HOW LOGGING RESIDUES IMPACT ECOSYSTEM RESPIRATION AFTER CLEAR-CUT?

Abstract

Drained peatlands are a source of CO2; to reduce such effect, afforestation is a common mitigation strategy. However, clear-cutting and logging residue retention, along with climate change, alter soil conditions, affect microbial activity, and reduce mitigation efforts. Moreover, lower water table levels, resulting from drainage, and higher soil temperature accelerate decomposition, leading to increased CO2 release, further undermining mitigation efforts. Interestingly, these relationships have not been studied in highly productive hemiboreal drained peatlands, which constitute a vast CO2 reservoir. This thesis investigates the impact of logging residue retention, along with soil temperature and water table level, on ecosystem respiration in hemiboreal drained peatland following the clear-cutting of Norway spruce forest over three years. The site was divided into two plots. At the rewetted plot, logging residues were removed, and later the plot was rewetted; at the other plot, the logging residues were retained, and it was afforested (afforested plot). At both plots, CO2 fluxes were obtained with the eddy covariance method, and soil temperature and water table level were measured with soil sensors. Logging residue biomass was estimated using the Marklund (1988) model. Linear regression models were applied to investigate the direct and moderating effects of soil temperature and water level table. The results reveal that the annual average of CO2 release at the afforested plot (4.55 kg CO2 m-2 yr-1 ) was almost twice as high as CO2 release at the rewetted plot (2.35 kg CO2 m-2 yr-1 ). Logging residue decomposition contributed to nearly 56% at the afforested plot and 26% at the rewetted plot in the first year of measurements. This increase can be attributed to the decomposition of logging residues, which boosts microbial and fungal activity and modifies soil conditions. Furthermore, a strong positive effect of soil temperature on respiration rates was observed at both plots (stand. coef 0.69 & 0.73, p < 0.01). This effect was moderated by water table level (stand. coef -0.043 & -0.104, p < 0.01). These results reveal that although higher soil temperature accelerates decomposition, leading to increased CO2 release, the higher water table level reduces oxygen availability in the soil and limits respiration sensitivity to temperature. These findings show that both substrate availability and environmental controls interact to influence CO2 release. This thesis contributes to a broader understanding of carbon cycling in managed hemiboreal drained peatlands and calls for further research on long-term effects and integrated management strategies.