The last time the world had today's level of 400 part per million of CO2 was four million years ago and the trees and plants at that time were adapted to the climate that went with it. Those trees had taken thousands or years to evolve to match those conditions. The trees we have today are adapted to a CO2 level of 280 PPM and a climate 0.8C cooler than today and are rapidly going into conditions 2C warmer and with dramatical changed rainfall conditions of either drought or flood.
We can expect to see much large numbers of trees and other plant life dying in the coming years.