Effects of environment and progeny on biomass estimations of five hybrid poplar families grown at three contrasting sites across Europe

The quantitative and qualitative estimation of above ground biomass production is essential for short-rotation forestry. Indirect methods of biomass estimation have been developed based on the establishment of regressions between biomass components and tree dimensions as well as canopy architecture. The difficulty encountered with this method is to know whether, and to what degree, the obtained equations can be used under contrasting environmental conditions and for different genetic backgrounds. In the present study, allometric equations were developed linking tree dry weight (stem, branch and aboveground tree dry weight) with stem circumference, stem height and number of sylleptic branches for five 2-year-old intra- as well as interspecific poplar (Populus) families growing at three contrasting sites in Europe, i.e. Northern Italy, Central France and Southern UK. The multiple regression method was used to find the most precise biomass equation which represents the genetic variation for each family at each site, independently, as well as a common equation involving all families and sites.The two F(1) P. deltoides × P. trichocarpa families and the F(1) P. alba × P. alba family were the most productive families irrespective of site. However, different patterns in biomass accumulation were observed between these crosses. The F(1) P. alba × P. alba family accumulated significantly more biomass in branches than in the stem as compared to both other families. In most cases, different transformations of the dependent variables were needed to improve the linear equations. Stem circumference was superior in predicting tree dry weight, but stem height and number of sylleptic branches were often needed to optimize the equations dependent on family and site. A standardized equation combining the five families and the three sites explained more than 95% of the observed variation for tree dry weight. However, there was a significant family effect, and at the UK site, a severe rust attack could have affected the relationships among the three predictors resulting in a significantly different equation for this site. When the UK site was excluded from the model, no overall site effect was observed. Thus, a single equation might be applied at family level regardless of the environmental conditions when stand density, age and plantation management are comparable.