The term 'TREE SCOT ALDER' can be summarized by the three crossword solutions ARN, which stands for 'Alnus Rubra', a species of alder tree native to the Pacific Northwest region of North America. This species is also known as the Red Alder and is the most common alder species in the region. It is a deciduous tree that grows up to 40 meters tall and has reddish-brown bark. The wood of this tree is used for furniture, flooring, and other woodworking projects.