Lilium longiflorum and L. formosanum are closely related species endemic to subtropical islands in the Ryukyu Archipelago and Taiwan. Stem leaf morphology, flowering rate within population and individual net production during the first year seedling growth were determined to clarify whether they can differentiate these two species, and reflect adaptive strategy during species and local population establishment. Four experimental populations of L. longiflorum and five of L. formosanum with different locality covering almost their native distribution were grown under greenhouse condition in the University Farm located in northern Kyushu, Japan. L. longiflorum showed distinct lower ratio of leaf length/width than L. formosanum. Flowering rate and net production tended to decrease along the population location, northward across the archipelago arc and toward higher altitude within the mainland of Taiwan, showing obvious geographic cline (geographically continuous variation). Low degrees of flowering rate and net production in northern L. longiflorum were associated with high frequency of individuals that obtained little net production during spring to summer, indicating an acquired dormancy status. L. formosanum native to about 3000m altitude showed higher resource allocation to the mother bulb. These variations of the growth habits within the species reflect region-specific adaptive strategy of L. longiflorum and L. formosanum for climate during the glacial period and that in highlands of the mainland of Taiwan, respectively. Higher annual net production and an earlier shift to the reproductive phase of L. formosanum is highly likely more advantageous in population establishment under disturbed and competitive environment, where L. longiflorum is rarely found.

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