Assessing Tree Species Dominance along an Agro Ecological Gradient in the Mau Forest Complex, Kenya

Abstract

Species relative dominance by basal area was assessed along an agro ecological gradient in the Mau Forest Complex (MFC). Trees were recorded per species and diameter at breast height (D 1.3 ) for size class D 1.3 ≥ 10 cm in 60 quarter hectare plots distributed in 7 agro ecological zones (AEZ) namely LH1, LH2, LH3, LH4, UH0, UH1 and UH2. Basal area per species was used to calculate species relative dominance i.e. the proportion of basal area by a species to the total basal area of the AEZ. Species associations were analysed as the group of highly ranked species in each AEZ. Sorensons similarity index was used to calculate the proportion of similar species among AEZ. Analysis of variance compared basal area among AEZ and Tukey’s multiple comparison test used to identify specific AEZ with differences. Tabernaemontana stapfiana (Britten) was ranked first in LH1, UH1 and UH0 with relative dominance values of 22.66%, 22.89% and 30.73% respectively. It was however not recorded in any other AEZs. Dombeya goetzenii (K. Schum) occurred in 6 of the 7 AEZs but had moderate dominance values in each of the 6 AEZs. The sum of dominance values per species in all AEZs indicated no species mono-dominance and different species dominated at different AEZs. Co-dominance resulted in species associations like Tabernaemontana - Allophylus - Eke-bergia - Albizia in LH1, Juniperus - Dombeya - Casearia - Prunus in LH2, Acokanthera - Cussonia - Olea - Teclea in LH4 and Tabernaemontana - Syzygium - Podocarpus - Neoboutonia in UH1. Species richness was highest in UH1 and had the highest similarity indices with those of other AEZs. The UH1 had a species similarity of 67% with LH1, 63% with LH2 and 56% with LH4. However, species in the very humid zone UH0 differed with those of the drier lower highland zones (UH0 vs LH3 and vs LH4 = 31% and 37% respectively). Basal area differed significantly among AEZ ( = 3.76) showing that they differ in stocking levels. Tukeys test showed that high potential zones of LH1, LH2, UH0, UH1 did not differ and similarly the lower potential zones; LH3 and LH4. The results show that the variation of species and forest stocking in the MFC is strongly influenced by AEZ and proposes future biomass mapping to be done along AEZ.