Is there a more efficient method of planting trees?

Unfortunately, the only way to currently plant trees in the game is one-by-one.

There is a Tree Brush which will allow you to place multiple trees at a time, but it looks like the mod hasn't been updated since the latest patch (which broke a lot of mods), so it's not currently working. Since it's a mod, it will disable achievements. There is a mod to prevent mods from disabling achievements, but it appears to have broken with the latest patch as well.