I want to very respectfully disagree with the conclusion of this post. Jessy asserts that, "I think he is suggesting that humans and nature are not separate entities. Rather, that they are united and should work together."
In my opinion the message is the exact opposite, that humans and nature are in fact very separate, disparate objects. However I do agree that there is and should be a working and complimentary relationship between both man and nature as represented by the tree and traveler. And I think the Yew tree is meant to symbolize and demonstrate just that.
In researching the Yew Tree to better understand the text I came across an interesting text which discusses the historic understanding and meaning of Yew Trees, especially in older times. But most important from this text to me was the notion that there is this two-way relationship.
It is not so much that they are separate entities, but rather, they compliment each other. On the one hand, humans seek shelter and wisdom - both of which seem to be provided by the Yew; at the same time, the Yew needs humans to avoid the loneliness of being an immobile tree. So rather than viewing these two as one in the same, we should look at both man and nature as separate entities that have a mutually beneficial relationship.