Diameter Distribution for Gmelina Arborea (ROXB) Plantations in Ukpon River Forest Reserve, Cross River State, Nigeria

Weibull distribution-based models were developed and applied in the characterization of trees for sustainable production of Gmelina arborea in Ukpon River Forest Reserve, Cross River State, Nigeria. Forty-two sample plots of size 20m X 20m were randomly laid in the 19 - 33-year G. arborea age series and complete enumeration of diameter at breast height (DBH)) over bark was carried out. The growth data sets were then processed into size-class sets. A three–parameter Weibull distribution function (with α, β, and λ representing location, scale and shape parameters respectively) was fitted into the size-class data set using percentile estimates method. The equations obtained under diameter distribution revealed that α was significantly related to S and N (r 2 = 0.53), common logarithm of β, was significantly related to Age (r 2 = 0.81) while the common logarithm of λ was significantly related to reciprocal of A and BA (r 2 = 0.63) where A, N, BA and S are age, surviving number of trees per hectare, basal area per hectare and, site index respectively. Leptokurtic, Mesokurtic, and Platykurtic distributions characterize plantations of 20 – 24, 26 – 28 and 30- 32 years respectively. Keywords : Diameter at breast height, Gmelina arborea , sustainable production ,Ukpon River Forest Reserve, Weibull distribution function.