Quantifying the drivers of forest-based climate change mitigation

Abstract

Besides offering numerous important ecosystem services, sustainably managed forests can help reduce atmospheric CO2 concentrations and thus mitigate climate change. Forest-based mitigation occurs through the carbon sink in the forest itself, the carbon sink in wood products, and through substitution effects when wood products replace carbon-intensive materials and fuels.The relative importance of each of these three mitigation dimensions depends on a multitude of factors. First, forest type and structure, site conditions, and climate change and associated disturbances determine the amount of carbon that may be sequestered over the next decades at a given site. Second, the type and intensity of management determines the trade-off between on-site carbon sequestration and carbon storage in wood products. Third, management, wood usage patterns, and the carbon-intensity of the economy determine the amount of avoided emissions via substitution effects.To assess their impact on the total forest mitigation potential, we conducted a factorial modeling experiment by varying all of the aforementioned factors. Specifically, we looked at the forest type (needle-leaved vs broad-leaved) and age (young vs mature), increased and decreased harvest intensities, increased material wood usage and cascading, decarbonization rates, climate change and disturbance scenarios, and salvage logging practices after disturbance.Under an assumed "closer-to-nature forest management" our results show a higher mitigation potential of young forests compared to mature forests, whereas the forest type does not have a clear effect. The importance of substitution effects outweighs the importance of the forest and product carbon sink on shorter time scales. This changes towards the end of the century, assuming that substitution effects decrease because the substituted materials can be produced in a less carbon-intensive way. Increases in harvest intensity consequently are also only beneficial for climate change mitigation on these shorter time scales, though they likely have adverse effects on other ecosystem services. Our results also show that increased material usage (as opposed to energy usage) of wood can be an important lever for mitigation. Finally, changes in disturbances strongly affect the mitigation potential, though the mitigation impact of a subsequent salvaging operation heavily depends on the forest type and the product portfolio created from the salvaged wood.In conclusion, our results quantify the impacts and interactions of the different factors that govern forest-based mitigation, while highlighting the complexity of the topic and the importance of the considered time-scales.