neighbor threatening to spray my new trees with round up!

I planted very small arborvitae trees 2 feet off my neighbors fence line, which is also the property line. The nursery said they would get three feet wide total (they arent even a foot wide right now), unpruned, but can be safely pruned back even more.

My neighbor is threatening to kill them with roundup when he sprays his fenceline.

if he kills my trees, is he responsible?

the fenceline is inbetween two horse pastures, and his horses are uncared for and unhealthy, and his fence presents a safety hazard to my horses. I already have my fenceline back to keep my horses from contacting his, and the reason for the trees is to eventually provide a natural barrier.