What is the driving force of photosynthesis

explanation:

for  example, forests have higher evapotranspiration than other ecosystems,  such  as grasslands, because of their deeper roots and greater leaf area. thus forests have a net moistening effect on the atmosphere and become a moisture source for downwind ecosystems.

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.