Impact of Human Physical Disturbance on Mangrove Forest Structure at the Gazi Bay, Kenya

Abstract

Human physical disturbance is prevalent in mangrove forests of the Western Indian Ocean. This study investigated the impact of human physical disturbance on the structure of mangrove forests by comparing forest attributes such as density, taxon richness, stem diameter and tree height between disturbed and relatively undisturbed sites. Physical disturbance was evaluated through tree harvesting intensity, roads and footpaths and other human activities, such as digging for fish bait at the sites. Disturbed sites recorded significantly (ANOVA, P<0.001) higher tree cutting intensity than comparable undisturbed sites, corresponding to lower forest complexity and changes in dominant tree species. Disturbance increased prevalence of Avicennia marina and Ceriops tagal species at the landward margin of disturbed sites. Disturbed sites also recorded significantly lower abundance of harvestable trees and stand volume (223 stem.ha-1, 14.56 m3 .ha-1, respectively) than undisturbed sites (288 stem.ha-1, 19.69 m3 .ha-1, respectively). In addition, accessible sites recorded lower marketable trees, heights, pole size classes (mazio 4-6cm, boriti >10cm) and species (Rhizophora and Bruguiera), being dominated by juvenile and stunted Ceriops and Avicennia. These results indicate that overexploitation of mangrove forests affects the species composition and structural complexity of the forest and hence may impair forest functioning and regeneration and subsequently, sustainable exploitation. Thus, human physical disturbance leads to exponential decline in forest complexity and requires management intervention.