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

Abstract

Maintaining or increasing forest carbon sinks is considered essential for mitigating the rise in atmospheric CO2 concentrations. In contrast, harvesting trees is perceived as having negative consequences for both the standing biomass stocks and the carbon sink strength. However, the forest carbon sink needs to be examined from a forest stand canopy perspective, where assimilation predominantly occurs in temperate forests. Here we show that a threshold of leaf area exists beyond which additional leaves do not contribute to CO2 uptake. The associated biomass can be harvested without affecting the forest carbon uptake. Based on eddy covariance measurements, we show that CO2 uptake (gross primary production – GPP) and net ecosystem exchange (NEE) in temperate forests are of a similar magnitude in both unmanaged and sustainably managed forests, on the order of 1500–1600 g C m−2 yr−1 for GPP and 542–483 g C m−2 yr−1 for NEE. A threshold located between 3 and 4.5 m2 m−2 LAI (leaf area index) can be used for sustainable harvesting with regard to CO2 uptake. Simulations based on the LPJ-GUESS (Lund–Potsdam–Jena General Ecosystem Simulator) model reproduce the saturation of GPP and NEP and the convergence on the LAI threshold range. Accordingly, in temperate managed forests, trees can be harvested while maintaining a high tree biomass and carbon sink of the remaining stand. In this case, competition between neighboring trees in unmanaged forests is replaced by harvest management and provision of wood products. No difference in the LAI productivity response was observed between managed and unmanaged sites.