Item with the cost of tree means that you only need to get those items in your inventory and they will automatically become the finalized item. Eg. Supposingly an item C need you to buy A, B, and C recipe. If C item has the cost of a tree, then you only need to buy A and B in your inventory and they will automatically become C. So in other words, a tree cost means that particular item doesnt require you to buy a recipe.