Form Factors of an Economically Valuable Sal Tree (Shorea robusta) of Nepal

Abstract

The accurate prediction of the volume of standing trees is a prerequisite for planning and decision making in sustainable forest management. In Nepal, limited information on form factor (i.e. the ratio of the volume of a tree to the product of its basal area and height) is available for economically important tree species. Thus, current management plans consider a simple approximation for all species irrespective of their height and diameter, which hampers the estimation of a sustainable harvest rate. Therefore, this study elaborates the form factor for Sal (Shorea robusta), an economically valuable tree of Nepal based on a random selection of 100 individual trees representing a wide range of diameters between 10 and 100 cm. Diameter and bark thickness were measured at every 1-meter interval of the length of the stem and branches until the diameter reached 10 cm. The analysis allowed for the estimation of an average form factor for Sal wood with 0.407 over bark and 0.336 under bark, while the form factor for the stem was 0.335 over bark and 0.281 under bark. The results indicate an increasing form factor until 70 cm diameter and a decreasing value for larger diameters, because of the large crowns of the mature Sal trees. We conclude that the default form factor of Sal (0.5) used in management planning results in an overestimation of standing tree volume. Using form factor according to diameter classes will allow a more accurate prediction of the standing volume.