Walnut trees are deciduous,30–130 feet of the species Juglans. The 21 species in the genus range across the north temperate Old World from southeast Europe east to Japan, and more widely in the New World from southeast Canada west to California and south to Argentina. Warmer, drier summers and extreme weather events considered possible as the climate changes would be especially troublesome - possibly fatal - for walnut trees, according to research at Purdue University.