To actually eat the trees? My mini donkeys gnaw on the bark of any trees in their pasture ... no pine trees though.

We occasionally throw white pine boughs into the pasture. Both the donkeys and goats will eat the needles. A few days later, I remove the stripped branches. They wont eat those, other than some nibbling.

I read somewhere once that pine needles are a good source of vitamin C. I've never tried to verify that its true.