Tall Tree Island is an island that first appeared in "Appetite for Destruction".
The appearance, geology, flora, or fauna (other than dragons) are not seen or discussed for this island. However, based on the name "Tall Tree", it might be assumed that the island was able to support old-growth forests before a single dragon destroyed it.
During the events of 'Appetite for Destruction", the Dragon Riders survey dragon populations on several islands. Fishlegs marks the dragons of Tall Tree Island with green paint. As the Riders each mark dragons, they realize that dragons from one island are ending up on the others. They discover that these islands are being systematically destroyed and the dragon populations are migrating. A Screaming Death is responsible, destroying islands as it makes its way to Berk.