What could cause trees to grow large, hollow bulges about halfway up their trunks?

I'd like to have a forest with trees that are "normal" except that as they grow a big, hollow bulge forms in their trunks about halfway up, giving them a shape something like this, with a relatively straight trunk above and below the bulge. The bulge would be almost entirely enclosed, though there could be a few small holes or one larger one. The trees would live and function normally.

For size, I'd like it to be possible for bulges to have a horizontal diameter of at least 30 feet on the inside. They would be taller than they are wide, as shown in the example images. On the outside, I'm imagining the bulge to be something like 3x the diameter of the trunk above and below the bulge.

Functionally I think this could work, as I believe the inner wood (heartwood) of a large tree is usually dead anyway, sometimes even rotting away, and it is only the outermost layers that are still alive. I'm mostly asking what would cause a tree to grow such a bulge. E.g. maybe there's a tendency for a gas or liquid (possibly some kind of waste?) to build up inside as the tree grows that eventually creates a hole to the outside and drains? But why?

posted about 5 years ago

nanoguy‭

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