Sometimes you need a lateral view of what you are looking at, a simple approach to this for me is a Gallery Run. Any collection of galleries will do, but London works well for me as you can hit three big galleries in one day The National, Tate Britain, and Tate Modern.
Task: Visit a number of galleries and take photos of, and notes about, Art which features your subject matter. Look for trends, patterns, omissions, peculiarities. See where your chosen subject is central to works or peripheral, represented directly or inferred. Observe how the history of Art deals with your subject, and Let the process give you ideas for your own work.
These are curated collections, so they are only a slice of existing work in any case, but their selection tell you something. Plenty of ornamental trees in National, Trees a little more foregrounded in Tate Britain, virtually no Trees at all in the Tate Modern collection.
The Trees of Tate Britain.
Again, you can explore the images taken and cut and combine them to interesting effect. Sometimes this suggests ideas.
Of course while you are about the gallery, or afterwards surveying the photos, the notebook is the place where observations are recorded for later consideration or action.
Here are some notes taken from the Gallery Run:
Trees as things to hang things off: Laundry, signs, bodies.
Trees as things stand next to.
Trees as Royal Property.
Trees as skeletal bodies.
Trees as things to sit under and sleep, dream, think.
Trees as paradise, as boundaries to the occult.
Trees as things to climb, cling to, sit in.
Trees as a consequence of ruin.
Trees as giants in the mist.
Woods as hunting grounds.
Woods as places to hide bodies.
Woods as places to conduct liaisons.
Woods as frames of the city and the farm.
Woods as blind puppet masters to the animals within.