Legal Question in Real Estate Law in California

I'd say there are two aspects to the question. First, as to responsibility for cleanup and removal, you are likely responsible unless there were some very unusual provision in the county's acquisition of its roadway property, such as a deed or easement granted by an owner of your property that contained as assumption of liability by the county. However, as to liability for accidents and injuries to users of the road, the general rule is that you are not liable if the trees were healthy and well maintained, in which case their falling would be an unforeseeable accident, but if you allowed the trees to become dangerous through old age, disease, etc., you could be held liable for accidents which happen when they (for example) blow over in a storm.