Daphne pseudomezereum and D. jezoensis appear to be the only woody species occurring in the northern temperate forest that undergo leaf dormancy in the summer (wintergreen, WG). The adaptive reason for this is unclear, particularly when their near relative, D. pseudomezereum var. koreana, exhibits normal winter dormancy (summergreen, SG). We attempted to reveal the adaptive and evolutionary basis for this unusual leaf phonological divergence.

We studied two populations each of WG and SG Daphne in western Japan by evaluating their habitat conditions and assessed their ecophysiological traits. While the light environment of SG is significantly brighter than that of WG (20-30% vs. <10% canopy opening), leaf traits such as LMA, chlorophyll and nitrogen concentrations were either not different between the two phenological types or showing WG with more sun-adapted values. Photosynthetic rates also showed that WG is similar to SG indicating no significant shade-adapted tendencies.

Our results suggest that SG Daphne may have evolved from a WG ancestor. Having been dispersed from sunnier rock outcrop habitats into lowland mixed deciduous forests, it experienced carbon deficit during summer in the dim understory. Lacking ecophysiological attributes in shade adaptation, these plants opted for a phenological solution to survival under seasonally fluctuating forest shade.