Night Elfs have a unique ability to be able to place their wisps anywhere on the map when harvesting lumber as wisps do not have to return the lumber to the main tree. As a result Night Elf players take advantage of this ability and will place some of their wisps on trees in strategically beneficial positions.
There is an amazing discord server that is run by Kevin (check his stream out here https://www.twitch.tv/kevinwc3) which focuses entirely on the Night Elf race and their game play. You can find that discord here https://discord.gg/5ZckHkxa8X. Kevin and some of the guys from that server provided a literal encyclopedia to warcraft3.info on wisp placement and the trees on these maps which are hard or impossible to attack.
Find SOME OF THE BEST SPOTS here https://warcraft3.info/articles/305/encyclopedia-for-wisp-positioning
This post (while 2 years old) is still extremely relevant given that many of the maps described are still in the w3c map pool or in some tourney map pools. However, sometimes maps are edited with even some small edits to tree locations resulting in some wisp placements being impossible now. As such some of the images in that article may not be relevant anymore. The only way to tell for sure is to test them out.
Furthermore, on the Night Elf discord and Gym discord even more positions have been suggested via screenshots and videos as potentially good spots to position your wisps. Check out the below video for several of the maps (which is also several years old).
Below are additional videos and screenshots for each of the ladder maps from the NE discord and from Gym’s Discord outlining trees that can be mined and are hard to attack or impossible to attack.