MODELING GROWTH AND YIELD OF GREVILLEA ROBUSTA GROWN ON FARM BOUNDARIES IN NANDI COUNTY, KENYA.

Abstract

Population pressure has led to changes in land use and livelihood strategies in most highlands of Kenya. This among other factors has contributed to increased integration of fast growing exotic trees species into tree-crop production systems on short rotation for socio-economic benefits. Grevillea robusta is one of the exotic species in Kenya planted along farm boundaries for fuel wood and timber production. . It is preferred by most farmers because of its fast growth and yield as well as its suitability to be integrated with crops. This study investigated growth and yield of G. robusta grown in line configuration boundary planting and their economic implication in Nandi County. The main objective was to model growth and yield of G. robusta grown on farm boundaries. Farmers growing G. robusta and knows its ages were identified and growth data (height, diameter at breast height-dbh) collected. A total of 985 G. robusta trees of ages ranging from 1-15 years in 39 different locations within the Upper and lower highland agro-ecological zone in Nandi County were measured. Four independent variables: height, dbh, location and spacing were modeled against age. The results showed significant differences among trees height (P=0.0001), basal area calculated from dbh (P=0.0001) and location (P=0.0001). Spacing was not significant (P=0.1993). Height took the form; Ht = α + β * (Ageθ) while dbh was Dbh = α + β * (Ageθ) where; Ht = tree height, Dbh = diameter at breast height (1.3 m), Age = tree age in years and α, β and θ are estimated regression coefficients. The local volume equation; was used to model volume because no farmer allowed their trees to be cut, where; V = Tree volume, Dbh = Tree diameter and Ht = Tree height. The study did not find overwhelming evidence (p>0.05) in support of different linear spacing distances. Thus further research, particularly as trees get bigger is recommended so as to advice on thinning for intermediate income. Boundary planting (line spacing) of G. robusta is a good system to be adopted in Nandi County.