The Subantarctic – where 43 plant species is a lot

Macquarie Island’s total of 43 native vascular plant species is low by most standards – some Tasmanian National Parks of similar size have more than ten times this diversity.* But amongst the Subantarctic islands, none come close to Macca’s species richness. The Kerguelen islands (7,215 km2 of which 6,450 km2 is ice-free) in the southern Indian Ocean have a land area around 50 times the size of Macca (128 km2), yet only 30 native plants. Other Subantarctic islands support even fewer species – Australia’s Heard Island for example has just eight. Further south, in the Antarctic proper, only two vascular plants survive in the harsh climate.

A water fern, Blechnum penna-marina ssp. alpina, one of five pteridophytes found on Macquarie Island. This widespread fern also occurs elsewhere in the Subantarctic, in Australia, New Zealand and South America.
The water fern Blechnum penna-marina ssp. alpina, one of five pteridophytes found on Macquarie Island, also occurs elsewhere in the Subantarctic, in Australia, New Zealand and South America. The fern is growing with one of the many moss species found on Macca.

The theory of island biogeography suggests that species richness on an island is a function of island size (land area) and isolation (distance from nearest major landmass). Amongst the Subantarctic islands, area does predict  plant species richness, but isolation does not. However temperature is as important or more so – the plant species richness declines with temperature. A third factor that may also be influential is glacial history, because it seems that vascular plants were wiped out during glaciations on some of the more southerly islands, such as South Georgia, and therefore needed to recolonise those islands following each glacial period. Isolation is perhaps not so important in the Subantarctic where every island is very isolated and the typical plant species are not limited by dispersal ability.

Macquarie Island, having no history of glaciation and a slightly milder climate than other Subantarctic islands, therefore has a relatively high richness of vascular plants, despite its modest size.

Not surprisingly, definitions come into play here: if we are to generously include New Zealand’s so-called ‘Subantarctic’ islands (arguably not Subantarctic because they support trees), then the numbers rocket up to 188 for the Auckland Islands.

Shrubby vegetation on Enderby Island, 50° 31' South, considered Subantarctic by some definitions. The red flowers are Southern Rata, Metrosideros umbellata.
Shrubby vegetation 2-3 m tall on Enderby Island, 50° 31′ South, considered Subantarctic by some definitions. The red flowers are Southern Rata, Metrosideros umbellata.

But that’s just vascular plants; like the temperate rainforests of Tasmania, Macca’s non-vascular flora dwarfs the angiosperms and ferns in species richness. There are at least 75 mosses, 60 liverworts and 55 lichens known from the island.

Macquarie Island’s native vascular plants – 22 dicots, 16 monocots and 5 pteridophytes – along with 3 introduced species are all illustrated in an online key.

* Tasmania’s Maria Island National Park has over 470 native vascular plants in an area slightly smaller than Macquarie Island; Douglas-Apsley National Park, a little larger than Macquarie Island at 160 km2, has over 270 native vascular plants, while the 169 km2 Freycinet NP has over 500 species.

Land of the Megaherbs

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
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.

IMG_1297
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.

Dendrosenecio kiliminjari is a giant herb (it's stem is not made of wood) from high elevations on Mt Kilimanjaro.
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. subantarctica2 is a miniature creeping shrub whose tiny woody stems are hidden. It is a fraction of the size of the dominant megaherbs on Macqaurie Island, and of its more temperate relatives.
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.

Who? What? Where? Why? How?

An exclosure plot showing effects of rabbit grazing on Macquarie Island.
An exclosure plot showing effects of rabbit grazing on Macquarie Island.

I’m a postgrad student at the University of Tasmania, studying plant ecology in alpine and Subantarctic environments. My studies focus on Macquarie Island, half way between Australia and Antarctica. (More about me here).

Why travel for days across the notorious Southern Ocean to then tramp around a soggy, windy and cold island looking at herbs and grasses? Well, ‘Macca’ is one of the wildest places on Earth – and is becoming increasingly wild as the ecosystem rebounds following the eradication of pest animals such as cats and rabbits – making it an ideal natural laboratory for studying ecosystem recovery and climate change impacts.

My other study sites are not so far south: alpine vegetation on the peaks of southern Tasmania. Evidently I like cold, windy, damp places. Or at least the plants which inhabit such places.

How to study vegetation change in these remote and wild environments? Read on…