Intraspecific divergence and a life-history trade-off along a successional gradient within Hawaii's Metrosideros polymorpha

The importance of successional gradients as drivers of within-species divergence has received little empirical attention. Two morphologically distinct varieties of the Hawaiian endemic tree, 'ohi'a lehua (Metrosideros polymorpha) are the canopy dominants at alternate stages of a successional gradient formed by the recurring disturbance of lava flows on East Hawai'i Island. Successional stages differ in temperature, light level, soil moisture, and bio-available nutrients, suggesting that the early-successional variety incana and late-successional variety glaberrima are adapting to divergent selection pressures. To test this hypothesis, seeds from three replicate 'pure' populations of each variety on east Hawai'i Island were germinated, and the resulting seedlings were grown under four combinations of light and nitrogen levels, representative of early- and late-successional and combined environments. A second set of seedlings was grown in early- and late-successional environments in the field for six months to test for 'home-field advantages' of the varieties in their native environments. A higher proportion of seeds germinated and germinated more quickly in low-light treatments. In contrast, seedlings of both varieties had increased growth and survival in response to high-light and high-nitrogen treatments and at the early-successional field site. Variety incana seedlings showed greater growth and survivorship in high-light treatments, while var. glaberrima showed greater survivorship in low-light treatments and at the late-successional field site. These results are consistent with seedling-stage intraspecific divergence along a successional gradient on East Hawai'i Island. Moreover, these varieties exhibit the classic plant life-history trade-off between fast growth in high light and high survivorship in shade, notably within a single species. Presence of this trade-off within a single, variable species suggests that it may contribute to iv intraspecific divergence and subsequently speciation. As 'ohi'a lehua is the canopy dominant of Hawaiian rainforests, the divergence of these varieties may play a role in shaping the community composition of successional environments on the windward side of Hawai'i Island.