The source mentions underground trees but properly calls them geoxyles but the source does not support the addition Some trees moved underground as a survival strategy in grasslands and savannah They didnt move underground  some lineages evolved a largely underground growth form that could be seen an analogous to an underground tree These species which are trees with extensive subterranean woody trunks moved below the soil surface in order to escape the damaging effects of seasonal grassfires or extreme cold as in the Arctic tundra The source does not call them subterranean woody trunks but rather quotes from another source which calls them a lignified complex of root and shoot tissue Although both are woody root and shoot tissues are quite different anatomically As above moved below the soil surface is an inapt way to describe evolutionary change Its also at least partly incorrect give the root and shoot tissues quote The source does not explain the origin only provides a reasonable speculation as to why drove the evolution of the life form Source makes no mention of tundra In fact the only mention of cold is the suggestion which they reject that this growth form evolved in response to snow in southern Africa In addition I have questions about whether this belongs in the tree article at all Maybe in woody plant albeit in a different form truer to the source