Much like how she describes the green grass as a couch they are lying on, so she describes the trees that surround them as if they are in their future home together. When she describes the trees as beams of cedar and rafters of pine, she is describing a home that is safe and well built. She is exalting in the enduring firmness of their relationship. This imagery is one of many indications in the Song that the woman is not just seeking sexual satisfaction, but a lifelong relationship of enduring marriage in which they build a family together.1
17 the beams of our house are cedar; our rafters are pine.