E xcess CO2 kills plants trees etc in the same way as excess oxygen causes problems for humans . Both are dependent on a balance of various gasses in the atmosphere . Plants need more co2 than people and people need more oxygen than plants . This was the shock scientists found when examining why during the early stages of the industrial revolution plants near high industrial concentrations were dying.
https://newatlas.com/environment/earth-forests-shorter-younger-climate/
https://science.sciencemag.org/content/368/6494/eaaz9463
