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. x 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.