Organ preformation in mayapple as a mechanism for historical effects on demography

1 The influence of past and current demographic characteristics (shoot type, reproductive condition, leaf size) on future demography were examined in mayapple (Podophyllum peltatum), a forest herb. Mayapple shoots are borne at the terminal ends of rhizome segments (one per segment), are annual, and are either vegetative or sexual. The number and type of shoots emerging next year (t + 1) are determined by developmental events that unfold in the current (t) and prior years (t - 1, etc.). Such protracted organ development is common to many other perennial species. 2 In 1990 (= year t), the growth of new rhizome segments, the success of segments in producing new shoot buds, and the types of shoots in these buds were recorded on plants for which the following data were available: the 1990 (t) shoot type and leaf area, the 1989 (t - 1) and the 1988 (t - 2) shoot types. Natural flower and experimentally induced fruit abortion in 1990 sexuals was also used to evaluate plant responses to reproduction. 3 The outcome of developmental events in year t, affecting shoots in year t + 1, was influenced by current (t) and historical (t - 1, t - 2) factors. Shoot size and reproductive condition in year t affected new rhizome size and growth and the new shoot type. Shoot type in year t - 1 influenced the number of new rhziomes that formed and whether or not branching occurred. Shoot type in year t - 2 had a weak effect on new rhizome growth and size. Thus, a complex and overlapping set of historical and current factors combined to determine future demography. The parallels between the historical influences in mayapple and other forms of carryover and maternal effects are discussed. 4 Reproduction elicited a number of demographic costs, but these were differentially expressed on the number vs. the type of new shoots. Reproduction had little effect on branching but had marked effects on shoot type. This differential sensitivity to reproduction at our site is a function of mayapple's developmental phenology. Branch determination begins in year t - 1 and is completed by early spring of year t, and so is not affected by fruit costs incurred later in year t. Shoot type determination occurs during fruit fill and is responsive to a plant's reproductive condition. The implications of our results for studies on the cost of reproduction are discussed.