Eco-evolutionary Modelling of Global Vegetation Dynamics and the Impact of CO₂ during the late Quaternary: Insights from Contrasting Periods

Abstract. Changes in climate have had major impacts on global vegetation during the Quaternary. However, variations in CO₂ levels also play a role in shaping vegetation dynamics by influencing plant productivity and water-use efficiency, and consequently the relative competitive success of the C₃ and C₄ photosynthetic pathways. We use an eco-evolutionary optimality (EEO) based modelling approach to examine the impacts of climate fluctuations and CO₂-induced alterations on gross primary production (GPP). We considered two contrasting periods, the Last Glacial Maximum (LGM, 21,000 years before present) and the mid-Holocene (MH, 6,000 years before present) and compared both to pre-industrial conditions (PI). The LGM, characterised by generally colder and drier climate, had a CO₂ level close to the minimum for effective C₃ plant operation. In contrast, the MH had warmer summers and increased monsoonal rainfall in the northern hemisphere, although with a CO₂ level still below PI. We simulated vegetation primary production at the LGM and the MH compared to the PI baseline using a light-use efficiency model that simulates GPP coupled to an EEO model that simulates leaf area index (LAI) and C₃/C₄ competition. We found that low CO₂ at the LGM was nearly as important as climate in reducing tree cover, increasing the abundance of C₄ plants and lowering GPP. Global GPP in the MH was similar to the PI (although greater than the LGM), also reflecting CO₂ constraints on plant growth despite the positive impacts of warmer and/or wetter climates experienced in the northern hemisphere and tropical regions. These results emphasise the importance of taking account of impacts of changing CO₂ levels on plant growth to model ecosystem changes.