It can be difficult to identify trees/shrubs at this time of year due to the absence of leaves. As mentioned previously, be certain that you are gathering from the correct tree/shrub. There are many good resources online. My favorite go-to book is Minnesota Trees, by David Rathke.
Oak trees are one of the few trees with leaves still hanging on to the tree at this time of year. There are both red oak and white oak trees - the red oaks have pointy lobed leaves and the white oak have rounded lobed leaves. I just came across this pdf from the University of Minnesota. Check out page 9 for oak identification:
Be certain to make any cuts to oak trees while dormant, before sap runs and the tree begins actively growing again. The chance of developing oak wilt from and open pruning wound is a high risk that would be avoided!
I make both a tincture and an oil from white oak bark.
Prickly ash is a native, weedy, understory shrub, not an ash tree at all, but is actually a distant relative of orange trees! My biggest tip - wear leather gloves when gathering and processing prickly ash. It is called "prickly" for a reason!
Beware - it is easy to confuse prickly ash trees/shrubs with black locust trees. I would suggest comparing pictures online. Both have 2 thorns on either side of the buds, but the buds of the black locust are hidden under the bark and the buds of prickly ash are clearly visible. Black locust also looks like a typical tree (with a trunk), whereas prickly ash typically grows in clumps (like a shrub) and the branches are moderately straight.
To the right are branches of prickly ash that I gathered last spring.
I make both a tincture and an oil with prickly ash.
More good information on prickly ash can be seen at: https://www.friendsofeloisebutler.org/pages/plants/pricklyash.html
I just realized I've never made a tincture or oil with white willow bark, one of the best known herbal remedies. Perhaps I'll have to try that this year.
Do you have some different barks you use often? Or any questions about some you would like to consider? Please share!