Costs of Reproduction in the Pink Lady's Slipper Orchid: A Four-Year Experimental Study

Abstract

The cost of current reproduction on future growth, survival, and reproduction is a central concept in evolutionary ecology, but the field evidence in plants for a cost of reproduction is weak because the appropriate experimental protocol (manipulation of the level of reproduction in an experimental group of individuals) has rarely been carried out. The pink lady's slipper orchid, Cypripedium acaule, is well suited to this design because naturally occurring flowering plants only rarely fruit, whereas virtually all hand-pollinated flowers develop into fruits. In two eastern Massachusetts populations, plants were randomly assigned to be hand-pollinated or left as controls, with the treatments repeated in four successive years. After 2 yr, treatment effects were weak at one site and absent from the other site. By the third and fourth years, the high cost of reproduction was clearly demonstrated in a lower growth and flowering rate of hand-pollinated plants in comparison with the control plants. An average-sized plant that produces a fruit in the current year results in an estimated 10%-13% decrease in leaf area and a 5%-16% decrease in the probability of flowering in the following year. Plants that are experimentally defoliated are still capable of maturing their fruit, even though these plants have a decreased probability of flowering in the following year. These results show that long-term studies in the field are capable of demonstrating a cost of reproduction.