On a tree trunk grow salad-green leave-like structures. At least, in this wet autimn they are clear gree, when it is dry they will turn brownish. They are from a lichens called Tree Lungwort (Lobaria pulmonaria), one of the biggest lichens of France.
Its thallus (the "leaf"), a long, lobed shape more or less hanging can get until 40 cm long. Here, the thalli are much shorter. If you look from nearby you see the thalli have different colours and shapes.
The lichen is a slow grower, and there are younger and older thallu present on the same tree.
Their surface is reticulated and on the ridges grow little soredia, here visible as a kind of grey-green powdery stuff. Soredia assume the vegetative reproduction of the lichen, they contain samples of the ascomycete and the cyanobacter that live together as a lichen, and that can move on towards another tree.
Tree Lungwort is an organism of older forests with old deciduous trees, in Dordogne Oaks and Hornbeams. Because it is sensitive to certain kinds of air polution, notably sulfur dioxide, it has disappeared from industrialized and urbanized areas. In Périgord it can still be found, here and there.