The increasing use of bioenergy and therefore the expanding market for biomass have boosted interest in the establishment of short rotation plantations, where biomass could be obtained either as a production goal or as a by-product. Biomass equations are specific to species and growing conditions; however, their development takes resources and is time consuming. In the Baltic States, several Populus hybrids differing by a number of traits are established in small areas; therefore, interest in generalised biomass equations is increasing. The aim of our study is to develop above-ground biomass equations for the hybrid aspen (Populus tremula x P. tremuloides) and the hybrid poplar (Populus balsamifera x P. laurifolia) in Latvia and to test their robustness regarding tree age and stand density.

   Sample trees were collected during the winter period in four stands located on mineral soil with normal moisture regime and similar fertility (corresponding to the Oxalidosa forest type) in the central part of Latvia. In total, 82 hybrid aspen trees from 12 to 19 years of age and 16 hybrid poplar trees from 62 to 64 years of age were sampled.

   Differences in the above-ground biomass in both Populus hybrids for trees with similar dimensions were non-significant; however, a noticeable difference in biomass allocation was found. Stem biomass formed 69% and 90% of the above-ground biomass (in leaf-less state) for hybrid aspens and hybrid poplars, respectively. Present biomass equations for the hybrid aspen significantly underestimate the real above-ground biomass by 8% to 24%. All of the developed biomass estimation models were statistically significant (p < 0.01), and the R-squared values ranged from 0.85 to 0.96 for hybrid poplars and from 0.96 to 0.98 for hybrid aspens, suggesting good explanatory power. The developed equation based on the hybrid aspen might be applied to the hybrid poplar without significant error.

Keywords: hybrid aspen, hybrid poplar, biomass allocation.