Shelf-like fungal growths on trees are known as conks
Fungus identification is best left to experts, but I think the “conks” shown in the pictures above are the species Phellinus Tremulae. The photograph in the upper right is a close-up of the porous bottom-side of one of them. So far I have seen these growths only on the aspen trees locally called “popples”.
Phellinus Tremulae fungus is notorious for causing heartrot, which is the decay of the heartwood of live trees. This weakens trees, and aspens infected with this fungus are susceptible to toppling over in the wind.
Phellinus Igniarius is a very closely related fungus that is usually found on willows. I mention it because of a strange use. Alkaline ash produced by burning this fungus enhances the effects of natural plant alkaloid compounds, like nicotine and caffeine. Alaskan Native Americans mix tobacco with this fungal ash to make a concoction they call “iqmik”. Chewing it provides a dangerously powerful dose of nicotine.
North MN Outdoors 