The Merge Dragons Spooky Trees sprout Necromancer Grass when there is free space around them although they have a limited supply. You can also have your dragons harvest the Spooky Trees for Decayed Logs, but be careful for they will turn into Decayed Logs after a certain amount of harvests.
You can merge the level 4 Spooky Old Trees for a Sunken Treasure Chest.
How To Get Spooky Trees In Merge Dragons?
Merge Fungus Logs
Rewards from levels on the World Map
Sometimes from tapping Haunted Houses
Will Spawn 10 Necromancer Grass, and harvest 3 Decayed Logs
Deathly Hollow Trees
Will Spawn 20 Necromancer Grass, and harvest 5 Decayed Logs
Spooky Old Trees
Will Spawn 30 Necromancer Grass, and harvest 8 Decayed Logs
Step By Step Spooky Trees Guide
In these images we will see the left Undead Tree stay in its full form. On the right we will see the Undead Tree shrink each time we harvest it until it becomes a Decayed Log.