Abstract
Abstract. We present an extension of the dynamic global vegetation model, Lund–Potsdam–Jena Managed Land (LPJmL), to simulate planted forests intended for carbon (C) sequestration. We implemented three functional types to simulate plantation trees in temperate, tropical, and boreal climates. The parameters of these functional types were optimized to fit target growth curves (TGCs). These curves represent the evolution of stemwood C over time in typical productive plantations and were derived by combining field observations and LPJmL estimates for equivalent natural forests. While the calibrated model underestimates stemwood C growth rates compared to the TGCs, it represents substantial improvement over using natural forests to represent afforestation. Based on a simulation experiment in which we compared global natural forest versus global forest plantation, we found that forest plantations allow for much larger C uptake rates on the timescale of 100 years, with a maximum difference of a factor of 1.9, around 54 years. In subsequent simulations for an ambitious but realistic scenario in which 650 Mha (14 % of global managed land, 4.5 % of global land surface) are converted to forest over 85 years, we found that natural forests take up 37 PgC versus 48 PgC for forest plantations. Comparing these results to estimations of C sequestration required to achieve the 2 ∘C climate target, we conclude that afforestation can offer a substantial contribution to climate mitigation. Full evaluation of afforestation as a climate change mitigation strategy requires an integrated assessment which considers all relevant aspects, including costs, biodiversity, and trade-offs with other land-use types. Our extended version of LPJmL can contribute to such an assessment by providing improved estimates of C uptake rates by forest plantations.
Highlights
It is increasingly clear that the stringent climate targets of the Paris Agreement cannot be achieved without negative emissions, i.e., net removal of carbon (C) from the atmosphere later during the 21st century, to compensate for emissions in the first half of the century (Gasser et al, 2015; Rogelj et al, 2018)
The target growth curves (TGCs) represent a compromise between the observations and the CSWC,max for natural plant functional types (PFTs), predicted by Lund–Potsdam–Jena Managed Land (LPJmL): the initial high growth rate is representative of the observations, while CSWC,max is closer to that of the simulated natural PFTs and notably lower than the level indicated by the observations
With respect to the relative growth rate k, the tropical forest plantation functional types (FPFTs) is comparable to the boreal FPFT
Summary
It is increasingly clear that the stringent climate targets of the Paris Agreement cannot be achieved without negative emissions, i.e., net removal of carbon (C) from the atmosphere later during the 21st century, to compensate for emissions in the first half of the century (Gasser et al, 2015; Rogelj et al, 2018). Of the many proposed techniques to achieve C uptake, the two options currently most discussed for large-scale implementation are bioenergy in combination with carbon capture and storage and afforestation (Williamson, 2016). Both approaches will require considerable amounts of land and compete with other land-use functions, for example, food production and biodiversity. C uptake occurs when natural vegetation is allowed to grow back on former croplands and pasture. While deliberately taking cropland or pasture out of production may involve costs, the direct management costs of natural regrowth are negligible. The carbon uptake rate of such natural regrowth, will usually achieve only a fraction of the potential C uptake rate at short timescales
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