Biomass can be estimated by different apparoaches. Volume and taper are important factors to assess the opportunties to utilize biomass from multipurpose short rotation forestry. The properties of biomass may vary from clone to clone and site to site. Therefore, accurate estimates of volumes, for instance, are needed based on clone and site specific. Several non-linear and liner functions were fitted to the data set processed from the cloned willow (Salix schwerinii E. L. Wolf) plantation in Siikasalmi, Finland, to estimate volumes and to compare the best fitted taper and volume approach based on the strategy of the number of radius measurements observation per stem (i.e. 3-15 radius measurements including RBH and H measurements) for cost-efficient estimates of volume. The functions were evaluated based on the fit statistics so as to select the best-fitted models. The nonlinear function of Demaerschak (taper function) with 2- parameters and Schumacher and Hall (direct volume function) were fitted to the data set more accurately to predict the volume than other taper and volume functions, since these had goodness-fit-statistics. Two best-fitted functions were also compared with compromising accuracy, using this strategy to select the suited to the cost-efficient estimation of volumes. The overall results suggested that the taper model was much more robust than the direct volume model when extrapolations were needed because a lower RMSE was found in the taper model approach. Without extrapolating the taper approach was better than the volume model only in the case of lower numbers of radius measurements (i.e. 3-5), which would therefore permit a cost-efficient estimate of volumes. The RMSE and B% were lower in these measurements.

Keywords: Willow, Volume, Radius, Taper function, Volume function