Preheat your oven to a gentle 200°F (95°C). Line a baking sheet with parchment paper – this makes sure your delicate trees don't stick!
In a large, clean mixing bowl, combine the room-temperature egg whites and cream of tartar. Using an electric hand mixer or stand mixer, beat them on medium speed until the mixture becomes wonderfully frothy, like the foam on a cappuccino.
Gradually add the granulated sugar, one spoonful at a time, making sure to mix thoroughly after each addition. Continue beating until the meringue forms stiff, glossy peaks that hold their shape perfectly when you lift the whisk.
Gently mix in a few drops of green gel food coloring until your meringue is beautifully and evenly green. Remember, gel coloring is best to keep your meringue light and airy!
Transfer the green meringue into a piping bag fitted with a large star tip. On your prepared baking sheet, pipe three dollops of meringue on top of each other, starting with the largest dollop at the bottom and getting smaller towards the top to create a charming Christmas tree shape. Don't forget to sprinkle on your festive decorations for ornaments!
Carefully place your Christmas trees into the preheated oven and bake for 45-50 minutes, or until the sides feel dry and they lift cleanly from the parchment paper.
To prevent any cracking and ensure a lovely chewy center, turn off the oven and let your meringues cool completely inside. You can leave them overnight, or simply crack the oven door open for about an hour until they are fully cool. Enjoy these light, melt-in-your-mouth treats!