Mystery numbers

A big blue '3' and then an arrow pointing to a green '1' near Uithuizermeeden on the cycling path that we followed on one of our recent hikes. There were no other numbers, and there was no explanation to what they could mean. Any suggestions?

I'm linking to signs, signs.

41

I have no idea why one tree is marked '4' and the other '1'.
What I do know that together they make 41 ;)