Holger Metzler, Samuli Launiainen, Giulia Vico

Boreal forests are often managed to maximize wood production, but other goals, among which climate change mitigation, are increasingly important. Examining synergies and trade-offs between forest production and its potential for carbon sequestration and climate change mitigation in forest stands requires explicitly accounting for how long forest ecosystems and wood products retain carbon from the
atmosphere (i.e., the carbon transit time). We propose a novel mass-balanced process-based compartmental model that allows following the carbon path from its photosynthetical fixation until its return to the atmosphere by autotrophic or
heterotrophic respiration, or by being burnt as wood product. We investigate four management scenarios: mixed-aged pine, even-aged pine, even-aged spruce, and even-aged mixed forest. The even-aged clear-cut based scenarios reduced the carbon amount in the system by one third in the first 18 yr. Considering only the amount of carbon stored in the ecosystem, these initial losses are compensated after 42-45 yr. At the end of an 80 yr rotation, the even-aged forests hold up to 31 % more carbon than the the mixed-aged forest. However, mixed-aged forest management is superior to
even-aged forest management during almost the entire rotation when factoring in the carbon retention time away from the atmosphere, i.e., in terms of climate change mitigation potential. Importantly, scenarios that maximize production or amount of carbon stored in the ecosystems are not necessarily the most beneficial for carbon retention away from the atmosphere. These results underline the importance of considering carbon transit time when evaluating forest management options for potential climate change mitigation and hence explicitly tracking carbon in the system, e.g. via models like the one developed here.