What is attacking my young trees and how can I stop it?

Last spring, I planted several young trees that I bought from a catalogue. They included a dawn redwood, a dappled willow, and a chestnut. The trees were each about 1-2 feet tall and less than a quarter inch thick. They were growing nicely, then something (I suspect it was an insect) decided to chop the tops off of them. The dawn redwood was hit first. It was a clean cut, about six inches from the ground. Whatever did it left the top of the tree lying beside it. (I dipped the top in Rootone and stuck it in the ground, and now I have two dawn redwoods.)

The original redwood put out new buds and started growing again. As soon as the new trunk hardened, it got lopped off again. The second redwood suffered the same fate several times. It's about an inch tall now, beginning it's second year of growth. It had a new shoot coming up a couple of weeks ago, but it got chopped off while still green. The dappled willow and chestnut have also been damaged repeatedly in the same manner. I lost two mimosa trees I had started from seed last spring to the same marauding lumberjack. It only appears to attack trees less than a quarter inch thick.

This photo is of my older dawn redwood. You can clearly see its multiple attempts at vertical growth.

I'd tend to suspect a 4-legged pest vs insects. Depending on the height they're being cut off at it could be either deer or rabbits. You might try putting wire cages/fencing around them until they get a bit larger. One insect I know that cuts off the tops of things like that is cutworms, but with them it's usually seedlings and they cut them off near the base, so this doesn't sound like them.

I have to agree this looks like a hairy four legged long eared rodent has been visiting you. I would have to reccomend quarter inch machinest cloth an a few pieces of rebar. Both are availible at any hardware store, Ace, HD, Lowe's etc. If you see deer track or sign (looks like large rabitt droppings or goat) use twist ties and put a piece of machinest cloth on top for a cover. Using this method you just keep adding sections between ground and top as tree grows. Normally only takes a good growth season or two and the critters will leave it alone. Also for bunnies you can use cat litter box leavings and put them in the vecinity of the trees. Providing those are availible to you. You can buy coyote or wolf urine at sporting/hunting supply stores to deter the larger varmints in most cases.

Thanks to both of you. We do have rabbits and deer--It just never occurred to me that they could be the culprits. I think the fact that they were often leaving the tops there after biting them off was why I ruled them out.

Deer have a tendency to taste test while they graze. When they bite off the plant and decide it does not suit thier pallet they spit it out. Wet the ground in the evening and then look for tracks in the mud.This is a good way to determine varmint then deal with accordingly.You may have to pull back ground debris for several feet as deer have a good reach.