A biologist pointed out the obvious. Plants need water, some soil nutrients, and CO2.
They absorb CO2 from the air through their skin pores, and if there is less CO2, they must widen their pores. But doing so allows more water to evaporate. Conversely, with more CO2, plants do not widen their pores, so they retain water.
Retaining water means they do not draw as much water out of the soil. So moister soil. And more plants can grow with that extra water. This can mean “de-desertification”, if there is enough CO2.
And over time, as plants die, their mulch retains moisture better than just dirt. As well as recycling nutrients.
A real boost comes with those plants that can fix nitrogen in the soil. They effectively fertilize other plants.
That's why the greenies hate dams...because of all the rotting vegetable matter under the water causes "global warming"....dams are a big contributor to CO2 world wide.