Clear-Cutting and Carbon Balance in Boreal Forests: Evidence from a Natural Experiment

Boreal forests store approximately one-third of the global carbon (C) pool, and the C sink capacity of Fennoscandian boreal forests has increased steadily over the past century. However, recent evidence indicates a marked decline in C uptake during the last decade, with some areas transitioning to net C sources. The drivers of these changes are complex, involving interactions between forest management and climate change.

Fennoscandian forests have been intensively managed for centuries, with only small remnants remaining virgin. Since World War II, stand-based forestry dominated by clear-cutting and planting has become the prevailing practice, enhancing timber production and tree C uptake. Yet, its long-term effects on soil C dynamics, ecosystem functioning, and resilience remain poorly understood.

The EcoForest project investigates the long-term impacts of clear-cutting on biodiversity, carbon dynamics, and ecosystem functions in Norway spruce (Picea abies) forests along climatic gradients. We established paired plots of mature forests: one previously clear-cut (CC) and one near-natural (NN), matched for macroclimate, topography, and soil properties. CC stands had higher tree density, while NNs exhibited greater structural heterogeneity, light variability, and crown length. Deadwood volume was three times higher in NNs than in CCs.

We monitored tree litterfall continuously for two years and measured soil respiration monthly during one snow-free season. Ground vegetation litterfall was estimated via destructive sampling. CC stands exhibited 12% higher annual soil respiration, 20% greater tree litterfall, and a tendency toward higher total aboveground litterfall (12%), whereas NNs had 45% greater ground vegetation litterfall. Deadwood from CC stands showed higher respiration rates in laboratory assays, likely due to differences in wood properties that, in turn, led to different fungal decomposer communities. Overall, current net soil C balance appears similar between CC and NN stands.

Our findings demonstrate that management history exerts a lasting influence on key ecosystem processes, including litterfall composition, deadwood decomposition, and soil respiration—factors often overlooked in current carbon models that treat forests as homogeneous units. By integrating these dynamics, models can better capture variability in carbon fluxes across clear-cut and near-natural stands. The EcoForest project provides a unique natural experiment, offering critical insights for improving ecosystem models and enhancing predictions of boreal forest carbon balance under future climate and management scenarios.