Survival costs of adult dormancy and the confounding influence of size in lady's slipper orchids, genus Cypripedium

Abstract

Adult whole‐plant dormancy is a phenomenon in which a perennial, herbaceous plant does not sprout for one or more years. Although previous studies have noted a cost of dormancy to survival, none have accounted for the potentially confounding influence of size variation on this relationship. I asked whether the probabilities of dormancy and survival in dormancy‐prone plants vary with size, possibly creating the appearance of a life‐history tradeoff between survival and dormancy. I censused sympatric populations of three lady's slipper orchid taxa, Cypripedium parviflorum, C. candidum, and C.×andrewsii, in an 11‐year study (1994 to 2004) at Gavin Prairie, Illinois, USA. Annual dormancy and survival trends were modeled using Cormack‐Jolly‐Seber mark‐recapture analysis, while stage‐specific survival and stratum‐transitions (i.e. probability of transitioning among stages conditional upon survival) were modeled with multi‐strata mark‐recapture analysis. Annual dormancy probabilities and transition probabilities to and from dormancy varied synchronously among taxa, while trends in survival did not. Both survival and dormancy varied with number of sprouts in the previous year, positively in the case of the former and negatively in the case of the latter. Accounting for plant size in this way eliminated any variation in survival by life‐history stage, with dormant plants surviving at the same rate as non‐dormant plants of the same size. Transitions among stages did not vary with plant size. My results suggest common climatic cues to dormancy, and suggest caution in inferring costs of dormancy from studies in which plant size is not controlled, as such costs may be artifacts of smaller plants being more dormant‐prone and less likely to survive.