Effects of Emergence Time on Survival and Plant Size in Natural Populations of Stellaria media and S. neglecta.

Abstract

Stellaria media (L.) Vill. is an annual agrestal weed often infesting horticultural fields, whereas S. neglecta Weihe grows in ruderal habitats such as roadsides and forest margins. Field censuses were carried out in representative habitats of the two species to compare their germination strategies. In vegetable and vine gardens, S. media individuals emerged after tillage from spring to autumn, and started flowering about 30 to 40 days after emergence. The cohorts that emerged earlier after tillage had larger plant size and higher survivorship to flowering than later-emerging ones. This trend suggests a potential selection pressure favoring quicker emergence after tillage. In ruderal habitats, S. neglecta individuals emerged in autumn, grew through the winter, flowered in the following spring and died before summer. The cohorts that emerged earlier in autumn suffered higher mortality, probably due to shading by summer plants and herbivory, but attained larger plant size if they survived. This trade-off between survivorship and plant size seems to determine the optimal emergence time in autumn.