I'm not sure what this is growing on. It's in one of those wells surrounding trees on a city street that I walk by several times a week. The tree is a non-native hardwood but the fungus is growing at the edges of the concrete, rather than at the base of the tree. There are two individuals. The larger fungus is 24 cm across. I'd guess that it's been there more than one season and I just never noticed before it began to put forth some new growth. 8/3/07: I've added some pictures of the tree. Turns out they're common street trees here in Santa Rosa. I believe it's Celtis occidentalis aka Common Hackberry. There's also a picture of the area with the G. applanatum outlined. This particular tree has a deep root system which makes me think it's not the host. It's hard to tell if the live oak seedlings are from underground roots or just seedlings that have sprouted.