Observing an experiment changes its outcome. Under the laws of quantum mechanics until someone observed that the tree had fallen it would theoretically exist in its pre-falling state, its fallen state and at every point in the arc of its fall.
The tree falling scenario is a popular but flawed dumbing-down of the Schrödinger's cat paradox, where a supremely unfortunate feline, a container of poison, a geiger counter and a radiactive element are placed together in a sealed box. Until the box is opened to observe the results of the experiment the cat can truthfully and paradoxically be said to be alive and dead in the same instant, that is if you accept quantum law.