The foundation of the symbolism for this Ketubah is that three make up the marriage - the husband, wife and Hashem. They are represented by the three trees. The trees all stand alongside and their branches have become intertwined to nest and support the text of the Ketubah, as if from one tree.
The idea that with Hashem being part of the marriage they'll create a strong foundation to grow their life and family from.
The doves are sitting settled in the comfort of trees - representing the wish for peace to remain in the couple's life. The stillness and calm within represents the root of the couple's unity.