The term 'alder relative' refers to a group of TREES and shrubs related to the alder tree. The crossword solutions for this term are ARN (3 letters), TREES (5 letters), TREE (4 letters), and BIRCH (5 letters). ARN stands for Alnus, which is the genus of alder TREES. TREES is a plural form of the word tree, which is a common name for woody plants. TREE is a singular form of the same word. Finally, BIRCH is a type of tree that is closely related to the alder tree.