From Dwarf Fortress Wiki
The Willow is a type of above ground tree that is only found near rivers, pools or other water. It is found in nearly any biome that has trees. Like the overwhelming majority of overland trees, willow wood is brown and produces brown products.
[PLANT:WILLOW] [NAME:willow][NAME_PLURAL:willows][ADJ:willow] [USE_MATERIAL_TEMPLATE:STRUCTURAL:STRUCTURAL_PLANT_TEMPLATE] [BASIC_MAT:LOCAL_PLANT_MAT:STRUCTURAL] [USE_MATERIAL_TEMPLATE:WOOD:WOOD_TEMPLATE] [STATE_NAME:ALL_SOLID:willow] [STATE_ADJ:ALL_SOLID:willow] [PREFIX:NONE] [TREE:LOCAL_PLANT_MAT:WOOD][TREE_TILE:244] [PREFSTRING:sad appearance] [PREFSTRING:fluffy catkins] [WET] [BIOME:ANY_TEMPERATE] [BIOME:ANY_TROPICAL_FOREST] [BIOME:GRASSLAND_TROPICAL] [BIOME:SAVANNA_TROPICAL] [BIOME:SHRUBLAND_TROPICAL] [BIOME:SWAMP_TROPICAL_FRESHWATER] [BIOME:SWAMP_TROPICAL_SALTWATER] [BIOME:MARSH_TROPICAL_FRESHWATER] [BIOME:MARSH_TROPICAL_SALTWATER] [SAPLING] [SOLID_DENSITY:420]