Influence of wildfires severity on tree composition and structure in Aberdare Afromontane forest ranges, Kenya

Abstract

Wildfires strongly impacts on the forest ecosystems, with poorly understood consequences on the composition, diversity and structure within Afromontane forests of Eastern Africa. The impacts of wildfire on vegetation were investigated one year after the fire occurrence season. The burn severity was evaluated using Sentinel-2 satellite derived spectral index known as dNBR (difference Normalized Burn Ratio). This was used to delineate wildfire into low and high severity classes. We investigated the effects of wildfires using 36 plots of 15 m radius distributed in high and low burn severities while others were laid in unburnt areas. Our findings indicated that tree height, basal area and density were significantly different along severity classes (p = 0.003, p=0.007 and p<0.000 respectively). However, diameter at breast Height (DBH) was not significant along severity classes (p=0.185). The most ecologically important species based on Important Value Index (IVI) within the fire affected areas were Juniperus procera, Nuxia congesta, Olea europaea subsp. cuspidata, Protea kilimandscharicum and Scurtia myrtina. Among the five dominant tree species, the density of Protea kilimandscharicum and Olea europaea subsp. cuspidata were significant along the fire severities (p<0.05). Interaction of vegetation zones and wildfire severity indicated that only height was significant (p= 0.000) while all the other tree variables and species diversity index were not significant; DBH (p=0.053), basal area (p=0.606), tree density (p=0.389), Shannon diversity index (p=0.843), Simpson dominance index (p=0.490) and Pielou's evenness index (p=0.248). Whereas this study was on a short-term post-fire effects on vegetation, it has shown remarkable findings which can guide restoration and management of fire affected landscapes. The study recommends evaluation of long-term regeneration and recovery to guide integrated fire management within Afromontane Forest Ecosystems.