Also what if people did this in the past already that means most forests could be burial grounds and people just see wood so they go Chopping down trees not knowing they're chopping down grave stones essentially
It's impossible, the tree would grow up, and the roots will grow big and wide, and no new grave can be dug cause of the roots, and there will be cemetery with 20-30 graves, and the next one will be 20-30 graves, and after 50 years or so, there will be no place for cemeteries at all... Even now, with this big cemeteries they put new corpses over "old ones" after some time... I think it was something like 100 years or so...
It's a great idea, but it wouldn't work. For one, as people have said, the roots would cause problems by destroying the caskets and preventing too many trees growing close together. Also, a lot of trees aren't compatible to grow near each other or in certain climates. And knowing human nature, people would insist that THEY WANT THIS TREE and try to plant one that doesn't work. Wish we could find a way to make this possible though! Those urns are a good idea.
And then we cut down the trees, but use them for paper which we use everyday! But we stain that paper, but we use them as newspaper! But they get old, and we burn them! YAYAYAYAYAYAYAY!
Wait this isn't game of thrones