There are no trees. No shrubs even. The tallest plants are ‘megaherbs’, which can just reach over head height. This is the unusual assemblage of plants which inhabit Macquarie Island.
In contrast to the Northern Hemisphere tundra where low-growing shrubs are prominent, the megaherbs and tussocks are king here.
Stilbocarpa polaris can grow to 2 metres tall with leaves up to half a metre across.
Macca’s most well known and most prominent plants are the tussock grass Poa foliosa with masses of long leaves sprouting from a root pedestal; Stilbocarpa polaris, Macquarie Island Cabbage (which looks more like a hairy rhubarb but is not related to either food plant); and Pleurophyllum hookeri, a rosette daisy with silvery leaves.
Pleurophyllum hookeri has underground rhizomes from which these silver rosettes grow. The leaves can grow to 60 cm long in summer but are replaced by shorter leaves over winter.
Tussock grass Poa foliosa forms pedestals composed of peat, roots and old shoots which elevates the living foliage to over head height on sheltered sites.
‘Megaherb’ is a poorly defined term in botany and is generally only applied to plants from the Subantarctic. It essentially refers to non-woody plants which attain unusually large sizes, often appearing as giant versions of their more modestly proportioned relatives. There are several potential benefits to the megaherb form which may explain why they are so prominent on the Subantarctic islands: the large leaves may help trap sunlight and reduce wind speeds, creating a leaf microclimate with relatively high temperature and low evapotranspiration which would aid growth in a cool windy environment; their greater height and leaf area means they can outcompete smaller plants for precious sunlight; and the large leaves are likely to collect airborne nutrients which would be an advantage on the infertile peat soils of Macquarie Island’s plateau (these are discussed in a study of Campbell Island megaherbs).
Although the term ‘megaherb’ is rarely used beyond the far southern latitudes, the growth form does occur in a few other places. Large-leaved herbs are quite rare globally and tend to occur where there are few or no shrubs, such as in the equatorial montane vegetation of Mt Kilimanjaro where tree groundsels and giant lobelias occur.
Giant groundsel Dendrosenecio kiliminjari is a giant herb (its stem is not made of wood) from high elevations on Mt Kilimanjaro.
Finally, I kinda lied about there being no shrubs – to be technically correct there is a ‘shrub’ on Macca. Coprosma perpusilla has wiry stems which creep through the moss, giving it the appearance of a herb (botanically described as a ‘subshrub’, it must be amongst the smallest even of this dwarf subset of shrubs).
Coprosma perpusilla ssp. subantarctica is a miniature creeping shrub whose tiny woody stems are hidden. It is a fraction of the size of the dominant megaherbs on Macquarie Island, and of its more temperate relatives.