Today, this can have a significantly different answer than a few years ago.
Let me explain why. Logically, an event would happen and you the observer would be irrelevant. Meaning, that tree would generate its sound and shockwave when falling no matter if you are there or not. This is physically valid and was practically demonstrated on numerous occasions.
But...
Now-days there's a new theory around. The theory says the whole world is a computer simulation. We could be the creation of a post-human civilization with vast computing power that choose to run simulations of their ancestors in the universe or the creation of way more advanced aliens with us being just entertainment characters inside their simulations.
If you wonder how is this related to the topic, well, here's how: in any computer game, for content to be loaded (map, landscape, characters, running interactions) the player must be in that zone looking at it. Nothing is loaded into other areas because it would make no sense to use many times more resources for nothing (as in load everything else). So the presence of the player is needed for things to happen. Parallel to that, one may say that the same is possible in our world: if nothing observes something, there is no point in that something doing anything or even existing (loaded in the simulation and active). This is supported by some physical experiments. For example, In a study reported in the February 26 issue of Nature (Vol. 391, pp. 871-874), researchers at the Weizmann Institute of Science have conducted an experiment demonstrating how a beam of electrons is affected by the act of being observed. Electrons at the submicron level, i.e., at distances measuring less than one micron when under observation, are being "forced" to behave like particles and not like waves and when not observed they can simultaneously pass through several openings in a barrier and then meet again at the other side of the barrier (called interference), as in they don't do what they suppose to. What that could mean is that the simulation theory is correct and things do something they were programmed to do only when observed, meaning "the map is being loaded for the player" This could be the evidence you mentioned in the question. There are further things supporting it, like the sudden disappearance of civilizations which also can be associated with a game map going too chaotic and being cleaned (as in let only the outer textures and rebuild everything else because the script is too damaged to be repaired). Of course, it can also be an error in determination/measurement/experiment (or the observation does somehow physically interfere with the experiment's particles).
