Twilight Mangrove is a tree added by the Twilight Forest mod. It is an old, ashy-looking tree that can be found commonly in Twilight Swamps and Twilight Streams, standing 11-15 blocks tall on average. Twilight Mangroves have large roots supporting the main trunk, oftentimes in the middle of water.
To use Treecapitator on it, cut the bottom block off the main trunk. The roots need to be cut down separately.