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