A growth and management model for coniferous plantations in East Africa

Thesis



A mathematical model is presented for growth and yield of Cupressus lusitanica, Pinus radiata and Pinus patula growing in the highlands of Kenya, Uganda, Tanzania, and Malawi, based on permanent sample plot data. Stand dominant height is predicted by a log/reciprocal function of age and site Index. Initial diameter distributions are predicted by spacing at a dominant height of 7 metres and generated by a Weibull function. The diameter distribution of stems is simulated by a vector of 10 cumulative percentiles which are incremented over each growth period by a function based on relative basal area, dominant height, and tree dominance. Relative basal area is defined in terms of maximum basal area, which. itself is modelled by a Chapman-Richards function in terms of dominant height. Dominance is defined as the ratio of tree diameter to stand dominant diameter. Thinnings are simulated by a novel and efficient algorithm that takes some trees from all size classes but is biassed to the smaller stems, although the degree of bias varies with thinning intensity. Tree volumes are determined by a new tree taper function using a quadratic/cosine model of relative height and relative sectional area. An algorithm is presented based on the taper function for subdividing stand volume into classes determined by log diameter and length. Validation of the model showed that diameter increment and height prediction, as well as the thinning routine were very accurate over a wide range of stand conditions; but the taper function was biassed towards underestimation of larger sizes and required re-fitting by a suitable method of weighting, whilst the initial distribution estimation required an additional variable to account for some of the variation occurring in practice. Two computer programs and a subroutine library are presented in the Appendix. VYTL-2 is a program to produce variable yield tables; whilst PYMOD is a forecasting program for multicompartment forests.

Authors: Alder, D; Alder, Denis

• Publication date: 1977
• DOI: 10.5287/ora-qonnjoxdz
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Subjects: Plantations; Management; Africa, East