Why Some Gravestones Are Shaped Like Tree Stumps

A tree's roots will mostly extend laterally a significant distance at a fairly shallow depth as it matures. Even smaller diameter roots can very easily damage the grave marker's foundation (concrete or granite bases on top of a foundation), as well as other nearby markers, roads, etc. I've even seen larger roots (from mature trees planted too close to burial spaces) break clear through old memorial bases and even bronze markers themselves on a few occasions. Beyond that, the roots would extend into adjacent burial spaces. They're narrow, closely-spaced (some more so than other, such as with lawn crypts where an entire section is excavated, and then they're installed en masse), and laid out in tight grids whenever possible. Things are tighter than one might think. All of this makes opening and closings in adjacent lots much more labor-intensive.

Double-depth burial spaces, along with side-by-side spaces, are also pretty common. Trees would have to be removed outright in order to entomb the companion.

The other issue is mowing and moving equipment into a section for burials. When they're installed correctly, flush bronze markers are generally much easier for maintenance crews to work around. Mower decks are set to clear markers with the vases down, so damage is less likely. Tree roots screw that up. Roots can lift up marker bases, and when there are large above-ground roots, they can damage the mower blades, or at the very least, wear them out quicker. A backhoe can easily crack a granite base if it's sticking up and gets run over. Those types of accidents can result in costly damage to markers, which is then on the cemetery to make right.

All of that said, trees are beautiful and can have a major impact on a cemetery's aesthetic. Though they can't be planted on top of burial spaces, they can be planted elsewhere around a section with a bit of planning. Some private mausoleums or columbaria can be surrounded by trees and extensive landscaping as well.

You might want to look at https://en.wikipedia.org/wiki/Natural_burial