A ratio method for predicting stem merchantable volume and associated taper equations for Cupressus lusitanica, Ethiopia

Volume ratio method is one of the methods used for estimating merchantable volume of a stem between a pre-specified stump height and top diameter. Volume ratios of merchantable volume to total volume were expressed as a function of the ratio of merchantable top diameter to breast height diameter (dm/D) and as a function of merchantable height to total tree height (hm/H). A number of mathematical equations were developed using dm/D and hm/H as independent variables for its estimation for Cupressus lusitanica. Fit index and standard error of estimate were used to evaluate and chose the best equations for each explanatory variable. The predictive abilities of the selected equations for estimating merchantable volume ratios were then examined using a test data. The results showed that the selected equations had an overall mean bias of −0.0111 and −0.0052, respectively. Development of merchantable volume ratio equations led to stem taper equations that predict diameter on a stem at any height and height at any diameter, based on given D and H. The performances of the derived taper equations were also tested. The overall absolute biases were 0.2276 cm and 0.2438 m for predicting merchantable top diameter and height at various relative heights and varying merchantable top diameters, respectively.