Trees (all green plants really) are the main reason why we are all alive. If you think about it, trees actually represent and in their actions fulfill many of the principles we talk about and admire, hope to achieve, etc
in MA and aikido perhaps more specifically: the general image to most people is that animals react to their environment and other organisms and trees just.. are. But a tree has no "fight or flight" response, it has to "blend in" and turn a conflict into harmony (eg. a fungal disease, or a wound).
From a more biological and less metaphorical point of view, trees have no nervous system and yet are in perfect chemical equilibrium with the environment; they are one of the few kinds of organisms (that we can see with the naked eye, at least) that convert solar energy into chemical energy. To me that is transcending in one way or another something animals cannot do that easily (or at all)....