Fast growing hybrids of Salix and Populus can be grown as coppice for short-rotation forestry as a source of renewable energy. Field trials of Salix genotypes of differing yields were assessed to identify anatomical and physiological differences, which determine biomass yield. The high yielding genotypes had the largest stem heights and diameters. Furthermore, high yielding genotypes tended to attain larger individual leaf areas, larger number of cells per leaf, and fast rates of leaf production and leaf extension. Large number of sylleptic branches were found on the highest yielding Salix genotype studied. Photosynthetic characterisation of genotypes and leaf area index investigations showed that light harvesting and leaf longevity may be more important to above ground biomass accumulation than measures of individual leaf photosynthesis. Many traits hypothesised as yield determinants in Populus were measured in a F₂ pedigree of P. trichocarpa x P. deltoides hybrids, Family 331, originating from North America. A replicated field trial was established and assessments were made of single stem and coppiced plants. Principal components analysis (PCA) and partial least squares regression showed that stem traits most important as determinants of yield were total basal stem area and stem height. The number of sylleptic branches influenced yield in coppice. The leaf traits most influential of yield were the number of leaves on the leading stem of the coppice stool and the area of a fully mature, recently expanded individual leaf. These are easily measured traits which can be used in assessments of phenotype for breeding novel genotypes for high yield. The strongest outliers in the PCA score plots may be those of interest to plant breeders, as they show extreme behaviour in traits related to yield. Broad-sense heritability was calculated and QTL analyses were conducted on the data set. The date of bud burst in the spring and stem height and diameter were highly heritable; these traits yielded QTL in addition to QTL for whole tree fresh weight, individual leaf area, stem extension increment and leaf extension rates.