Saturating response of photosynthesis to increasing leaf area index allows selective harvest of trees without affecting forest productivity

Abstract. Maintaining or increasing forest carbon sinks is considered essential to mitigate the rise of atmospheric CO₂ concentrations. Harvesting trees is perceived as having negative consequences on both the standing biomass stocks and the carbon sink strength. However, harvesting needs to be examined from a forest stand canopy perspective since carbon assimilation occurs in the canopy. Here we show that a threshold of leaf area exists beyond which additional leaves do not contribute to ecosystem fluxes. The associated biomass can be harvested without affecting the forest carbon fluxes. Based on eddy covariance measurements we show that CO₂ uptake (GPP) and net ecosystem exchange (NEP) in temperate forests are of similar magnitude in both unmanaged and sustainably managed forests, in the order of 1500–1600 gC m^-2 y^-1 for GPP and 542 – 483 gC m⁻² y⁻¹ for NEP. A threshold of about 4 m² m^-2 LAI (leaf area index) can be used as a definition of sustainable harvesting with regard to CO₂ uptake. Simulations based on the LPJ-GUESS model reproduce the saturation of GPP and NEP and convergence on the LAI threshold range. Accordingly, in managed forests, trees can be harvested while maintaining a high tree biomass and carbon sink of the remaining stand. In this case competition between neighbour trees in unmanaged forests is replaced by harvest management. In unmanaged forests, competition for light, nutrient and water cause self-thinning, thereby limiting the carbon sink strength.