Glumprongs are a type of above ground tree. They are found only in evil regions, and are notable for their unique purple color and high density wood. Because of this high density, they can be detrimental when used to make items that need to be hauled frequently; they are however prime material for your ballista bolts.
If embarking in an evil biome, glumprongs can provide an early and enticing source of wood to the practical player, and purple to the purple-loving player. However, they are so slow to haul that any dwarf tasked with dragging a felled glumprong to the wood stockpile is in a prime position to be attacked by beak dogs or worse.