I think the first thing we have to do is decide what we mean by 'more important'. Whether something is important or not is essentially a question of value: what do we place value in/consider valuable, what don't we place value in/consider valuable. In philosophy, theories which attempt to explain why we value certain things (and indicate what we should value) are known as 'value theories' or 'theories of value'.
Value theories come in all shapes and sizes, and there really isn't enough time or space to into these things in detail (never midn that I'm not exactly qualified to go in-depth with any real confidence), so I'll just skim some the basic issues/questions related to value theory.
First of all, we have to ask whether value is relative or absolute. Is a tree's value subjective, i.e. does its value change depending on who or what is 'looking at it'? For example, if value is relative, then a tree may have great value to a badger living in a den underneath it, whereas the same tree may be worth nothing to a farmer looking to clear land. These may seem like obvious, common-sense statements, but the point is that if value is relative, then nothing is valuable in and of itself. This has profound consequences for 'common-sense' or normative notions of right and wrong. If value is relative, and Man 2 is more valuable to Man 1 as a slave than he is free, then some would argue that Man 1 does nothing wrong by enslaving him. This latter point morphs into moral theory, but gives you an idea of the consequences. Relative theories of value are usually referred to as 'Agent-relative' value theories.
If value is absolute, then things possess a given value which exists independently of experience; i.e. a tree has value, even if it is of no use to us or we simply do not consider it valuable. For example, many believe that life or freedom are things with inherent value, and on that basis argue that slavery is morally wrong. Again, this ties into (and is affected by) your particular moral theory, but value theory and moral theory really are two sides of the same coin. Absolute theories of value are usually referred to as 'Agent-neutral' value theories.
We then have to ask if something's value is inherent/intrinsic or instrumental. In other words, is something valuable in and of itself (e.g. "life is sacred", "Live free or die"), or is it valuable as a means to an end, i.e. acheiving something else we consider valuable (e.g. "By creating a culture in which life/freedom is valued, we will have a better quality of life overall)? Some would say things have an inherent value and an instrumental value. For example, freedom might be viewed as having inherent value, but it could also be argued that societies which promote freedom are better in terms of other things we consider valuable (e.g. quality of life).
There are other distinctions concerning value, but these are less important and slightly more technical.
Personally, I view value as relative, not absolute, but not subjective. In my view we can justifiably place different values on the same things according to our circumstances. However, I also think the value still has a direct or indirect link to the objective world (e.g. the value of food as a means of survival). I don't think things have inherent value, or any worth in and of themselves.
So basically, if you ask me whether or not a human life is more important than a tree, I have to ask three questions:
1) Which tree's life are we talking about?
2) Which human's life are we talking about?
3) Who's asking?
Personally, I don't think this question (or any similar question) can be answered in a meaningful way without that information. But that's just my opinion!
P.S. Forgive any spelling mistakes, I was in a rush and did not have time to proof-read this. Off to see Chromeo live, w00t!