Shapechanger. The mimic can use its action to polymorph into 12-foot, fully-decorated evergreen tree or back into its true, amorphous form. Its statistics are the same in each form. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.

Adhesive (Object Form Only). The mimic adheres to anything that touches it. A creature adhered to the mimic is also grappled by it (escape DC 14). Ability checks made to escape this grapple have disadvantage.

False Appearance (Object Form Only). While the mimic remains motionless, it is indistinguishable from an ordinary holiday tree.

Grappler. The mimic has advantage on attack rolls against any creature grappled by it.

Innate Spellcasting. The mimic's innate spellcasting ability is Strength (spell save DC 14, +6 to hit with spell attacks).  A creature that is restrained by a spell the mimic casts is considered to be grappled by the mimic.  The mimic can innately cast the following spells, requiring no components or concentration:

At will: druidcraft, thorn whip

3/day: ensnaring strike, entangle

1/day each: grasping vine

Pseudopod. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 13 (2d8 + 4) bludgeoning damage. If the mimic is in object form, the target is subjected to its Adhesive trait.

Bite. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 13 (2d8 + 4) piercing damage plus 9 (2d8) acid damage.

Ornament Barrage (Recharge 5–6).  The mimic hurls ornaments from its branches in a 15-foot cone. Each creature in that area must make a DC 14 Dexterity saving throw.  On a failure, 4 (2d6) ornaments adhere to the target, or half as many ornaments (rounded down) on a success.  Each ornament is actually a miniature mimic.  An ornament mimic is a Tiny monstrosity with AC 6, 1 hit point, a 2 (−4) in every ability score, and a speed of 1 foot.  An ornament mimic can be killed by normal means or scraped off using an action. At the end of each of the target's turns, the ornament mimics excrete acid, and the target takes 2 (1d4) acid damage per ornament mimic adhering to it.