Mechanical site preparation and ecosystem carbon storage in boreal forests.

Abstract

Description of the subject. Carbon responses to forest management that cause mechanical soil disturbance remain difficult to predict in boreal forests, where thick organic horizons and contrasting edaphic conditions modulate carbon turnover. Objectives. The objective of this study was to quantify ecosystem carbon stocks following mechanical site preparation (MSP), including live biomass of planted trees and other vegetation, dead organic matter, and soil carbon in organic and mineral horizons down to 55 cm, and to determine how disturbance intensity and site conditions shape long-term C dynamics. Method. We assessed vegetation carbon, FH horizon and mineral soil carbon (0-55 cm) across four long-term MSP experiments (10-25 years post-treatment) representing a gradient of disturbance intensities in boreal forest plantations. Analyses combined experiment-level mixed- effects models with cross-site comparisons integrating climatic and edaphic gradients. Results. Deep trenching reduced FH carbon stock by more than 80%, and mounding by 59%, while mineral soil horizons showed no detectable change across experiments. Species-site interactions contributed to response variability, with conifer plantations (Picea sp.) maintaining higher FH carbon stocks and hybrid poplar sites exhibiting 39-58% lower FH carbon stocks, especially on coarse-textured, cold, drought-prone podzols. Conclusions. MSP effects on soil carbon were strongly context-dependent. The absence of detectable mineral soil carbon responses suggests multi-decadal carbon adjustment times. Refining carbon-sensitive management will require deeper mineral sampling, species-specific belowground measurements, and experimental design capable of capturing microsite heterogeneity and vertical redistribution processes.