Quick and accurate biomass estimation of willow (Salix spp.) grown under short rotation intensive culture (SRIC) is essential for carbon accounting and management decisions. Currently, most estimates of tree biomass, including willow, rely on measurement of stem diameter. This is a suitable approach for single-stem species but for measurements of multi-stem species such as willow, there is an increase in the time and effort required as well as the need to include site, clone and age specific information. Therefore, we developed a new method which calculates optical stem density from digital photographs taken at predetermined locations and angles within a plantation during the fall or winter when the willow is without leaves. We then calibrated a mathematical model using destructive sampling to convert the measurements of optical stem density into estimates of biomass. The method produced very strong relationships (adjusted r(2) = 0.97) between the predicted and actual harvested biomass for the plots studied. Being new, the method still requires further testing and possibly adjustments for different planting designs and clones. (C) 2008 Elsevier Ltd. All rights reserved.