The tree in the centre of the pool is a full tree (not cut down), its shown this way so you can see the rest of the map under its canopy. Of course you could have the tree cut down, if it works for you!
If the party is hostile to the grove, tree roots can burst from the ground to attack or grapple the party.
The water could be magically charming, if a player steps into it they make a saving throw or are charmed to walk slowing towards the tree until they take damage or save.
Have your party play a game of ‘The Floor (water) is Lava’ and have them move across the stones to reach the tree, with corresponding skill checks.
If a party member gets completely submerged in the water they have a chance to turn into a Satyr (Goat person!) over the next 5 rounds. Do with this as you will, depending on how evil you are feeling!
The Forest Guardian stands on the raised cliff and commands his minions to protect his grove.
The party notice blood in the water as they make it to the clearing. The King/Queen of the forest (any giant forest beast) has been killed in the pool, the forest is angry! The party is a convenient outlet to this anger.