Relationship between tree species and soil quality in ancient forest ecosystems in Wallonia

Abstract

Ancient forests are defined as stands that have been forested since the eighteenth century to the best of our cartography knowledge. Although their tree species composition and management practices differ, these forests share a common feature: their soils have remained under continuous forest cover for over two centuries. This protected them from major degradations to their physical and chemical integrity due to land use change, providing a window to understand how forest soils are may evolve in the long term. It has been previously demonstrated that soil quality, important for both ecosystem functioning and ecosystem services, can be influenced by tree species. In Wallonia the dominant tree species are oak (Quercus robur), beech (Fagus sylvatica) and Norway spruce (Picea abies). However, no study so far specifically studied the effect of tree species on soil quality in ancient forests in Wallonia. In addition, how this effect varies according to soil layers is unclear. Consequently, the aim of this study was to evaluate whether there is a link between soil quality and the dominant tree species in the Ardennes’s ancient forests, across soil horizons. Soil quality was assessed through several indicators, including microbial metabolic diversity, microbial biomass, microbial C/N ratio, microbial and metabolic quotients, soil respiration, pH, and soil carbon content Results demonstrated clear tree species effects on most soil quality variables. Overall, the two deciduous species (oak and beech) showed more similarities to each other, while spruce consistently showed different properties. Compared to deciduous species, spruce soils contained significantly more organic carbon, were more acidic, likely hosted more fungi relative to bacteria, and had lower-quality litter in the O horizon. Oak was consistently distinct from spruce, whereas beech occasionally showed patterns closer to spruce than to oak. For instance, the microbial metabolic profile of oak was clearly different from the one of spruce, indicating different microbial substrate preferences, while beech displayed an intermediate position. Finally, significant differences were observed among soil horizons for all parameters analysed These results support the current understanding that spruce tends to have less favourable effects on soil quality compared to beech and oak. They also suggest that comparing soil samples within the same horizons may be more adequate in the present case than sampling using fixed depths. Future research could include sampling in autumn instead of spring to capture potential seasonal effects, and analysing microbial genetic diversity in addition to metabolic diversity. In the longer term, testing mixed stands rather than monocultures, as well as incorporating additional predictors such as leaf area index or tree age, would provide a broader understanding of tree–soil interactions in ancient forests