A key to any form of self-expression, artistic or otherwise, is the faculty for observation. This is because at its root nearly all expression is a form of communication, and if the subject of the communication is not mutually intelligible then it fails. The fastest route to mutual intelligibility is proper reference of common experience, i.e. the world around us. The failure to observe and communicate shared experience creates a lot of bad, bad art.
Not to beat this horse much longer, but the reason you need to go back to the world and draw actual trees that you actually experience in person is while you may have an intuitive and/or symbolic understanding of what a tree is, but you seem to have no real understanding of a tree as a process or a holistic system of objects and processes, or, in other words, why trees do what it is that trees do? Why do trees have roots? bark? leaves? why do they grow up rather than sideways (except when they grow sideways, and then, why do they do that?) The answers to these questions, far from being academic, will help inform and improve your art, I promise.
By way of illustration and entertainment I present an excerpt from Mark Twain's essay "Fenimore Cooper's Literary Offences":
If Cooper had been an observer his inventive faculty would have worked better; not more interestingly, but more rationally, more plausibly. Cooper's proudest creations in the way of "situations" suffer noticeably from the absence of the observer's protecting gift. Cooper's eye was splendidly inaccurate. Cooper seldom saw anything correctly. He saw nearly all things as through a glass eye, darkly. Of course a man who cannot see the commonest little every-day matters accurately is working at a disadvantage when he is constructing a "situation." In the "Deerslayer" tale Cooper has a stream which is fifty feet wide where it flows out of a lake; it presently narrows to twenty as it meanders along for no given reason, and yet when a stream acts like that it ought to be required to explain itself. Fourteen pages later the width of the brook's outlet from the lake has suddenly shrunk thirty feet, and become "the narrowest part of the stream." This shrinkage is not accounted for. The stream has bends in it, a sure indication that it has alluvial banks and cuts them; yet these bends are only thirty and fifty feet long. If Cooper had been a nice and punctilious observer he would have noticed that the bends were often nine hundred feet long than short of it.
The whole essay which is worth the read and easily available online is, in fact, a rebuttal to willful amateurism, something that I know that I, for one, need to be careful of.
