This is a list of toxic plants to alpacas. I'm terrible with plant, tree and bush names so can't give you certain trees to plant. Make sure you block off all access to the tree while it is growing, otherwise the alpacas will destroy it.
That list is so long it makes me want to leave the field totally bare!!! lol We just planted three varieties of maple in our field, none of which are red maple (although, according to that list, issues have only been noted in horses). We will be fencing our young trees (we don't have our alpacas yet) in the weeks ahead.