Effect of Early Thinning Treatments on Above-Ground Growth, Biomass Production, Leaf Area Index and Leaf Growth Efficiency in a Hybrid Aspen Coppice Stand

The bioeconomy strategies in the EU are expected to lead to increased consumption of woody biomass. The empirical knowledge of asexually regenerated hybrid aspen ( Populus tremula L. × P. tremuloides Michx.) coppice stand production and responses to silvicultural treatments is still poor. In hemiboreal Estonia, four different management treatments (corridor thinning with ~67% removal, cross-corridor thinning with ~89% removal, single-tree thinning with ~97% removal and control with no management activity) were applied in a 2-year-old hybrid aspen coppice stand, and effects on tree above-ground biomass and leaf characteristics were investigated during three post-thinning years. Hybrid aspen mean annual increment of above-ground biomass peaked at 6.3 (range: 3.6–8.5) Mg ha −1 in year 4, suggesting 4–5 years as an optimal age for bioenergy harvest. The above-ground growth characteristics of dominant trees did not differ from control area. The current annual increment of the height, biomass and leaf growth efficiency (LGE) of dominant trees under single-tree thinning remained even lower compared with the other treatments. Dominant trees were more efficient in resource use, as their LGE values were 21–50% higher compared with the stand average value. Poor growth, high mortality and low LGE in single-tree thinning indicate that the low density of remaining trees created an imbalance between leaf area and parent root system. The hybrid aspen coppice stand showed a high biomass production during early development. We recommend modest early thinning in vegetative hybrid aspen stands to ensure a sufficient balance between leaf area and parent root system.