Strong negative impacts of whole tree harvesting in pine stands on poor, sandy soils: A long-term nutrient budget modelling approach

Abstract

Global environmental changes such as climate change, overexploitation and human population growth increase the interest in woody biomass from forests as a resource for green energy, chemistry and materials. Whole Tree Harvesting (WTH) can provide additional woody biomass, mainly for bioenergy, by harvesting parts of the crown not harvested under conventional Stem-Only Harvesting (SOH). However, WTH also increases nutrient export, potentially depleting soil nutrients and threatening future stand productivity. Here we assess the impacts of WTH in Corsican pine stands (Pinus nigra ssp. Laricio var. Corsicana Loud.) with a rotation period of 48years on poor, sandy soils in Belgium. We performed a detailed nutrient budget assessment before and after thinnings and clear-cuts under scenarios of WTH and modelled the long-term changes in ecosystem nutrients under both WTH and SOH. Our results demonstrate a strong immediate impact of WTH on aboveground nutrient stocks (mainly in clear-cuts). In clear-cuts with WTH, half of the base cations (calcium, potassium, magnesium) in the trees and forest floor were exported. The amount of available cations in the soil is not sufficient to immediately compensate for this export. Only one fourth of the amount exported were available for biota in the top 50cm of the soil. We also modelled long-term development of ecosystem nutrients (available nutrients in the soil and nutrients in trees and forest floor) and found that the available soil calcium, potassium and phosphorus stocks are insufficiently replenished by deposition and weathering to sustain WTH on the long term. We found no indications of potential depletion of ecosystem cations and phosphorus for the next ten rotation periods under SOH management. Our results thus support a less intensive management in pine stands on poor, sandy soils, for instance, by adopting SOH and/or longer rotation periods.