Life-history of a pioneer plant species, Aconogonon weyrichii, was studied at two habitats, and open site and an alder-forest site by setting three quadrats (1×1m in each size) at the former and two at the latter, in Yufutsu Mire, northern Japan. At the open site, the majority of shoots, which developed in spring, were retained until early October. Shoot growth stopped in early June and flowering occurred from early June and flowering occurred from early July to early September. At the forest site shoots emerged densely in spring but shoot density gradually decreased with the decrease in ambient light intensity due to alder leaf emergence. From early June to early July, shoots grew rapidly and completely suppressed other forest-floor plants, but shoot longevity was shorter than that at open site. Flowering occurred simultaneously in early July, and flowering duration was shorter (about 60 days) than that at open site (80-90 days). Therefore, A. weyrichii growing at open site are considered to be adapted as "ruderals" whereas those growing at forest site are adapted as "competitors". Individuals transplanted from the open site to the forest population initially showed rapid growth but by late July had died due to shading. Individuals transplanted from the forest site to the open site showed a similar growth pattern and growth form to open site types. It is assumed that the observed between-site variation in A. weyrichii is and example of ecological plasticity.
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