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

Changes in climate have had a significant impact on global vegetation patterns during the Quaternary. However, variations in CO₂ levels also play a key role in shaping vegetation dynamics by influencing plant water-use efficiency, and consequently, the competitive success of employing the C3 and C4 photosynthetic pathways. In this study, we use an eco-evolutionary optimality (EEO) based modelling approach to examine the respective impacts of climate fluctuations and CO₂-induced alterations on vegetation shifts. We consider two distinct periods, the Last Glacial Maximum (LGM, 21,000 years before present) and the mid-Holocene (MH, 6,000 years before present) and compare these to contemporary conditions. The LGM, characterised by generally colder and drier climate, had a CO₂ level close to the minimum threshold for effective C3 plant operation. In contrast, the MH had warmer summers, increased monsoonal rainfall in the northern hemisphere, with CO₂ levels lower than the present day. We simulate vegetation changes at the LGM and the MH using a light-use efficiency model that simulates gross primary production (GPP) coupled to an EEO model that simulates leaf area index (LAI) and C3/C4 competition. We show that low CO₂ at the LGM is as important as climate in reducing tree cover, increasing the abundance of C4 plants and lowering GPP. Global GPP is also lower than today in the MH (although increased compared to the LGM), reflecting CO₂ constraints on plant growth despite the positive impacts of warmer and/or wetter climates experienced in the northern hemisphere and tropical regions on plant growth. These results emphasise the importance of taking account of impacts of changing CO₂ levels on plant growth in order to simulate ecosystem changes correctly.