Divergent Norms of Reaction to Temperature in Germination Characteristics among Populations of the Arctic-Alpine Annual, Koenigia islandica

Abstract

The timing of germination is a critical life-history trait for annual plants because it coordinates development from seedling to reproduction with the growing season. Arctic/alpine annuals are subject to the constraints of an especially brief growing season in which temperature both acts as a germination cue and limits growth. The evolution of phenotypic plasticity in response to temperature is thus expected, measured as norms of reaction. Strong selection in combination with differences among habitats in appropriate temperature cues leads to the expectation of population divergence in norms of reaction. Their form may be complex; no a priori mathematical distribution can be assumed. Here, we use nonparametric smoothing (Loess) to detail norms of reaction of germination to temperature, and find significant genetic divergence among six widely distributed arctic and alpine populations of the annual Koenigia islandica from Norway, the U.S.A., and Canada. Germination plasticity is discussed in relation to temperature regimes recorded under field conditions; however, any adaptive inference based on lab observations must be interpreted cautiously. The present results indicate strong population differentiation in plasticity, and highlight the necessity to consider evolved differences in life-history traits when evaluating the vulnerability of arctic and alpine species to a changing climate.