GROWTH PREDICTION IN SOME PLANTATIONS OF EXOTIC TREE SPECIES IN THE NORTHERN GUINEA AND DERIVED SAVANNA ZONES OF NIGERIA

Abstract

Growth studies which include diameter distribution using the Weibull function, development of stand volume models, and construction of provisional top height-age and total volume production-age curves for the average site class were carried out in plantations of Eucalyptus cloeziana, E. tereticornis and Pinus caribaea. The study locations were Kabama (Zaria) and Afaka (both typical of Northern Guinea savanna zone of Nigeria), Nimbia (Derived savanna zone) and Miango, Vom and Ta-Hoss (on the Jos Plateau). E. cloeziana represented only at Afaka had 4 experimental sample plots with age range of 4.2 - 15.5 years. E. tereticornis represented at Kabama and Afaka had 6 sample plots with ages from 3.7 - 15.5 while P. caribaea represented in all the study locations except Zaria had 43 plots with ages between 4.7 - 29 years. In fitting the Weibull distribution to the individual diameter plot data, WWINGO, the most suitable Weibull (program) subroutine for determining the Weibrill parameters in the stands, was capable of describing the diameter distribution in the plots as from the minimum age of 4 years for E. cloeziana and 6 years each for E. tereticornis and P. caribaea. On testing the Weibull parameter predictive models constructed (based on stand attributes and site factors) in new stands, while those of E. cloeziana and E. tereticornis gave good predictions as from the respective minimum age limits stated above, those of P. caribaea did not give reliable predictions until the age of 10 years. With reference to the stand volume model developed for each species, that of E. cloeziana gave the best prediction with the root mean square error (RMSE) of 2.25 m3/ha, followed by that of E. tereticornis with RMSE of 4.7 m3/ha and that of P. caribaea with RMSE of 10.1 m3/ha. The models predicted well within the range of field data when tested in new stands. Out of the three functions used in fitting the provisional top height-age (site index) curve for the average site class for each species, the Gompertz model, in most cases, gave better fittings than the Logistic while the polynomial function was the poorest. Based on the best-fit model for each species and some other criteria, a provisional average site class was defined as top height of 27.4 m at a reference age of 13 years for E. cloeziana, 24.8 m at the age of 15 years for E. tereticornis and 23.4 m at age 20 for P. caribaea. The Gompertz model was also more reliable than the Logistic in fitting the provisional average total volume production-age curve for each of the species. A maximum total volume production of about 240 m3/ha at age 13.4 years, 286 m3/ha at age 19.4 years and 726 m3/ha at age 30 could be obtained for E. cloeziana, E. tereticornis and P. caribaea, respectively, on an average site. The applications of the Weibull parameter predictive models in forest management, and the expected optimum rotation ages for the species in view of the growth figures obtained from the other growth models were discussed.