Quantifying Invisible Losses: Greenhouse Gas Budgets from Three Timber Harvesting Systems and Nine-Year Recovery of Compacted Skid Trails

Background: Forest soils in Austria's Flysch zone are highly productive but susceptible to compaction from mechanized timber harvesting. Soil compaction alters soil structure, porosity, aeration, and greenhouse gas (GHG) dynamics.

Objectives: This study compared the effects of different timber harvesting systems on soil GHG fluxes of carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) and quantified recovery following soil disturbance.

Methods: At the Steinplattl experimental site (Klausen-Leopoldsdorf, Austria), we performed a controlled before-after study in a thinned beech stand using three harvesting systems: tracked harvester and forwarder (H+), non-tracked harvester and forwarder with tracks on uphill axles only (H-), and manual felling with cable yarding (MC). From 2022-2024, we measured soil GHG fluxes tri-weekly in treatment plots (H+, H-, MC) and uncompacted control plots (C) using closed-chamber technique. Random forest models trained on environmental predictors (soil temperature, moisture, precipitation) were used to generate daily soil GHG flux predictions for calculating GHG budgets per hectare for each treatment over 435 days post-harvest (March 10, 2023 to June 17, 2024).  Additionally, we assessed long-term recovery using 24-hour soil GHG monitoring (six chambers per treatment, 5-minute measurement cycles) at skid trails from non-tracked operations (H16) in a 2016 thinning and adjacent uncompacted forest soil (C16).

Results: Ground-based harvesting substantially altered all three soil GHG fluxes, while cable yarding effects varied by GHG. Cumulative N₂O budgets in ground-based systems (H+ and H-) were more than 3-fold higher than controls, with peak emissions comparable to fertilized cropland. MC showed intermediate N₂O fluxes with emission peaks occurring primarily after rainfall events. Soil CH₄ uptake was severely reduced in all treatments, with H+, H-, and MC showing 94%, 89%, and 51% reductions compared to C, respectively. CO₂-equivalent budgets revealed that H+ generated the highest climate impact (~77 t CO₂-eq ha^-1), 32% above controls, though high spatial variability precluded statistical significance. Long-term monitoring revealed that 9 years after trafficking, H16 skid trails showed persistent GHG alterations compared to C16. N₂O emissions remained elevated with episodic hot moments after rainfall, CH₄ uptake remained reduced under dry conditions but approached zero during wet periods, and CO₂ emissions remained elevated.

Conclusions: Compared to H-, H+ systems mitigate soil physical impacts but generate elevated GHG emissions. MC minimizes disturbance but exhibits high N₂O emission potential after rainfall. Effects on soil GHG dynamics were most pronounced for H+ and H- during the first post-harvest year.  However, even after 9 years, skid trails did not recover to pre-disturbance conditions but rather stabilized in an altered state, characterized by a persistent vegetation shift from beech understory to graminoid-dominated communities and thicker litter layers accumulating in wheel track ruts. These changes resulted in elevated CO₂ emissions in H16, while CH₄ uptake rates remained reduced and episodic N₂O hot moments continued during periods of high soil moisture and temperature. Our results emphasize the importance of permanent skid trail networks and site-adapted technology selection for sustainable forest management on compaction-susceptible soils.