Floral Variation across Three Varieties of the Landscape-Dominant TreeMetrosideros polymorpha(Myrtaceae): Insights from a Hawaii Island Common Garden

Abstract

Premise of research. Metrosideros polymorpha is a landscape-dominant tree species endemic to the Hawaiian Islands. Although the group is famous for its ecological amplitude and variation in vegetative characters, little attention has been paid to variation in its “shaving brush” flowers, which occur in inflorescences of diverse sizes and colors. We aimed to determine whether the floral variation observed in natural populations is heritable and how this variation is distributed across environments and varieties of the species. Methodology. We measured seven floral traits in 93 adult trees representing three varieties of M. polymorpha in a common garden on the island of Hawaii and examined variation in these traits across elevations (of the source population) and varieties. Pivotal results. Flower length increased with elevation, while flower width decreased with elevation. Nectar cup diameter and stamen length did not vary across elevations. Variation in flower color decreased with increasing elevation, with only dark red–flowered trees observed from the two highest elevations. Much of the variation observed in floral traits was partitioned among the three varieties in the garden. Stigma-anther separation (SAS) was significantly lower in var. incana, while long flowers and smaller inflorescences characterized var. polymorpha; var. glaberrima had the least distinct flowers. Conclusions. We observed a distinct floral morphology in the high-elevation var. polymorpha consistent with pollination predominantly by birds. The lower SAS observed in var. incana may be consistent with weaker selection against self-pollination in low-density populations of this early-successional variety on new lava flows. These results indicate that, in addition to divergence in vegetative traits associated with adaptation to contrasting abiotic conditions, varieties of M. polymorpha on the island of Hawaii show heritable differences in floral characters that may be associated with biotic factors.