Living trees often make very good anchors. In the most basic rigging, a loop of webbing is tied around the tree, and the rappelling rope is threaded through the rapide or rappel ring on the webbing. More rigging techniques are discussed in Rigging Part 1. If the tree is still alive, that means it still has roots. And that generally means you're safe to rappel off of it. If you're nervous, you could having a few people try to push the tree over. For greater strength, rig the tree near the base. The BSA requires that "trees used as anchor points in a climbing anchor system must be well-rooted, healthy and at least 6 inches in diameter at the point of attachment, and protected from damage."