From Dwarf Fortress Wiki
Black-caps are a type of underground tree. They are found in the lower levels of caverns near the water. Unsurprisingly, they yield black logs when cut down. Due to their color, live black-caps saplings are indistinguishable from trampled ones.
[PLANT:BLACK_CAP] [NAME:black-cap][NAME_PLURAL:black-caps][ADJ:black-cap] [USE_MATERIAL_TEMPLATE:STRUCTURAL:STRUCTURAL_PLANT_TEMPLATE] [BASIC_MAT:LOCAL_PLANT_MAT:STRUCTURAL] [USE_MATERIAL_TEMPLATE:WOOD:WOOD_TEMPLATE] [STATE_NAME:ALL_SOLID:black-cap] [STATE_ADJ:ALL_SOLID:black-cap] [PREFIX:NONE] [STATE_COLOR:ALL_SOLID:BLACK] [DISPLAY_COLOR:0:0:1] [TREE:LOCAL_PLANT_MAT:WOOD][TREE_TILE:6] [PREFSTRING:gloomy appeal] [WET][DRY] [BIOME:SUBTERRANEAN_WATER] [UNDERGROUND_DEPTH:2:3] [SOLID_DENSITY:650] [TREE_COLOR:0:0:1] [DEAD_TREE_COLOR:0:0:1] [SAPLING_COLOR:0:0:1] [DEAD_SAPLING_COLOR:0:0:1]