Major habitat shifts are well documented for the Hawaiian flora, but examples of shifts to the highest Hawaiian habitats by plant lineages of lowland tropical ancestry have been lacking. We sought to determine whether Hawaiian Artemisia (Compositae–Anthemideae), which includes lowland and subalpine species, represents such an example, by investigating the origin and relationships of the Hawaiian taxa.
Hawaiian Islands and continental settings world-wide (except Antarctica and Australia).
Molecular phylogenetic analyses of Hawaiian Artemisia in the context of world-wide diversity were conducted using nuclear and chloroplast DNA spacers. The timing of divergence events associated with inferred dispersals was estimated with calibration from fossil pollen records. Historical biogeographical analyses based on molecular trees and ecological modelling of the distributions of extant taxa were used to aid the interpretation of geographical and habitat shifts associated with diversification and long-distance dispersal.
Our findings indicate that the Hawaiian endemic species (A. australis, A. kauaiensis and A. mauiensis) constitute a clade sister to Southeast Asian A. chinensis, which, like the Hawaiian endemics, has ribbed fruit walls and, unlike other members of Artemisia except A. kauaiensis, has a distinct pappus, which is often associated with dispersal ability in Compositae. The clade encompassing A. chinensis and Hawaiian Artemisia was resolved to be most likely of Asian origin. The natural occurrence of A. chinensis in littoral habitats of Taiwan, Okinawa, and the Ryukyu and Bonin islands, and of A. australis in similar settings in the Hawaiian Islands, is likely to reflect the ancestral ecology of the Hawaiian clade, with subsequent colonization of inland, higher-elevation habitats, including subalpine shrubland, where A. mauiensis is endemic.
An ecological shift in Hawaiian Artemisia from tropical coastal habitats to drier and colder subalpine slopes is consistent with evidence from recent studies for repeated colonization of the Arctic by diverse lineages of Artemisia. Artemisia appears to be prone to such anticlimatic ecological shifts, which may explain this exceptional example of an ancestrally lowland tropical lineage in the Hawaiian high-montane flora.