'flade' in this context is 'terræn'(Clausens tekniske ordbøger and Eurodic). The area(s) of the forests that were damaged. Therefore I would suggest surface, area, expanse, level, depending on the context.

'Fald' could mean both damage, or 'skråne' drop or tumble.

Lone

Resembles what happened here in Washington when Mt. St. Helens blew up.

May sound a bit of an understatement (I flew over southern Jutland the day after the storm in December and saw the extent of the damage to the plantations myself!) but I do believe it is the correct forestry term. You can differentiate between degrees of damage thanks to the flexibility of the English language - e.g. total destruction, single trees (or clumps) still standing etc. Depends on the context.

Robin Brown. Network Communications A/S