Hi Dave. A few more thoughts.
Yes, these trees do stand upright for many years. Of course, we typically drop them where there are several feet of water over top of the trees (safety reasons in public waters). If you let them sink, and release them upright, they do stand up. I don't think they would come up with any type of "normal" fishing line, but those flathead catfish guys might dredge one up? :-) Remember, the wood also waterlogs, and they get pretty heavy. When we were doing studies on yellow perch spawning, we'd lift them with a 3/8 or 1/2 inch poly rope.
Obviously, you never can say never, as some might tip. However, last summer I went over a 200-tree reef that we placed through the ice about 10 years ago. Many, many of those trees (no way or really telling) are still standing upright as shown on my electronics while fishing.
I don't have a good feel for just how much those 12 inch bases weigh when they have 6-8 inches of concrete. Let me check in with my buddy at the Game, Fish and Parks office tomorrow. He's a practical guy, and may have a guess for you. I also wonder if the 12-inch width wouldn't be a lot more stable than a 6-inch diameter on the base. We got those carpet tubes at a carpet store -- nothing special, to the best of my recollection.
I have at times just sunk trees in piles, using cinder blocks or concrete blocks for weight. I'm not sure that those light cinder blocks can take down more than one Christmas tree. A heavy cement block can take down two or three, depending on how big they are. If you have some cedar trees, they take quite a bit more weight. I remember one time working with a bass club in Kansas. I had one cement block cabled to a good size cedar tree. I pushed it over, and it floated! That's a tad bit embarassing, and hard on your credibility! Now, it's mostly just funny.
If you attach trees to the blocks with wire or cable, I'd suggest you stack them pretty several trees deep. As they settle, you want to maintain some vertical height on the overall structure. Also, I've seen people nail, screw, or wire Christmas trees to pallets to create 3-d structures (pallet on bottom, four trees at corners, pallet at top, etc.). Those are good ideas, too.
I like the idea of 10-tree groups (depending on whether you stack them or not), and I really like putting the trees on the edges of drop-offs, etc., as you said. Good plan.
Again, I'll bet you'll get some more ideas here over the next day or so.
Dave
