Megaherb

Close-up on megaherbs on Enderby Island

Originally, the term was coined to describe large-leaved herbs that form meadows in the subantarctic islands[1] but has also been applied to describe tropical alpine vegetation forms[2] found in the Andes, East Africa and New Guinea.[3] They are large herbs that can reach dimensions of over one metre, often feature strikingly colourful flowers,[4][5] large leaves and long stalks,[6] and are important components of the ecosystems of the subantarctic islands.[7] Beyond these traits, megaherbs can have different forms;[8] for example, some genera have perennial leaves and others are deciduous.[9] Intensely coloured flowers are not unique to megaherbs but also occur on other plant species in the same environments[10] and either arose by chance or are evolutionary adaptations to the environment.[11]

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[12] where another megaherb, Stilbocarpa polaris, also occurs.[13] They reach their maximum extent on that island.[14] The only subantarctic islands lacking them are the Falklands and South Georgia, but Poa flabellata on South Georgia could be considered a megaherb.[15] 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.[16]

The most extraordinary of the megaherbs is the Pleurophyllum meadow, a community dominated by the large-leafed herbaceous composite, producing a floral display second to none outside the tropics

— Joseph Dalton Hooker, quoted in Fell, 2002[1]

They are the best known plants of the New Zealand subantarctic islands[17] and are important components of the biodiversity of the region.[2] Another name is "megaphyllous herbs".[3]

Common taxa forming megaherbs are Asteraceae, Apiaceae, Brassicaceae, Gentianaceae and Liliaceae; sometimes Poaceae (grasses) are also included.[15] 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.[17][18] Additional genera are Gentianella[19] and Pringlea.[7]

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.[20] 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.[21]

Other species called "megaherbs" include Phormium colensoi on New Zealand.[22] The Mountain buttercup (Ranunculus lyallii) is also considered a megaherb[23] and Astelia solani has been referred to as one.[24] The species Aciphylla latifolia, Bulbinella rossii, Myosotidium hortensia, Pleurophyllum spp. and Stilbocarpa in the Chatham Islands have been referred to as "macrophyllous forbs".[25] Finally, Angelica archangelica in Iceland has been considered an example of a Northern Hemisphere megaherb.[15]

The origins of the megaherb growth form are enigmatic. Several different subantarctic genera independently evolved this trait, which is defining for the genus Pleurophyllum.[18] Stilbocarpa megaherbs evolved from ancestors with smaller leaves.[26] These plants survived the last glacial maximum on the subantarctic islands and spread northward after its end.[27] Related species also occur on New Zealand.[28]

Megaherbs have been described as a form of gigantism.[19] The evolution of the megaherb form may be a consequence of specific conditions in the subantarctic islands.[18][4] The large leaves could be intercepting nutrient-bearing aerosols and trap heat in cold environments. Rosette growths reduce wind speeds and wind-driven evaporation and cooling.[28] The leaves may also act to absorb heat from diffuse radiation[16] and higher temperatures have been measured in megaherbs than the surrounding environment.[29] At the same time, the lack of herbivores, plentiful water and nutrients brought by e.g seabirds and steady temperatures facilitate the growth of large plants.[26][15] On Adams Island megaherbs are well developed where they are fertilized by guano. In return, birds use them as cover and source of insects[30] and are burrowed by nesting birds.[31] Such burrowing may influence the establishment of megaherb communities.[32] They are among the first plants to resettle former albatross nests on Adams Island.[33] The simultaneous occurrence of large leaves, large underground storage tissues, large seeds and large seed output is also found in megaherbs and appears to reflect unusual adaptations, as resource trade-off would normally prohibit their simultaneous occurrence.[7]

Herbs with large leaves occur in other places such as Chile, Hawaii, Kenya and New Zealand but there are traits specific for subantarctic megaherbs[34][27] and there are environmental differences, such as lower and more steady insolation.[35] Their evolution may be driven by similar environmental factors that are encountered on high mountains and in polar climates,[26] such as cold and windy weather, and may thus be examples of convergent evolution.[36]

Wētā, flightless crickets of New Zealand, have been observed to pollinate megaherbs and may constitute their main pollinators.[37] Moths have also been observed pollinating megaherbs.[38]

Megaherbs were first described by the British botanist Joseph Dalton Hooker, who coined the term in 1847.[1] Campbell Island is known for its grasslands and its megaherbs.[39] The Fairchilds Garden site on Adams Island was noted already in 1891 for its megaherbs.[30] The characteristic appearance makes Pleurophyllum a plant that could be used in horticulture, but attempts to cultivate it outside of the subantarctic environment have largely been unsuccessful.[18]

The term "megaherb" is sometimes used to describe plant species from other continents that have features similar to subantarctic megaherbs.[46] It has also been used to describe members of the family Heliconiaceae, which are tropical species,[47] and for the Amazonian species Phenakospermum guyannense.[48]

• 
  Bulbinella rossii, Stilbocarpa polaris, and two species of Pleurophyllum
• 
  B. rossii, Anisotome latifolia, and Pleurophyllum
• 
  B. rossii
• 
  A. latifolia and B. rossii
• 
  Pleurophyllum speciosum, the Campbell Island daisy
• 
  Pleurophyllum hookeri and B. rossii
• 
  A hybrid between P. hookeri and P. speciosum
• 
  S. polaris and B. rossii
• 
  Gentianella antarctica
• 
  Gentianella concinna

• Island gigantism
• Antarctic Floristic Kingdom
• Antarctic flora
• Megafauna
• Megaflora

• Article on a recent botanic expedition to Campbell Island to view megaherbs
• Photograph of megaherb community on Enderby Island, one of the Auckland Islands
• Another megaherb photograph, Enderby Island
• Another megaherb photograph, Campbell Island
• Another megaherb photograph, Campbell Island
• Megaherb photoset on Flickr