I do not have experience with Paulownia but I'm very interested in the concept of growing/using trees for forage. I look forward to following your research. I am in SW Michigan in Zone 6a and am compiling a list of potential forage trees.

Tree prunings I have fed to my sheep (after checking for toxicity, of course) are autumn olive( this agressive plant is already growing here but is too invasive/agressive to plant as a forage tree, in my opinion), mulberry, apple, and poplar. All were relished by the animals.

I suppose some of the characteristics one looks for in  good forage trees are similar to what one would look for in selecting grasses, legumes and forbes(does it grow on my site, is it palatable, nutritional value, yield, tolerance of repeated harvest etc.) but what criteria are particular to selecting trees for forage?

What trees have been used historically for forage?