They're interested in carbon sequestration. Trees store carbon for long periods. In a short amount of time grasses grow, fixing CO2, then die and decompose, in the process releasing most of that fixed CO2. Trees grow (sequester carbon) for decades or centuries, then are often cut up to build paper and houses, maintaining that storage. Additionally, a lot of work is being done on determining the effects of increased global CO2 and temperature on the worlds plants. Grasses have a C4 metabolism, which is not carbon limited, increasing CO2 will not appreciably increase growth. Most trees have C3 metabolisms, and will increase growth with an increased atmospheric CO2. Increasing atmospheric CO2 also lowers water use, the plants don't need to "breath" (open stomata) as much. In many parts of the world, forests are a large contributor to weather, less transpired water means less rain.