Rising atmospheric CO2 concentrations: the overlooked factor promoting SW Iberian Forest development across the LGM and the last deglaciation?

A cross the last deglaciation, the atmospheric CO2 concentration (CO2) increased substantially from ∼ 180 to ∼ 280 ppm, yet its impact on vegetation dynamics across this major climatic transition remains insufficiently understood. In particular, Iberian pollen records reveal an intriguing feature that can be related to an often-overlooked role of CO2 in shaping vegetation responses during the last deglaciation. These records reveal the near disappearance of forests during the cold Last Glacial Maximum (LGM) and Heinrich Stadial 1 (HS1) phases and an unexpected recovery during the Younger Dryas (YD) cold phase when CO2 increased. Here, we present high-resolution tracers of terrestrial (pollen, C29: C31 organic biomarker) and marine (alkenone-derived Sea Surface Temperature, C37: 4 %, and long-chain n-alkanes ratios) conditions from the southwestern (SW) Iberian margin Integrated Ocean Drilling Program Site U1385 (“Shackleton site”) for the last 22 cal kyr BP. This direct land-sea comparison approach allows us to investigate how the Iberian Peninsula vegetation responded to major global CO2 changes during the last deglaciation. Our results show that cool and moderately humid conditions of the LGM supported a grassland-heathland mosaic ecosystem, but low CO2 likely caused physiological drought and suppressed forest development. HS1, the coldest and most arid period, combined with sustained low CO2 values, almost suppressed forest growth in favour of Mediterranean steppe. In contrast, the warmer Bølling-Allerød, characterised by a temperature optimum and variable but generally wetter conditions, along with the rise of CO2 above 225 ppm at ∼ 15 cal kyr BP, contributed to substantial forest development. During the YD, sufficient moisture combined with increasing CO2 enabled the persistence of a mixed grassland-forest mosaic despite cooler temperatures. Our study suggests that during cool and humid periods (LGM and YD) different pCO2 values led to contrasting SW Iberian vegetation responses. In contrast, during periods of relatively high CO2, temperature and precipitation played the main role in shaping the distribution and composition of the vegetation.