Reproductive Biology And Population Ecology Of Senegalia Senegal (L.) Britton Within Lake Baringo Woodland Ecosystem, Kenya

Abstract

Senegalia senegal (L.) Britton is an economically important dryland tree species known for gum arabic production and soil fertility enhancement. However, the species is currently under considerable ecological threat due to overexploitation and habitat degradation. The present investigation was designed to determine the effects of anthropogenic disturbances on reproduction and population structure of S. senegal within Lake Baringo woodland ecosystem. The study assessed impacts of anthropogenic disturbance on genetic diversity, natural regeneration and population structure, phenological events, mating systems and gene flow patterns of S. senegal. Eleven natural populations of S. senegal within Lake Baringo woodland ecosystem were sampled for genetic diversity study. The populations were grouped into two disturbance levels based on population disturbance index (PDI). Within each population, 30 trees were sampled from which fresh leaf tissues were collected. Data on natural regeneration and population structure was collected from 60 plots measuring 20 x 20 m systematically distributed within four natural populations. Senegalia senegal trees within these plots were measured for height and diameter at breast height (dbh) and categorized as seedlings, saplings or adult trees. Thirty adult trees in the four populations were assessed at a fortnight interval for a period of 24 months for phenological patterns. For mating systems study, seeds collected from 20 adult tree from two populations with contrasting disturbance levels were sampled. All S. senegal adult trees and seedlings from the two populations studied for mating systems were sampled for geneflow analysis. Twelve nuclear microsatellite markers previously developed for S. senegal were used to investigate the mating patterns and gene flow of the species within the ecosystem. The markers were also used to determine the impact of human disturbance on the species genetic diversity. Higher seedling (1778±452) and sapling (1294±274) densities were observed in lightly disturbed population than in heavily disturbed populations (seedling, 1206±235; sapling, 717±197). Lightly disturbed xvii populations revealed a ‘reversed J-shape’ size-class distribution (SCD) indicative of normal and stable population structure. The leafing, flowering and fruiting pattern of S. senegal were bimodal and followed the rainfall patterns within the ecosystem. Significant variations in floral and fruit morphology were evident. Lightly disturbed populations had higher gene diversity (HE = 0.635), mean number of alleles (NA = 7.31) and allelic richness (AR = 6.72) than heavily disturbed populations (HE = 0.577; NA = 7.02; AR = 6.56). The species displayed a predominantly outcrossing mating pattern. However, heavily disturbed population showed significantly lower outcrossing rates (tm = 0.833; p < 0.05) than the lightly disturbed population (tm = 1.000; p > 0.05). Multilocus analysis showed that biparental inbreeding and correlated mating events occurred in the heavily disturbed populations. This suggests that human disturbances may be promoting establishment of seeds produced through consanguineous mating. This suggestion is supported by the estimate of correlation of paternity (rp = 0.329) and the mean coefficient of co-ancestrality within families (Θ = 0.233), which indicated that some of the matings in the heavily disturbed populations led to about 23 % of the offspring being full-sibs. Long-distance pollination events and seed dispersal occurred within the woodland. The average pollen dispersal distance ranged from 110 to 400 m. Eventhough mating among near neigbors was common within the populations, the small population sizes were mitigated by the occurance of long distance gene flow through pollen from outside the populations. The results of the present study suggest that S. senegal is ecologically and genetically vulnerable to human disturbances and there is need for in-situ conservation strategies to protect the species from further genetic degradation.