Monitoring and Assessment of Leaf Litter Dynamics in a Mixed Mangal Forest of the Cross River Estuary, Nigeria

Abstract

The leaf litter decomposition of Nypa fruticans, Rhizophora racemosa and Avicennia africana were studied across a tidal gradient in a mixed mangrove forest of the Cross River estuary Nigeria. Leaf litter decomposition was measured along the tidal gradients (low, mid, high) using litter bags. A single exponential model was used to study the decomposition rates of the leaves. Leaf decomposition varied significantly (P < 0.001) among species and spatially across tidal gradients over the study period. Decomposition was fastest in A. africana and slowest in N. fruticans and spatially, it was fastest at the low tide level and slowest at the high tide level. Tidal effects were much larger than species differences in the decomposition rates. The time (days) required for the loss of half the initial dry mass (T50) of the decomposing leaves at the low, mid and high tide levels were A. africana, 46, 57 and 77, R. racemosa 69, 86 and 115, and N. fruticans 86, 99 and 115 respectively. Estimates of leaf litter turnover rates showed that the actual litter turnover based on the relative measure of leaf litterfall and biomass on the forest floor (Kt) were much shorter than the projected litter turnover based on the leaf decomposition rates (Kd). The estimated actual residence times were less than one day while the projected residence times ranged from 83 to 142 days across tidal gradients, suggesting involvement of other ecological processes in litter loss and their possible transport into the Cross River estuary. The nitrogen contents and nutritional value of the decomposing leaves increased with time during the study. The increase varied significantly (P <0.001) among species, as well as temporally (P < 0.001) and spatially (P ≤ 0.05). The average C:N ratio decreased from 27.7 to 22.1 in A. africana, 26.4 to 23.9 in R. racemosa and 32.8 to 23.6 in N. fruticans. The overall changes in nutrients during decomposition indicated net mineralization. Mangrove leaf litter dynamics, trophic value and organic matter exchange of the system have implications on the productivity of the Cross River estuary and ultimately the Gulf of Guinea. Knowledge of these processes is critical for the maintenance and long term sustainability of the mangrove and surrounding ecosystems.