No Trespassing signs popping off the trees

Trees grow radially, dry wall screws don't move and the tree will grow and pop off the sign unless you leave the screws long. Nails might slip but normally you need to leave them nailed with about an inch of nail showing to account for tree growth. I was trained to use long aluminum nails to attach signs to trees so that some day if that log ends up in mill it don't destroy a tooth on a saw. Of course a standard metal detector will not pick up aluminum.

We use roofing nails with the big head they tend to push out as the tree grows
or until the plywood rots and falls off

I used to survey long ago and got to see the results of nailing in trees. Sometimes the nails pulled out but most of the time they held fast and the tree grew around the sign or popped it off. I am volunteer boundary maintainer for the Appalachian Trail in Maine and I was taught long roofing nails with about a 1" gap. This unfortunately makes the corridor markers easy to steal but the area where my section is located is very remote and I expect there are far more moose and bears than people who find them as its decidedly a very steep and cliffy chunk of trail.

I just read the other day on what states use instead of signs. Here in Illinois it is purple painted on your line- trees-posts what ever. You also must say for one year on a sign that there is no trespassing posted at the easiest point of your land to enter. From what I have read they choose purple because color blind people can see it.

In northern NH no trespassing signs are pretty rare. Most undeveloped land is taxed under "current use" which is a major tax incentive for not developing it. There is 10% surcharge for posted land. The other big reason is that there is recreational liability statute in NH that absolves the land owner of liability if someone from the public gets injured on the property. If the land is posted no trespassing the statute does not apply and the owner is liable for anyone getting injured on their property and most likely will need to buy insurance to cover themselves. The bar is set quite high for anyone trying to sue an owner that doesn't have his land posted, it pretty much comes down to unless the owner deliberately set up a trap to injure the public, the public can not sue and generally the case is kicked out of court at the preliminary hearing. There are usually a couple of cases a year where lawyers try to argue their way around the statute usually using the "attractive nuisance" argument but rarely do they even make it to court. Maine has a similar statue on the books but I am less familiar with it. Over the years I have heard of some quite gruesome accidents where people on snowmachines, off road bikes and ATVs run into steel cables stretched across roads as gates or barbed wire fences and generally the owner is not liable.

An owner can still allow the public in the property but can really cut down on abuse by gating the entrances and banning motorized vehicles. In NH ATV and snowmobiles trails are both by permission only of the owner and the clubs that maintain the trails are pretty careful to make sure they have landowner permission. Most hunters will rarely hike more than mile or so past a gate as they need to lug out any game so depending on access it generally not an issue with large landowners.

I would probably just reflag the line but it is smart to be proactive on this as every state has different laws regarding "squatter rights". Generally the clock start ticking again when property changes hands unless the prior owner acted to formalize the rights. In most states only a surveyor can make permanent boundary markings like paint and ax blazes but as far as I know, replacing the flagging is legit.

There should be steel posts in the ground from the survey. Wooden posts that are flagged can be placed along side to make it easier finding them. Our property's original survey had a blaze on a white pine (1950-60?). The pine is still there but the blaze is higher and the tree is bigger!

We have had issues with Ministry of Transport and a local roads board about property boundaries. Years ago, one of the maintenance crew came onto our land with the excuse of brushing ... he was snooping to see what equipment we had there. Cut down about a dozen naturally seeded white pines in the process (of course he didn't bother to cut the scrub willow that is more of a nuisance). He totally ignored the survey pegs ... there are 4 bends on the highway frontage and 3 on the side road. Turns out the side road is actually on our land - having grown from a single dirt lane (that ran along the edge) to a two lane gravel that keeps getting wider every time they work on it. They appropriated the 10% they were allowed already. Ministry figures they have the right to keep on doing this but I'm thinking not ... we suggested a swap of crown land on the north side. Nope... guess we have to take them to court to get it sorted out.