An electric fence is probably the best option but I've heard of people
driving nails through ply-wood and placing the boards around fruit trees,
nails up, so bears won't walk over the boards to the fruit trees. Bears
only go after ripe fruit so I assume you could move boards tree to tree. I
expect that bears would pull their paws away before any serious punctures
occurred, having been a bare foot hippie for many years. I was certainly
quick enough to avoid that kind of injury.