No need to find them every time. If forest is a smart object, and it should be, as soon as it’s spawned, make sure to add it to some list that you will keep in Game Mode (for single player), or GameState (for multiplayer). When you have location coordinates in the list, you can calculate which one is closest one, and send AI’s to chop wood.
This will be optimized, as you don’t have to use GetAllActorsOfClass at all. Calculating distance between vectors is kinda cheap, and you should be good.