Daphne is the only temperate genus with a diverse range of leaf phenological expressions that includes winter-deciduous, summer-deciduous and evergreen species. This diversity is most strikingly displayed by the two closely related Daphne species - D. pseudo-mezereum (Dp) and D. pseudo-mezereum var. koreana (Dk) where one sheds its leaves in summer and sprouts in autumn (Dp) while the other (Dk) is bare in winter. The evolution of such diametrically opposite leaf phenological expressions strongly implies an adaptive origin. To understand the functional significance of the phenological divergence, we assessed the coupling between patterns of carbon gain and local environment in four populations of these two species. We found high maximum photosynthetic rates in Dp at early spring and late autumn while Dk maintained similar values but during a relatively short summer. However, during the summer, Dk habitats were brighter and moister than that of Dp. These results are consistent with an adaptive explanation encompassing stresses imposed by light, soil moisture and temperature. We infer that the summer-deciduous habit was selected to avail Dp with young physiologically active leaves that maximize carbon gain as light levels improve in autumn and to do so during the least profitable mid-summer.