The term 'birch tree' refers to a type of tree that is native to many parts of the world. The crossword solutions CATKIN (6 letters), ALDER (5 letters), and ALDERS (6 letters) all refer to different aspects of birch trees. Catkins are small, cylindrical flower clusters that are found on some species of birch trees. ALDER is a type of birch tree that is found in temperate climates. Alders are the plural form of ALDER, referring to multiple ALDER trees.