Megaherbs are a group of herbaceous wildflowers growing in the New Zealand subantarctic islands and on the other subantarctic islands. They are characterised by their great size, with huge leaves and very large and often unusually coloured flowers, which have evolved as an adaptation to the harsh weather conditions on the islands. They suffer from overgrazing due to introduced mammals.
Appearance and occurrence
Originally, the term was coined to describe large-leaved herbs that form meadows in the subantarctic islands but has also been applied to describe tropical alpine vegetation forms found in the Andes, East Africa and New Guinea. They are large herbs that can reach dimensions of over one metre, often feature strikingly colourful flowers, large leaves and long stalks, and are important components of the ecosystems of the subantarctic islands. Beyond these traits, megaherbs can have different forms; for example, some genera have perennial leaves and others are deciduous. Intensely coloured flowers are not unique to megaherbs but also occur on other plant species in the same environments and either arose by chance or are evolutionary adaptations to the environment.
They live in the wet, windy and cold environments of the subantarctic islands, where they coexist with cushion plants and tussock grasses but trees are absent. A single species, Pleurophyllum hookeri, covers almost a third of Macquarie Island where another megaherb, Stilbocarpa polaris, also occurs. They reach their maximum extent on that island. The only subantarctic islands lacking them are the Falklands and South Georgia, but Poa flabellata on South Georgia could be considered a megaherb. On Campbell Island they appear to occur preferentially in nutrient-rich (eutrophic) terrain. Researchers since the 1940s often incorrectly assumed that they are limited to cliffs and ledges, as grazers had extirpated them from other areas.
They are the best known plants of the New Zealand subantarctic islands and are important components of the biodiversity of the region. Another name is "megaphyllous herbs".
Taxa
Common taxa forming megaherbs are Asteraceae, Apiaceae, Brassicaceae, Gentianaceae and Liliaceae; sometimes Poaceae (grasses) are also included. In the New Zealand subantarctic islands, four endemic genera make up megaherbs. Among these are Anisotome and Bulbinella with colourful flowers and Pleurophyllum and Stilbocarpa with large leaves. Additional genera are Gentianella and Pringlea.
The Chatham Island forget-me-not (Myosotidium hortensia) naturally occurs on the Chatham Islands but has been brought to New Zealand. It has large leaves and forms rosettes up to one metre wide. The Kerguelen cabbage (Pringlea antiscorbutica) is a megaherb species that occurs on Heard Island where it is a key species in herbfields, Iles Crozet, Iles Kerguelen, Marion Island and Prince Edward Island.
Other species called "megaherbs" include Phormium colensoi on New Zealand. The Mountain buttercup (Ranunculus lyallii) is also considered a megaherb and Astelia solani has been referred to as one. The species Aciphylla latifolia, Bulbinella rossii, Myosotidium hortensia, Pleurophyllum spp. and Stilbocarpa in the Chatham Islands have been referred to as "macrophyllous forbs". Finally, Angelica archangelica in Iceland has been considered an example of a Northern Hemisphere megaherb.
Evolutionary history and ecology
The origins of the megaherb growth form are enigmatic. Several different subantarctic genera independently evolved this trait, which is defining for the genus Pleurophyllum. Stilbocarpa megaherbs evolved from ancestors with smaller leaves. These plants survived the last glacial maximum on the subantarctic islands and spread northward after its end. Related species also occur on New Zealand.