The term 'Holiday TREES' can be broken down into two crossword solutions: FIRS (4 letters) and TREES (5 letters). The solution FIRS is an acronym for the type of tree most commonly used as a holiday tree, the fir tree. Fir TREES are evergreen conifers that are native to many parts of the world, including North America, Europe, and Asia. They are popularly used as Christmas TREES due to their symmetrical shape and ability to retain their needles for long periods of time. The solution TREES refers to the fact that fir TREES are a type of tree. TREES are woody plants that have a single trunk and branches that grow from it. They are important for providing oxygen, shade, and habitat for wildlife.