Complex sentence: Researchers are aiming to use genetic techniques to make poplars grow fatter and with a smaller canopy, so that more trees can be grown more quickly in a small space, and to make the plants contain a higher proportion of cellulose to lignin.
Split into simpler sentences: Researchers are aiming to use genetic techniques to make poplars grow fatter and with a smaller canopy, for example, so that more trees can be grown more quickly in a small space. And they would like to make the plants contain a higher proportion of cellulose to lignin.