Distribution and population structure of East African Greenheart (Warbugia ugandensis, sprague) plant in Mount Kenya forest, Kenya
Abstract
Effects from past climate, natural disturbances and human activities are significantly affecting current day processes in tropical forests with indigenous trees. Warburgia ugandensis is highly valued for its medicinal properties, timber, poles, and fuel wood. Consequently its population and distribution has been declining due to environmental and human factors and it is listed as a medicinal plant at risk from commercial exploitation in East Africa. The objectives of the study were to determine the present distribution and population structure of Wugandensis in Mt. Kenya forests.
To establish the plant species associated to potential natural vegetation types where W ugandensis occurs and to investigate the threat of W ugandensis in Mt. Kenya forests. This study was conducted in several forests around Mount (Mt) Kenya (5,199 m asl, 0° 9' 00' Sand 37° 18' 00' E in Central Kenya, about 150 Km North East of Nairobi). The study was carried out from September 2009 to march 201O.The study area was stratified into four blocks based on potential natural vegetation types: moist montane, moist intermediate, dry montane and dry intermediate natural vegetation type. Dry montane was the only vegetation type with W ugandensis and therefore four forest blocks were selected for this study; Kangaita, Kahurura, Ontulili and Gathioro forests.
Belt transects measuring 25m wide and 500m long were marked and subdivided into twenty sub-plots of 25m by 25m from which four sub-plots were systematically selected for sampling. Rainfall data for all the sampled blocks were obtained from metrological records in Nanyuki and Nairobi while altitude data was obtained by use of GPS and topographical maps of the area. Data was analyzed by general linear models (GLM) using SPSS 11.0 (2001) statistical software. Soil was analyzed for texture, nitrogen, phosphorus, potassium, organic carbon.
W. ugandensis tree height differed significantly in different forest. There was significant negative correlation between rainfall and canopy diameter, tree height, and diameter at breast height which was consistent with the abundance of the species in dry montane forests. There was negative but significant correlation between altitude, soils and W ugandensis trees sizes. Thus smaller trees were at higher altitudes. All the forests were dominated by mature trees, very few saplings while seedlings were rare. However plenty of shoots sprouted from the tree stumps.
The species associated to W ugandensis in the forest were Mystroxylon aethiopica with about 18% of the studied species, followed by Podocarpus latifolius 14.3%, Olea africana and Olinia rochetiana with 11.9% and 11.7% respectively. The main threat to W. ugandensis was human impact with 86% of damages while other factors contributed 14%. The major form of damage was debarking which threatens trees' survival. Warburgia ugandensis distribution and population is mainly determined by climatic factors like rainfall and also human factor through its exploitation for herbal medicine.
Key words: Distribution, Threats, Population, Structure, Warburgia ugandensis, Mt. Kenya, potential natural vegetation type (PNVT).
Publisher
University of Nairobi, Kenya