Seed crops large enough to justify commercial collections generally occur at 2 to 4-year intervals (table 3) but may vary considerably due to numerous factors. Strobilus production in balsam (Powell 1977) and Spanish firs (Arista and Talavera 1995) occurs in alternate years. This was previously thought to be due to an endogenous rhythm unrelated to environmental factors (Greenbank 1963), but it has been shown that good cone crops in grand fir require both a cool, moist summer the year before seed maturation and a warm, dry summer the year of seed maturation (Eis 1973).