Ecosystem Carbon and Nutrient Balances in Short-Rotation Hybrid Aspen Coppice Under Different Thinning Methods

Coppice plantations have gained a high interest for biofuel production and carbon uptake in short rotation cycles. There is a limited knowledge how such intensive coppice management affects soil fertility and nutrients supply to maintain carbon sink. We studied ecosystem carbon and nutrients balance and allocation during a 5-year period in hybrid aspen coppice under different thinning methods in hemiboreal Estonia. The benchmark value for the changes was defined before the coppice emerged after the clear-cut of the previously planted hybrid aspen plantation. The studied systematical thinning treatments were as follows: corridor thinning with removal of 67% of the trees (CT), cross-corridor thinning with removal of 89% of the trees (CCT), and unthinned (UT) coppice. The UT and CT treatments resulted in a positive carbon balance at the ecosystem level. In all treatments, a decrease of soil acidity, organic C, total N, K, Mg and Mn contents, and an increase of soil Cu and B contents were observed in the 0–20-cm deep layer. The concentrations of leaf N, P, and K were higher in UT than in the two thinning treatments, indicating that the aspens had not entirely recovered from the changed root to shoot ratio 2 years after thinning, whereas the leaf mass fraction of medium- and small-sized trees had already increased. Bioenergy harvest from the UT site in a 5-year rotation would cause 5–18% removal of NPK from the total ecosystem pool. Overall, hybrid aspen coppice showed positive ecosystem carbon balance after the first 5-year period; however, further monitoring of soil properties is needed as we found decrease of soil organic C and nutrients concentrations in short term.

Graphical Abstract