Bioresource Technology41 ( 1992) 59-63 Non-destructive Estimation of Biomass of Salix dasyclados Theo Verwijst & Nils-Erik Nordh The Swedish University of Agricultural Sciences, Department of Ecology and Environmental Research, Section of Short Rotation Forestry, Box 7072, S-750 07 Uppsala, Sweden (Received 25 January 1991; accepted 25 June 1991 ) Abstract Common non-destructive methods for biomass estimation of willow developed for Salix viminalis stands perform poorly when applied to Salix dasyclados, because of differences between these species in stem shape and branching pattern. A method has been developed by which the weight of a single stem, consisting of several shoots with apical dominance, is related to the sum of the cross-sectional areas of the shoots. This method gives a ,good fit of the data with a linear regression model, and performs better than models which include stem height in the predictor variable. Key words: Biomass estimation, Salix dasyclados, willow. INTRODUCTION During the last decade, short rotation forestry research in Sweden has focused on the use of willow (Salix spp.)to produce biomass for energy purposes (Sir6n, 1983). Descriptive methods for stand characterisation for assessment of biomass dynamics (Nilsson, 1981, 1982) were developed simultaneously with programmes for breeding and selection of willow clones (Sir6n, 1983; Ager et al., 1986; Gullberg, 1988). Methods for biomass estimation were developed using Salix viminalis, but the earlier breeding programmes also released clones of other Salix species. Some of these species display growth forms which differ markedly from the erect physiognomy of Salix viminalis. An extreme example of such different growth form is found in the Salix dasyclados clone 77075, which produces Bioresource Technology 0960-8524/92/S05.00 © 1992 Great Britain 'bow-shaped' stems, often with an almost horizon- tally oriented lower stem part. Clone 77075 pro- duces curved branches, starting at different heights above the ground. Many of these branches show apical dominance. Clone 77075 and some other clones with the same growth form are currently being tested on a large scale and an annual biomass estimation is needed for the evaluation of these trials. Some of the stands are relatively small and destructive sampling would influence their future develop- ment. Furthermore, destructive sampling is time- consuming, especially in stands which have grown during more than one season, and therefore a suit- able non-destructive method is preferable in most cases. The non-destructive biomass estimation methods developed for Salix viminalis require stem weight to be related to stem diameter. If stems of Salix dasyclados clone 77075 are har- vested close to the soil surface while diameter is measured at a higher level, a series of diameter scores has to be related to one stem weight. Relating one weight score with a series of dia- meter scores can be avoided by measuring the diameter at harvest level, but this may cause large errors due to the taper of the stems, which is more pronounced at the stem base (Nilsson, 1982). The number of diameters to be measured per har- vested stem will largely be a function of the height above the ground at which diameter is measured. The inclusion of stem height as a predictor var- iable for weight brings about comparable prob- lems. Biomass estimation on Salix dasyclados has been carried out in Finland (Hyt6nen, 1985; Hyt6nen et al., 1987), but the clones used in these studies did not have a branching pattern which 59 Elsevier Science Publishers Ltd, England. Printed in