Soil responses to timber harvesting in coniferous forests of the Valle d’Aosta region

Timber harvesting can exert significant direct and indirect impacts on forest soil development and properties. It alters microclimatic conditions and reduces forest litter inputs, influencing humus formation, soil biological activity, and nutrient cycling. Moreover, increased soil exposure can enhance erosion processes, leading to the loss of organic and surface horizons. Together, these processes can substantially modify soil physical, chemical, and biological properties, with consequences for forest ecosystem functioning. The aim of this study was to assess the effects of timber harvesting on soil properties and development, and the underlying pedogenetic processes involved, approximately ten years after the end of harvesting activities.

The study was conducted in the Valle d’Aosta region (northwestern Italy). Six forest parcels classified as spruce-dominated coniferous forests, located between 1270 and 1850 m a.s.l. and predominantly east- or west-facing, were selected. Timber harvesting operations were completed by 2017, mostly in 2013, using the most common regional systems (cable yarding and winch-equipped tractors). Within each parcel, soil profiles were described and sampled by horizons in the harvested areas and adjacent control (non-harvested) areas. Soils and humus types were classified according to WBR and European humus forms reference base, respectively. Differences between soil profiles were evaluated considering carbon stock (C stock), pH, carbon to nitrogen ratio (C/N), thickness of organic horizons, soil structure, and biological activity.

Several variables, including pH and C/N, did not show consistent trends in relation to timber harvesting, likely due to the dominant influence of environmental factors, such as elevation, parent material, soil type and precipitation. In contrast, timber harvesting significantly affected organic horizons, resulting in a substantial reduction in OF thickness and C stock. A decrease in C stock was also observed in mineral horizons, particularly within the upper 40 cm, although it was less pronounced than in organic horizons. Consequently, the relative contribution of organic and mineral horizons to total carbon stock was also altered, with a reduced organic-to-mineral C stock ratio in harvested soils. These effects were mainly attributed to reduced litter inputs and altered microclimatic conditions, that shifted the balance between organic matter accumulation and decomposition toward degradation process. Moreover, the development of herbaceous vegetation in the harvested areas, whose deep root systems promoted microbial activity and organic carbon decomposition, further contributed to carbon losses in mineral horizons.

These changes also affected pedogenetic processes, particularly in Podzols, with the exception of the most strongly leached. Indeed, reduced acidic litter inputs from coniferous species led to a slowdown or cessation of podzolization processes, while herbaceous root systems provided continuous organic matter inputs to the E horizon. In addition, the increased biological activity, evidenced by abundant earthworm presence in harvested soils, promoted horizon mixing.

Therefore, by driving a shift from coniferous forest to herbaceous vegetation, timber harvesting can strongly influence soil properties and pedogenetic processes at different soil depths, over relatively short timeframes compared to soil formation rates. Enhanced biological activity emerged as a key driver of soil structural modification, humus transformation, and carbon stock reduction.