Most of the oysters I've found are almost always on a dead alder.

I am finding both Pleurotus ostreatus and Pleurotus dryinus in TN. I found my first Agrocybe aegerita 2 weeks ago growing on a maple tree. I can't understand why more people don't talk about or eat this species, it's delicious! Here is a link to the ones I found.
groups.msn.com/SouthernShroomin/shroomso...hoto&PhotoID=268
Dave