With most of their branches safely underground and just their leaves and perhaps some twigs poking up above the surface, these subterranean versions of their above ground ancestors are close to indestructible. Some can live for more than 10,000 years.
Of course almost all trees have roots that bury through the soil in search of nutrients and water, and so live underground to some extent. But the underground savannah trees, sometimes called geoxylic suffrutices, are different.
Beneath flimsy leafy shoots, some grow large woody structures as much as one metre wide, while others form branched networks of stems measuring up to 10 metres across. Their shoots are so small and thin that it makes little difference to the tree if it occasionally loses them to wildfire - they can quickly regrow.
There seems to have been a global trigger around 8 million years ago that led to an increase in wildfires, the spread of savannahs and the appearance of underground trees. Scientists know that seasonal variation became more pronounced at that time. The hotter, drier dry seasons would have made wildfires more frequent, and this may have been a factor.
While fire is something grasses that use C4 photosynthesis can live with because they quickly re-sprout in the aftermath, for most trees it spells death. As a result, savannahs can almost literally fuel their own spread.
These grasses thrive in the wet season, but when they shrivel up in the dry season, they can easily catch fire in the sun, generating wildfires that will destroy some of the neighbouring forest. Come the next wet season, it's the C4 grasses that are quickest to take advantage of the space formerly occupied by forest. The savannahs grow and the forests shrink.
http://www.bbc.com/earth/story/20141103-why-some-trees-live-underground
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