Short-term effects of pre-commercial thinning on carbon cycling in fertile birch (Betula sp.) stands in hemiboreal Estonia

Pre-commercial thinning (PCT) is a common silvicultural practice for directing the development of the young stand in Nordic and Baltic countries. However, its impact on the stands carbon (C) cycling is still poorly studied. We carried out a comprehensive case study for estimating net ecosystem production (NEP) in unthinned control plot and in moderately and heavily thinned plots (2,500 and 1,500 trees ha⁻¹ remaining, respectively), in the next growing season after PCT in young Betula pendula stands on mineral soil (Site 1) and Betula pubescens stands on drained organic soil (Site 2). Thus, the study demonstrates post-thinning changes in C cycling in stands of two species of the same genus. The control plots of both sites served as C sinks: NEP 1.5 and 3.6 t C ha⁻¹ yr⁻¹ in Site 1 and Site 2, respectively. However, the thinned plots acted as C sinks on Site 1 (1.8 t C ha⁻¹ yr⁻¹) and as C sources on Site 2 (−1.4 and −3.1 t C ha⁻¹ yr⁻¹ in heavily and moderately thinned plot, respectively). The declined net primary production of trees after PCT was compensated for by the production of herbaceous vegetation and stump sprouts. Soil heterotrophic respiration (Rh) was the largest flux in the C budget of both sites and all treatments. Despite the increasing trend of Rh with increasing thinning intensity on Site 2, no statistically significant difference in the annual Rh flux occurred between the treatments in either site.