VARIATION IN SEED SIZE OF A SOUTHWESTERN RIPARIAN TREE, ARIZONA WALNUT (JUGLANS-MAJOR)

Seed and nut size of Arizona walnut have high variance among years, attributable to differences in spring rain, with large viable seeds produced only in years with above-average rain. Trees in drier habitats (riparian terraces) produce larger seeds on average than those on stream-banks. In drought years on terraces, seed weight is lowest on large trees that produce many female flowers. Seed weight declines significantly with tree age and size in terrace habitats. Between-habitat differences may reflect evolutionary processes that have selected for larger but fewer seeds in drier habitats, while competition between seeds may be a proximate cause of variance in seed weight among years and within habitats. -from Authors