If left untreated, wood will begin greying, which is caused by UV rays. Any wood that is left outside exposed to the elements will eventually rot. The speed of the decay depends on one thing only: Is the wood allowed to dry?
Four conditions rot needs to occur:
So what is the one thing that causes decay?
Fungi. The simplest of all plant life. Microscopic fungus spores are all around us. Wherever they land, if the four conditions above are present, then you will have rot, and that rot will continue as long as those conditions are present.
Even if you remedy the conditions, the fungus is still present and will resume growth (aka rot) when the conditions are more favourable.