Modelling the nutrient cost of biomass harvesting under different silvicultural and climate scenarios in production forests

Intensifying the use of forest biomass to produce fuelwood, through the removal of harvest residues or reductions in rotation length, increases nutrient outputs and can ultimately lead to reduced soil fertility. We developed a modelling approach for the evaluation of different forest management options under future climate scenarios. This approach allows management systems to be evaluated in terms of their nutrient costs by quantifying several variables: nutrient outputs (N, P, K, Ca and Mg) resulting from harvesting, ecosystem N and P balances, and changes in organic C, N and P stocks in the soil. In addition, we calculated a “nutrient cost index” (in kg-harvested-biomass g-exported-nutrients−1). As part of this study, we looked at the effects of harvesting branches, foliage and stumps in addition to tree stems, as well as the effects of changing rotation length in Pinus pinaster, Pseudotsuga menziesii and Fagus sylvatica forest stands, under contrasting Representative Concentration Pathway climate scenarios (RCPs). Comparably to previous studies, our simulations showed that removing harvest residues and, to a lesser extent, reducing rotation length have high nutrient costs. Climate was also found to have an impact, mainly caused by larger amounts of standing tree biomass, and therefore larger biomass harvests and increased nutrient outputs in the scenario which involved elevated atmospheric CO2. Using contrasting forest management systems and climates, we showed that our modelling approach can be used to guide forest managers in their choice of future silvicultural practices (rotation length, conventional stem-only harvest versus intensive harvest, thinning regime) based on future climate scenarios. Finally, our approach can be used to determine, more accurately than simple allometric relationships, the amounts of nutrients that would need to be applied in order to compensate for losses.