18. Comparing Relationships between Plant Body Sizes and Minimum Reproductive Threshold Sizes for Species of Contrasting Life History

Abstract

Two traits are fundamental in defining plant life history strategies: How big can a species get? And how big does it need to get before it can reproduce? Previous research has shown that there is a general positive relationship between these two traits, across species, and this can be accounted for as a trade-off. In this project, I explored whether this relationship differs among herbaceous species with perennial versus annual or biennial life histories. Perennials, because of their capacity to grow across several years, might generally be expected to display a relatively large MAX (maximum potential body size) and hence large MIN (minimum reproductive threshold size) compared with annuals or biennials that live only one or two years. In addition, annuals/biennials might be expected generally to have a smaller MIN for a given MAX, compared with perennials because of selection in the ancestral past — i.e. in frequently disturbed habitats, where annuals and biennials are common, predictable early death (from disturbance) has imposed strong selection to produce at least some offspring quickly, regardless of how small/suppressed the plant might be. I tested these predictions for resident plants sampled from natural populations of 105 species found in the vicinity of Kingston, Ontario. Remarkably, the results support neither prediction, and point to an alternative consequence of selection in shaping plant life history strategies; i.e. small MIN for a given MAX has also been favoured in perennials but for a different reason — as a strategy for competitive fitness.