Well, I know that with the Ancient Celts the tree of life was a big thing to their culture, to the extent they believed they were trees, or came from trees. They saw them as the symbol for balance, and would leave a big tree to stay on their land because of their beliefs that they were magic (as they provided a multitude of uses such as food and medicine); because of this, cutting down a tree of life was seen as an offense.
So in a way, the Celts were kind of like caretakers of the trees, but not necessarily forests.
According to a bit of further research, I've found somewhat indirect answers to your questions.
The trees were also seen as a symbol for rebirth and long life, because they lose and regrow their leaves each year. So everlasting, yes I guess you could say that. As for indestructible, that's debatable.
It's also said that the trees were a gateway to the spirit world; that the tree of life's roots connect to the world below, while it's branches connect to the world above and it's trunk is the link that connects both of them to the Earth, which is how the Celts supposedly communicated with the gods. In this sense, I would personally say that it's safe to assume it can be a defense against impurities and corruption, especially when you factor in medicine being available from trees.