Cervere is in there as a large non-plant creature in case I run into a fire-heavy mage (as is Renewing Rain bound to the Mage Wand). I thought about using the Grizzly, but if I'm up against a fire mage then the position control game with Thornlashers and Tanglevines / Vinewhip Staff becomes much harder to play, which in turn makes it difficult to use the Grizzly to full advantage. Theoretically, Cervere allows the build to transition to a more aggressive playstyle against fire mages; plus, Eagle Wings on a naturally Fast, Elusive creature seems way too awesome to pass up.
Tataree is really only in there because she's so cheap, both in book points and in casting cost; she was the last card that I added after I couldn't think of anything better for a single book point. She does give me an extra action marker and more mana for the Vine Tree, but I'm sure there's something more efficient for this book.