The Christmas trees are supposed to create structure for fish. I knew somebody once who told of doing the same with old bedsprings.That's what I generally do, along with fallen branches and other tree trimmings. It'll rot down eventually.
Wouldn't it be easier to chip old Christmas trees? That's what they do around here.
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Come to think of it, he was doing that so that he could "harvest" fishing lures after people snagged the bedspring and cut/broke their line. I'm not sure if it actually worked...