Development of tree volume equations for common timber species in the tropical rain forest area of Nigeria

Volume equations were developed for common timber species in Nigeria's tropical rain forests. Data consisted of merchantable volume, stump diameter, diameter at breast height and merchantable height for 77 timber species. Number of observations per species ranged from 5 to 142, and diameter at breast height ranged from 20.0 to 230.0 cm. Schumacher–Hall's volume function was fitted to the data of each of the 33 well-sampled species (n ≥ 30), using weighted least squares. The coefficients from these species-specific equations were used as input variables for species grouping. Species grouping was obtained using a two-stage approach of cluster analysis followed by discriminant analyses. First, cluster analysis was used to group the 33 well-sampled species into five clusters, and then discriminant analysis was used to assign each of the remaining 44 species (with n < 30) into one of the five clusters. The species groups obtained did not follow any particular taxonomic pattern, as there were species of the same genus that fell into different clusters. The Schumacher–Hall's volume function was fitted to the data for each species group. The resulting equations possessed desirable statistical properties and model behaviours, and can be used to estimate merchantable volume for common timber species in the tropical rain forest areas of Nigeria.