Growth, yield and silvicultural management of exotic timber species in Kenya

This study presents the growth, yield and the silvicultural management of Cupressus lusitanica, Pinus patula and Pinus radiata, the three most important timber species growing in the Kenya highlands. The study Is based on 163, 176 and 164 permanent sample plots for the three species respectively. The stand dominant height development was predicted as a function of stand age and site index, defined as dominant height at reference age of 15 years. The Chapman-Richard's growth function was used for C. lusitanica and P. radiata while a linear quadratic equation was used to describe dominant height development for P. patula by geographical regions. Height development for the two pine species was found to be significantly different (up to age 20 years) in the Shamba and grassland establishment sites. Stand basal area before thinning was predicted as a function of stand age, dominant height and number of stems using a Weibull-type growth equation. In thinned stands basal area was predicted through a basal area increment nonlinear equation. For P. radiata, basal area increment was predicted as a function of basal area at the beginning of the growth period (1 year) and age. For C. lusitanica and P. patula, a third term-stand density index, defined as the percent ratio of average spacing between trees to stand dominant height was included. The Weibull probability density function was used to characterize stand diameter distribution with the Weibull parameters predicted as a function of stand parameters. Stand volumes were determined from tree volume equations for the respective species while the mean DBH of stems removed in thinnings was predicted from mean stand DBH before thinning and weight of thinning. Using the above functions, a growth and yield simulation model EXOTICS was constructed. Written in FORTRAN IV G-level which is compatible with IBM System/360 and System/370, EXOTICS is an interactive whole-stand/distance independent model with an added capability for providing diameter distribution (by 3 cm diameter classes) to give final main stand yield by size classes. The model is intended to facilitate silvicultural management of the three species in the Kenya highlands. On validation, EXOTICS was found to have no bias within the range of validation data, and 95% confidence limits of 16%, 20% and 17% for C. lusitanica, P. patula and P. radiata respectively. Using EXOTICS, the current silvicultural management schedules in Kenya were studied. The thinning regimes were found to have marked effects on the current annual volume increment. It was therefore concluded that at the present level of silvicultural management, Moller's theory that thinning has no appreciable effects on total volume yield does not hold for the three species in Kenya. The current thinning policy aimed at production of large-sized sawlog crop in as short a rotation as possible at the expense of some loss in total yield is discussed and found to have been overtaken by events. A policy based on the concept of maximum volume production is advocated. A thinning experiment (using C. lusitanica) demonstrated that total merchantable volume could be increased by between 5 and 10% (using 20% thinning intensity) depending on site quality class. Within the range of stockings maintained in plantations in Kenya, thinning intensity was found to be the most important consideration, with stocking before thinning having very little effect on both mean annual volume increment and total merchantable volume yield up to age 40 years.Forestry, Faculty ofGraduat