Abstract
Short rotation plantations are often considered as holding vast potentials for future global bioenergy supply. In contrast to raising biomass harvests in forests, purpose‐grown biomass does not interfere with forest carbon (C) stocks. Provided that agricultural land can be diverted from food and feed production without impairing food security, energy plantations on current agricultural land appear as a beneficial option in terms of renewable, climate‐friendly energy supply. However, instead of supporting energy plantations, land could also be devoted to natural succession. It then acts as a long‐term C sink which also results in C benefits. We here compare the sink strength of natural succession on arable land with the C saving effects of bioenergy from plantations. Using geographically explicit data on global cropland distribution among climate and ecological zones, regionally specific C accumulation rates are calculated with IPCC default methods and values. C savings from bioenergy are given for a range of displacement factors (DFs), acknowledging the varying efficiency of bioenergy routes and technologies in fossil fuel displacement. A uniform spatial pattern is assumed for succession and bioenergy plantations, and the considered timeframes range from 20 to 100 years. For many parameter settings—in particular, longer timeframes and high DFs—bioenergy yields higher cumulative C savings than natural succession. Still, if woody biomass displaces liquid transport fuels or natural gas‐based electricity generation, natural succession is competitive or even superior for timeframes of 20–50 years. This finding has strong implications with climate and environmental policies: Freeing land for natural succession is a worthwhile low‐cost natural climate solution that has many co‐benefits for biodiversity and other ecosystem services. A considerable risk, however, is C stock losses (i.e., emissions) due to disturbances or land conversion at a later time.
Highlights
On a global level, bioenergy currently holds the largest share of all renewable energy sources (REN21, 2018)
The results are presented in the following ways: First, we compare the relative performance of natural succession (nSucc) in relation to BE, with displacement factors (DFs) being varied from 0.1 to 1
At a very low DF (0.2 or lower), C benefits from nSucc generally exceed those from BE, regardless of the timeframe, and at 0.3, it takes more than 70 years for BE to achieve higher cumulative C savings
Summary
Bioenergy currently holds the largest share of all renewable energy sources (REN21, 2018). Due to the reduction in forest C stocks associated with increased use of forest biomass relative to a counterfactual scenario with lower harvests, it often takes considerable periods of time until forest bioenergy provides net C savings in comparison to fossil‐based reference systems (“fossil fuel parity time,” see Cherubini, Bright, & Strømman, 2012; Cintas et al, 2017; Gustavsson, Haus, Ortiz, Sathre, & Truong, 2015, 2016; Holtsmark, 2012; Hudiburg, Law, Wirth, & Luyssaert, 2011; Jonker, Junginger, & Faaij, 2014; Lamers & Junginger, 2013; McKechnie, Colombo, Chen, Mabee, & MacLean, 2011; Sterman, Siegel, & Rooney‐Varga, 2018; Vanhala, Repo, & Liski, 2013; Zanchi, Pena, & Bird, 2010, 2012). Depending on different influencing factors (management practices, tree species, types of fossil fuels being displaced, which parts of trees are used for energy and other uses, etc.), parity times vary from less than a year to several decades or even centuries (e.g., Agostini, Giuntoli, & Boulamanti, 2013; Bentsen, 2017; Buchholz, Hurteau, Gunn, & Saah, 2016; Mitchell, Harmon, & O'Connell, 2012)
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