Because trees are larger and heavier than other plants, they need a much deeper soil system for their roots. (rooflitesoil.com)
At the same time, organic carbon frozen in deeper soil layers will decompose and enter the atmosphere. (newscenter.lbl.gov)
Another advantage bioenergy grasses and switchgrasses have over corn is their deep root system which allows them to draw water and nutrients from deeper soil levels and allows them to be more resilient in poor growing seasons. (sciencedaily.com)