Accounting of forest carbon sinks and sources under a future climate protocol-factoring out past disturbance and management effects on age-class structure

Today, forests in the northern hemisphere are a sink for carbon dioxide (CO2) from the atmosphere, partly due to changes in forest management practice and intensity. Parties of the Kyoto Protocol had the option to elect to account for direct human-induced carbon (C) sources and sinks from land management activities since 1990. The effect of age-class structure of a forest landscape resulting from past practices and disturbances before the reference year 1990 should be excluded, but methods for "factoring out" the effects of this age-class legacy on carbon emissions and removals are lacking. The legacy effect can be strong and can even overwhelm effects of post-1990 management. It therefore needs to be "factored out", i.e., removed from the direct human-induced post-1990 effects. In this study we examine how the contributions to forest biomass carbon stock changes of (1) past (pre-1990) disturbances and harvest and (2) recent (post-1990) changes in forest management can be differentiated in present and future observable carbon dynamics in managed forest ecosystems. We also calculate the consequences of different accounting rules for the magnitude and direction of accountable C stock changes in European countries in the period 2013-2017. Different accounting approaches are compared in terms of applicability and their ability to provide incentives for management changes to increase carbon sinks and reduce carbon sources. We demonstrate implications of the various ways of accounting for a sample of European countries with different initial age-class structures. The current forest age-class distribution in countries determines whether and how many credits can be created by the various accounting approaches. We suggest an approach that includes a dynamic, forward-looking baseline as reference and list options to define such a baseline. Accounting of recent management change against such a baseline factors out the contribution of the legacy effect to accounting results and only rewards the effect of recent changes in forest management practices in support of climate change mitigation. We demonstrate that relatively simple, state-of-the-art forest models can factor out effects of past practices and past disturbances on present and future carbon stock changes. Factoring out of past practice effects is thus technically feasible but the numerical results are highly dependent on the choice of baseline which will be subject to negotiation. It is possible, however, to select a dynamic baseline that represents "business-as-usual", and to isolate and account for only the changes in management. Changes in accounting rules will always be advantageous for some countries and disadvantageous for others, but using a dynamic "business-as-usual" baseline effectively removes the legacy of pre-1990 age-class effects, and thus overcomes one of the acknowledged shortcomings of the current accounting approach