Effects of fire on the population structure and abundance of Anogeissus leiocarpa and Vitellaria paradoxa in a West African savanna parkland

Abstract

The West African savanna experiences a prolonged dry season with Harmattan winds which facilitate large and persistent biomass burning from November to April. The fires are mostly caused by humans, mainly in pursuit of the day to day livelihood activities. We examined how fire influences the population structure and abundance of two economically important woody species Vitellaria paradoxa (Shea tree) and Anogeissus leiocarpa (African Birch) in six land use types in the Guinea savanna, Ghana. We calculated the stand basal area, mean densities of juveniles and adult trees, Lorey's mean height of adult trees and Simpson's index of dominance. Eight diameter size classes of each species were analysed by comparing their observed distributions to a three-parameter Weibull distribution across the land use types. A total of 3366 individuals of A. leiocarpa (n = 1,846) and V. paradoxa (n = 1,520) were enumerated. The basal area of A. leiocarpa and V. paradoxa in sacred groves (16.9 m2 ha−1) and unburnt woodlands (20.6 m2 ha−1) was higher than the estimates in the other land use types. High mean densities of A. leiocarpa and V. paradoxa were found in sacred groves (22.7 ± 29.7 stems ha−1) and fallows (15.3 ± 2.2 stems ha−1), respectively. Mean density of juveniles of both A. leiocarpa (248.0 ± 89.1 stems ha−1) and V. paradoxa (68.0 ± 29.7 stems ha−1) were higher in unburnt woodlands than in the other land use types. A. leiocarpa was absent in fallows and burnt crop fields. An inverse J-shaped distribution was found in sacred groves for both species.The absence of A. leiocarpa in burnt crop fields and the decrease of some size classes of V. paradoxa in both burnt and unburnt crop fields indicate the need for sustainable conservation of both species. Furthermore, the inverse J-shape distribution found in sacred groves for both A. leiocarpa and V. paradoxa implies that these species thrive best with minimal anthropogenic disturbances. Although species conservation is achieved through conventional protection, traditional or cultural conservation practices which avoid the indiscriminate use of fire should be highly promoted to ensure sustainable conservation of species.