Hey, welcome! Another wisco buckthorn fighter- I've been stewarding a
local park that is also suffering from the buckthorn takeover.
I've found the DNR of Minnesota to have a really helpful resource for dealing with buckthorn. Perhaps not immediately relevant to you, but maybe to other buckthorn fighters. As is
this study from DePaul University, a particularly relevant excerpt is this:
In addition buckthorn may modify soils in other ways. For example, a greenhouse experiment by Klinosky et al. (2013) compared the effects of European buckthorn foliage and Acer saccharum, a native tree to southern WI that has a similarly high nitrogen content in its foliage. They concluded that buckthorn inhibited germination of other plant species, and showed that litter produced by buckthorn inhibits the growth of the seedlings of four native trees (Klionsky et al. 2013).
Unfortunately, animal browsing and interfering vegetation are the leading causes to slow regeneration per my city's forester. I would definitely recommend animal deterrents- be that as simple as a
fence for rabbits, a dog for the squirrels or just hunting- really whatever works best for you. Natural deterrents could be another route to go with.
Regardless, I think repeated plantings are sometimes just the nature of things. I've had some luck without repeated plantings with black locust, beech and aspen in a few spots I'm trying to regenerate. That being said, those spots are near ferns, black cap raspberry patches or other natural deterrents. The spots that aren't protected or "warded" by plants certain animals don't like, those are the spots I've had to come back to and replant.
Hope this helps some, and best of luck!