Well, Talk to your LHS. Don't try to return the kit, explain that you only want the missing tree. If you go in there and want a refund or another kit, they will think you are trying to screw them. If they are any good, they will contact the distributor and get you a new sprue. If not, you can contact Borgfeldt directly and explain it to them. Their sticker with a -800 no is on the box and an email addy. They will likely ship you the sprue quickly with no cost. However, they may say that you will have to pay a small fee for the replacement sprue. No one that I know that was missing an entire sprue has had to pay. They do sell replacement parts by the sprue and may have a policy that allows only that.
BTW -Borgfelt is the Canadian distributor for Tamiya and has an excellent customer service reputation.