Conceptual pitfalls in the forest carbon debate

Abstract

Forests stock up to 90% of the global terrestrial plant biomass carbon (C). Any rise or fall of that stock, but also its utilization for substituting fossil resources can influence the rate of atmospheric CO₂ enrichment. By employing the term ‘C sequestration’, the ongoing debate suffers, however, from an implicit confusion between (1) processes, rates or fluxes of C (e.g. tree growth) with (2) pools, stores or stocks of forest biomass C. Stock formation is driven by turnover, C duration, residence time, or tree demography, and not by the rate of influx of C, including tree growth. Enhanced tree growth must not be treated as a rise in C stock, without accounting for turnover, also removing often assumed benefits of CO₂ fertilization for stock formation, should tree growth be C limited, another questionable assumption. A carbon ‘sink’ is a potential volume that can be filled with C, but it does not represent a stock either, without accounting for C residence time. ‘Buying time’ by lengthening rotation has a cost in terms of reduced utilization of forest products for substitution of fossil resources. Finally, management cessation for biodiversity benefits, should be qualified by its conservation value, rather than by making a case for C storage benefits, without accounting for natural forest gap dynamics, and again, without pricing-in the inevitable cost of the cessation of the substitution of fossil C by renewable C. All this calls for a strict separation of the meaning of carbon fluxes and carbon stocks, and avoiding ambiguous terms such as C sequestration and C sink.