can I plant to sapling on top of a dead tree, so the old root system can be used by the sapling, or is this a very bad idea?
depends on why the old tree died
Your sapling has roots of its own and, like those of the dead tree, they need a place to grow down into. If you put the sapling on top of the dead tree the sapling's roots won't be able to find some nice fresh soil.
Much better to put the sapling well away from the dead tree. Why did it die? Was it diseased or just old? Anyway, the old tree will have made the soil around it very dry and lacking in nutrients. Give your sapling a nice clean fresh place to grow if you can and it will appreciate it very much.
The sapling would not use the old root system . I think you are confused about grafting, where a top is grafted on to a live rootstock. If the old tree is dead, you need to clear it out before replanting.
If the old tree died of honey fungus or something like that, you would have a major problem.
thank you so much for all the replies. will put sapling a good long way away, tree died of age. what a fantastic site!