like re: deforesting
Where does he get his information on WestEuropean deforestation? Italy was probably deforested long before any collapse. Most of the Mediterranean Basin long before that (deforestation of Crete and So. Greece helped the downfall of Mycenaean Civilization - The Ceders of Lebanon plundered long before the Romans got to it.)
And Germany was hardly deforested, for instance. I mean, I'm sure the situation wasn't helped during the ca. 600 years of Roman Empire..

EROI.
Germany is a long way from Rome.
Imperial Rome managed to rid northern Africa and the middle east of all elephants, lions, and many other beasts, entertainingly killed at your local arena. By the time Rome fell, it cost too much to get wood from Gaul, and to get wheat and olive oil and pottery up to the northern provinces. In the mean time, relatively poor northern and northeastern tribes were sucked in to the apparent affluence of The Roman Empire. Millions of Roman citizens enrolled into slavery (what made them do so? What was so terrifying about freedom? Hunger?). An explosive (or implosive) cocktail if ever there was one.
Many lessons are to be learned from the demise or the Roman empires.