Stack each kind of cookie together in pairs, using about ½ teaspoon of white frosting to hold the cookies together.
Dab ½ teaspoon of white frosting on top of a stack of 3-inch (7 cm) cookies, then place a stack of 2-inch (5 cm) cookies on top. Dab ¼ teaspoon of frosting on top of the 2-inch (5 cm) cookie, then place a 1-inch (2.54 cm) cookie stack on top. Dab another ¼ teaspoon of frosting on top, then finish with a stack of ½-inch (1.24 cm) cookies. Set on the prepared baking sheet and repeat with the remaining cookies and frosting to make 12 “trees.”
Transfer the baking sheet to the refrigerator to firm up the frosting, about 15 minutes.
Carefully pipe the green icing around the bottom ledges of each cookie tree, then decorate with white and red pearl sprinkles. Repeat with the remaining ledges between each cookie layer, then pipe a small circle of icing at the top of each tree and finish with a single sprinkle. Return the pan to the refrigerator to firm up the icing at any point if needed.