Different groups of green plants have significantly different photosynthetic cycles (notably C3, C4, and CAM) which respond very differently to changes in CO2 c

Different groups of green plants have significantly different photosynthetic cycles (notably C3, C4, and CAM) which respond very differently to changes in CO2 concentration. For C4 plants (about 3% of plants) the current CO2 concentration is at the point of saturation. C3 plants (85% of plant species, including most trees and grasses, and staple crops such as wheat) have rates of photosynthesis proportional to CO2 concentration to about 1000ppm or so. (The desert plants you mentioned use CAM cycle, but overall their contribution to CO2 consumption is small.)