Neither wood ash application nor phosphorus-fertilisation mitigated the negative effects of whole-tree harvesting on a temperate oligotrophic forest

Background and Aims Concerns about climate change and carbon economy have prompted the promotion of alternative energy sources, including forest-based bioenergy. An evaluation of the environmental consequences of intensive harvests (stumps and roots, and also branches and foliage) for energy wood supply, and use of wood-ash recycling as a compensatory practice, helps in the analysis of the use of forest biomass for energy production. Methods After 11 years, we made use of records from a split-plot experimental site crossing four different intensities of Biomass Harvesting (Stem-Only Harvest [SOH], Aboveground Additional Harvest [AAH], Belowground Additional Harvest [BAH], and Whole-Tree Harvest [WTH]) and three Compensation Methods (control [C], wood ash application [A] and phosphorus fertilisation [P]) to evaluate their effects on tree growth, soil fertility, chemical properties and soil carbon. This site is located in a maritime pine forest on a poor soil, under a warm temperate climate (SW France). Key results Despite their low additional biomass exports (+10% for AAH to +34% for WTH), the non-conventional harvest practices exported much higher quantities of nutrients than the conventional SOH technique (for example +145% for N and K in WTH). Consequently, these treatments had several effects on the soil nutritive status. Additional biomass harvests impacted the soil organic matter content, with negative effects on P-organic and soil cation exchange capacity. However, data suggested that tree growth and foliage nutrient content had not yet been significantly impacted by harvest treatments, whereas tree nutritional status was improved by P-fertiliser or wood ash. As expected, we observed a positive effect of wood ash application on soil acidity and nutrient content but, like additional harvests, wood ash application decreased the pool of soil organic carbon (∼10% of the initial stock with ∼7% of N-total losses). Conclusions Overall, this factorial experiment showed that exporting more forest biomass due to the additional harvesting of tree canopies, stumps and roots had negative consequences on the ecosystem. Additional harvests have aggravated the poverty of the already oligotrophic soil, which in turn may decrease tree growth and the soil organic carbon content in the future (but without any impact on either soil acidity or on trace metal contents). Importantly, applying nutrients as fertiliser or wood ash did not compensate for the negative impact of biomass exports and the method of wood ash recycling in forests could even decrease the soil organic carbon. ### Competing Interest Statement The authors have declared no competing interest.