Form factor functions for nine commercial tree species in Bhutan

Abstract

Standing timber volume in combination with the expected volume increment rates derived from volume functions are essential for developing sustainable forest management plans. Tree volume cannot be measured directly. It is derived from the diameter at breast height, tree height and a so-called form factor, which reduces the volume of a cylinder to the actual tree form. In this paper, we test four different types of form factor functions (Pollanchütz, Short Swedish, Meyer and F. Evert) for estimating total merchantable timber volume of nine commercial tree species in Bhutan: Abies densa, Picea spinulosa, Pinus wallichiana, Tsuga dumosa, Pinus roxburghii, Castanopsis tribuloides, Quercus glauca, Quercus lanata and Quercus lamellosa. The data for fitting the form factor functions come from 395 felled trees. The resulting functions are evaluated using independent validation data. Fitted statistics for evaluation include: Root mean square error and mean absolute deviation. Although all form factor functions performed similarly, we suggest that the Pollanschütz function because of its consistency in the estimated form factors for all tree species. The evaluation of the calibrated form factor functions by species exhibited consistent and unbiased predictions.