I'm not sure that we have an expert on medieval forestry qualified to speak to all periods, if any exist. I will say that Europe was more forested earlier in the period and much less so later in the period, but trying to quantify it seems to me a fool's errand. We don't have the kind of sources to be able to say, for instance, "in 500 AD Gaul was 38.7% forest."

That said, it may be useful to understand that from about 900-1300, the European economy and population grew rapidly and markedly, which placed a great premium on arable land, to the point that bogs and marshes were drained and land reclaimed from the sea! Many forests fell to the axe; by 1100 AD or so, western Europe had been significantly deforested, and the remaining woodlands became precious resources, especially to a nobility with a passionate devotion to hunting. This led to the establishment of royal forests in England and later France, where the ordinary people were forbidden to cut wood and take game. I don't believe there would have been significantly more forest than there is today.