Why does the verse speak of green tree for lighting a fire and not a dry tree or wood that burnes easier!?
It might be due to the fact that the green tree is able to catch carbon, and to emit oxygen, and by such chemical processes store up some of the sun's energy, where as a dry and dead tree cannot do that.
This process also illustrates the resurrection or revival of energy too.
Any how, although kindling a fire seems to be very simple a matter to us, it really is one of the most wonderful phenomenon in the nature, the process of which is vital for the life of creatures upon the earth.