Are the climatic ranges of plant species impacted by atmospheric CO2 ? An attempt of quantification with a dynamic vegetation model

The observed present-day climatic ranges of plant species are frequently used by palaeobotanists and palynologists to reconstruct the climate evolution in the past. This is, for instance, the case of the widely used “Coexistence Approach” method, which has provided a wealth of palaeoclimatic data on many periods of the Neogene. Such vegetation-based palaeoclimate reconstruction methods rest on the uniformitarian assumption that the climatic tolerances of plant species, or the way their establishment and growth respond to climate parameters, have not changed markedly over time. This hypothesis can be questioned, because climatic tolerances and growth of plant species may depend on many factors likely to change over time. A first example is that other abiotic and biotic factors allowing the plant presence have probably changed in the course of time. Another example is genetic evolution that may affect climate resistance and end up to some adaptation of the populations as climate is changing.

Atmospheric CO2 may also modify the plant response. It is not accounted for in the vegetation-based palaeoclimatic reconstruction methods, but may alter the tolerance of plant species to aridity through stomatal closure or stomatal density changes. Moreover, a rise of atmospheric CO2 stimulates photosynthesis through the well-known CO2 fertilisation effect. How far this effect impacts plant growth and how long it can persist is still much debated in the scientific community. It likely depends on the nutrient abundance in the soils. However, if CO2 stimulates growth, it will also facilitate the colonisation of extreme environments by plant species. Indeed, their growth rate between two successive extreme climatic events will be enhanced and, so, the accumulated biomass will be larger and the likelihood to find their signature in the palaeovegetation records will increase.

In this contribution, we attempt to quantify this impact of CO2 on the climatic ranges of plant species by using the CARAIB dynamic vegetation model. This dynamic vegetation model can be run at the species level. We use a set of tree species from various climatic zones over different continents, for which the model has proved a good ability to simulate the present-day distribution. The model is run for different levels of atmospheric CO2, but with exactly the same climatic inputs. The simulated tree species distributions versus different climate variables (mean annual temperature, coldest month temperature, mean annual precipitation, precipitation of the driest month, etc) are then analysed and compared among the different CO2 configurations.