I also was thinking about how trees only have a canopy of green, whereas a field of grain has just as much green covering the land...just not separated from the ground by trunks. And I have seen a fully grown field of rapeseed or canola. That is a six foot thickly woven dense mat of nitrogen fixing green stuff, which is used to produce precious oil, some of which can substitute for fossil fuels in the way of biodiesel or can be manufactured into plastics or lubricants.