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In the decaying tree pics, is that a cedar tree or a Douglas fir (Christmas tree)? All of the Christmas trees around me in South AL are all Douglas firs. I don't know that it matters because I think they are both awesome for CNBG cover and I can't wait to round up a bunch of Christmas trees after the season this year.
I have used both and underwater there is not much difference in longevity. By mixing up types (assuming you can get them free off the street after Christmas) you get better results IMO because they decay differently thus providing diverse cover (some very tight some a bit sparser).