Yeah, I've heard the "Yggdrasil is a yew" theory as well. It's toxin is apparently hallucinogenic in small amounts, and it also sheds it's toxin during the warmer months, meaning people sitting under it will have "spiritual journeys". The possible explanation I've heard for why it's called Askr (Ash) Yggdrasil in the kenning is that Ash trees are synonymous with men, because of Ask from Ask and Embla, so they're basically saying the tree has a masculine presence. We may never know, though.