Why do naturalists suggest that we plant more trees?

The naturalists suggest that we plant more trees because plants release CO2 through respiration and its use for photosynthesis, thus decreasing the amount of CO2 and maintaining balance in the atmosphere. 

That is true tge cactus as a structural adaptation to its envirinment has it leaves reduced to spines. but this doesnt mean it cant produce food it produces food because of the presence of chlorophyll in its stem