BIOMASS ALLOCATION AMONG REPRODUCTIVE STRUCTURES IN THE DIOECIOUS SHRUB OEMLERIA-CERASIFORMIS - A FUNCTIONAL INTERPRETATION

1. The allocation of resources among various reproductive structures and functions can suggest the relative importance of past selective pressures in moulding reproductive patterns. 2 To determine how male and female plants of the dioecious shrub Oemleria cerasiformis differ in allocation to reproductive functions, biomass was partitioned among reproductive structures for 20 plants of each sex. 3 Structures contributing to pollinator attraction (petals and hypanthium) were heavier in males than in females. In males, these structures constituted 63% of total reproductive biomass; inflorescence stems and bracts constituted 28%, and the androecium only 9%. In females at flowering, petals and hypanthium constituted 50% of reproductive biomass, inflorescence stems and bracts 37%, and the gynoecium 13%. 4 In females, fruits constituted 87% of the total reproductive biomass at average fruit set (13.7% of pistils). Even with the lowest observed fruit set (4% of pistils), fruit comprised 75% of reproductive biomass overall, and pollinator attraction no more than 4%. Fruit biomass was distributed about equally between pulp (offspring dispersal) and stone (offspring provisioning and protection). A flower can produce up to 150 times its own weight in fruit, thus fruit set is the main factor determining how biomass is distributed among reproductive structures in females. 5 These data support the view that the primary factors influencing the evolution of reproductive allocation are, in males, pollen limitation, and in females, the necessity of provisioning and dispersing offspring.