Concept designing impaired.

Whenever I sit in front of my trees with a pencil, I choke. When I have no paper or pens in my hand I am full of inspiration, but I can't draw it. So this is my question, how do you do concept designs?

Practice drawing real trees as they currently exist to develop your drawing skills. You will find that you start to change things to make the tree look better as your skills develop. At some point you will be able to draw a vision of the tree based upon the potential. It has taken me many years after my mother (also my art teacher) died to get to the point where I can get a vision of the future tree on paper that I understand. I am still not good enough for others to see my vision very often.

Try graphite sticks rather than pencils. Practice with them in drawing abstract shapes (it'll come pretty easy) and once you get the hang of them I think you'll find that you're able to sketch objects in a very fluid way. It may not result in a lot of detail, if that's what you're aiming for, but you'll find that you can pretty easily construct a decent approximation.