There are accounts that trees in some areas of the western front had absorbed enough mustard gas that they would occasionally poison people trying to clear them some 50+ years after the war...
I had a science teacher tell me something like that back around 1970. I was young and empty-headed and I pictured the scenario as a low-lying dip in the land, with some sort of lingering fog of mustard gas just waiting for an unwary traveler to stumble down the hill and die, 50 years after the fighting stopped. As I grew older I decided that the teacher made the whole thing up (because my scenario was preposterous). But perhaps the tree aspect is more believable.