Spray a large bowl with cooking spray and pour Trix cereal into bowl. Set aside.
In a small saucepan, melt butter over low heat. Add marshmallows and stir continuously until completely melted.
Pour melted marshmallows over cereal and stir to coat. Use hands to form cereal mixture into balls. Place balls on a baking sheet coated with cooking spray. Allow balls to set for a few minutes before gently inserting a popsicle stick into each one.
If desired, use green playdough to display each tree.