystem functions of mangrove forests and (b) mangrove root production. To explore these objectives two experiments were established in the mangrove forests at Gazi bay, Kenya: (i) tree harvest and (ii) mangrove productivity studies. For the tree harvest experiment, ten 12 m x 12 m plots were established in March 2009 in a Rhizophora mucronata (Lam.) forest. Five plots were randomly selected and all trees within them were girdled in November 2009 and then cut in May 2010. Gas fluxes of CO2 and CH4 were sampled using the chamber technique at monthly intervals from June 2009 to April 2011. Surface elevation dynamics were observed using surface elevation stations (SES). Other variables measured included, macrofaunal abundance and diversity and natural regeneration patterns. For the root productivity experiment, twenty eight 10 m x 10 m plots were established in four mangrove forest types; with each type comprising of Avicennia marina (Forsk) Vierh., Ceriops tagal (Perr) C. B. Robinson, R. mucronata and Sonneratia alba (Sm) forests. Ten of the plots were established in A. marina and R. mucronata forests in Makongeni; while 18 plots comprising all the four species were established at Gazi; six plots each for A. marina and R. mucronata and three plots each for C. tagal and S. alba forests. Root production was estimated using the root in-growth technique (two in-growth trenches per plot), while the aboveground productivity was estimated from measurements of girth increment. Girth increment was measured using dendrometers installed on selected trees, one per plot, in combination with periodic girth measurements of 10 trees per plot. Environmental variables such as height above datum, salinity, grain size and redox potential were measured at the beginning of each experiment and during treatment periods for the tree harvest experiment. Treatment significantly elevated carbon emissions from the mangrove sediments by 14.2 ± 10.3 tCO2 ha-1 (rate of 9.8 ± 7.1 tCO2 ha-1 yr-1) within two years. Similarly, treatment significantly induced subsidence of -51.3 ± 24.3 mm (at a rate of -32.1 ± 8.4 mm yr-1) compared to 11.1±10.1 mm (at a rate of 4.2 ± 1.4 mm yr-1) in control plots in over 2 years after treatment. Decomposition of labile roots in the treated plots was most likely the driver of high emissions of carbon in the treated plots. Soil compaction due to collapse of aeranchyma tissue in roots might have been responsible for subsidence in cut plots. Natural regeneration was drastically affected by cutting, with treated plots having sparse seedlings 450 days after treatment. Gap-preferring ocypodid crabs colonized and became more abundant than sesarmids (usually found in closed canopy forest) in treated plots.

There was significant variation in mangrove forest productivity between Makongeni and Gazi sites, with the mangroves in the former having higher production than those of the latter. Rhizophora mucronata forest at Makongeni had a higher aboveground biomass (AGB) than all other forest types. On the other hand A. marina forest at Makongeni had the highest belowground biomass (BGB) production. Differences in microtopographical settings and soil factors might have influence the variation in forest productivity between the two sites and between the forest types.

These results underscore the importance of putting in place management options that ensure maintenance of continuous canopy cover and fast regeneration in mangrove forests under wood extraction. In addition, mangrove areas at the seafront should be protected. These results also support other work showing that mangrove forests often allocate a higher proportion of carbon to belowground roots than other forests. A high investment in belowground carbon helps facilitate surface elevation and peat formation, which not only forms important carbon sinks but may also enable mangroves to keep pace with projected sea level rise. Therefore, mangrove management in Kenya and the Western Indian Ocean region should explore options that consider trade-offs between mangrove utilization and minimizing loss of ecosystem functioning such as coastal stabilization and protection. In addition initiatives such as the payment for ecosystem services (PES) schemes e.g. reducing emissions from deforestation and degradation (REDD+) and should be explored as some of the strategies to reverse the declining trend in mangrove forest cover.