Quantitative Trait Loci (QTL) analysis can be used to understand the genetic control of complex traits, but QTL mapping experiments usually give poor resolution of position, and verification of the presence of QTL is rare making further study difficult. Increased competence of QTL mapping can be achieved in four ways; by repeating experiments over several years, repeating across different environments, comparison across different pedigrees or even different species, and the study of highly correlated traits. Adaptive traits such as those related to phenology and biomass can be particularly difficult to map due to small additive effects and susceptibility to environment, but many such traits are easy to score and highly correlated, therefore are ideal for this four stage approach to QTL verification. A hybrid poplar pedigree (Populus trichocarpa T. & G. x P. deltoides Marsh) has been established in three environments across Europe, within the project POPYOMICS. Work has been carried out to assess traits highly correlated to phenology and biomass (Rae et al 2004). QTL have been mapped for stem, leaf and cell traits over a number of years using both single and multi-trait analysis to give improved mapping ability.