Megaherbs are a conspicuous, unique and important part of the subantarctic island flora, including on Motu Maha Auckland Islands. But what exactly are megaherbs? Te Papa Botany Curator Heidi Meudt shows some examples of megaherbs she saw on the Strannik 2023 Auckland Island Expedition, and discusses what makes them so special.

What is a megaherb?

Scientists on the Ross Antarctic Expedition (1839–1844) were awestruck by some very large-leaved herbaceous plants that they found in the subantarctic Motu Maha Auckland Islands and Motu Ihupuku Campbell Island.

The expedition’s botanist, Joseph Dalton Hooker, and its captain, James Clark Ross, have both been credited with calling these plants megaherbs, as both appear to have used the term in their trip diaries.

In addition to the subantarctic species, there are examples of megaherbs in different parts of the world, including mainland Aotearoa New Zealand (think Ranunculus lyallii, the Mt Cook lily), the Rēkohu Chatham Islands (Myosotisdium hortensia, the Chatham Island forget-me-not), as well as Hawai’i, Chile, and even tropical Africa!

So what exactly is a megaherb? Saldivia et al. (2022) offer a fascinating and informative summary of the use of the term, and how different botanists have defined megaherbs over the last 180 years.

Just as with the terms tree, shrub, and herb, there is no universally accepted, precise definition of megaherb. It might be more accurate to call these plants macroforbs or mesoforbs, or maybe even island giants (think wētāpunga | giant wētā), but the term megaherb seems to have stuck, for now.

Regardless of what we call them, megaherbs are essentially very big herbaceous plants. Herbaceous plants – nicknamed herbs – are in turn vascular plants that do not have persistent woody stems. Herbs can be short lived (annual) or long-lived (perennial); they can be grass-like (graminoids) or wide-leaved (forbs). The subantarctic megaherbs are perennial and wide leaved forbs. 

Some, most notably Pleurophyllum criniferum, are summer green (i.e. they die down over winter).

Subantarctic megaherbs

So what makes the subantarctic megaherbs unique? Nicholls and Rapson (1999) tried to pin it down by studying two of the megaherb species on Campbell Island. They concluded that the combination of a suite of characteristics sets the subantarctic megaherbs apart:

• leaves that are large, corrugated, hairy, leathery and in a rosette
• a fleshy root system
• stereome tissue (a rigid herbaceous plant tissue that is not woody)
• colourful flowers

Ten species in five genera are usually considered to be subantarctic megaherbs: Pleurophyllum (P. speciosum, P. criniferum and P. hookerii), Anisotome (A. latifolia and A. antipoda), Bulbinella (B. rossii), Pringlea antiscorbutica, and Azorella (A. polaris and A. robusta (Snares Islands); A. lyallii is now restricted to islands around Rakiura Stewart Island). 

Because these genera are from four different plant families, it’s clear that the megaherb growth form as evolved independently in multiple, unrelated genera.

How and why did the subantarctic megaherbs evolve?

The megaherb species listed above have evolved from small-leaved relatives, many of which are today found in other southern hemisphere areas such as Aotearoa, Australia and South America. Some authors suggest certain megaherb ancestors may have also once populated some parts of Antarctica (e.g. Wagstaff et al. 2007, 2011).

Several botanists have hypothesized that the characteristics listed above allow megaherb species to thrive in the harsh subantarctic environment, short growing season, and highly acidic, peaty soil.

The megaherb growth form might be adaptive to the special combination of moisture, nutrients and light in subantarctic habitats, giving these large plants an advantage over other, smaller herbs.

View these close-ups of leaves of different megaherbs on Auckland Island showing some of the characteristic features.

For example, their large, hairy corrugated leaves might channel water and nutrients to the stem and roots, and suppress nearby plants competing for the same resources.

The leathery texture and presence of hairs on the leaves might help reduce water loss in the strong winds, and a fleshy root system can also retain water and nutrients.

The large leaves and large, colourful, compact groups of flowers (inflorescences) also act giant solar panels (Little et al. 2006), which can heat the plants to temperatures several degrees higher than the air around them!

What megaherbs did we see?

On the Strannik 2023 Auckland Island Expedition, we saw all seven of the native megaherbs that are found there, including: Anisotome antipoda, Anisotome latifolia, Azorella polaris, Bulbinella rossii, Pleurophyllum hookeri, Pleurophyllum speciosum, and Pleurophyllum criniferum. We found flowering or fruiting plants of each of these species.

View a close-up of flowers of four different megaherb species on Auckland Island.

Because Auckland Island still has pigs (who love to dig up and eat certain megaherbs), most of the megaherb species we saw were largely restricted areas that the pigs could not access.

This included cliffs (especially those on the western side of the island), places with large rocks, and other inaccessible sites.

With the exception of Pleurophyllum hookeri and Anisotome antipoda, the remainder of the megaherb taxa on Auckland Island can be found down to sea level.

However, we saw these species most in the island’s alpine areas, which might reflect impacts of pigs at lower elevations.

Although our research team only visited Auckland Island on this expedition, other pest-free islands in the archipelago – including Adams Island and Disappointment Island – are a testament to what the megaherb populations on Auckland Island were once like, and may once again become once pigs are eradicated.

Finally, although megaherbs are doing well on other, pest-free subantarctic islands including Motu Ihupuku Campbell Island, the possible effects of climate change on megaherbs is unknown.

Acknowledgements

Thanks to Brian Rance for contributing to this article, as well as to Colin Meurk and Matt McGlone for sharing their thoughts on the term megaherb and suggesting alternate terms.

Further reading

• Little, L., Eidesen, P. B., Müller, E., Dickinson, K. J., & Lord, J. M. (2016). Leaf and floral heating in cold climates: do sub-Antarctic megaherbs resemble tropical alpine giants?. Polar Research, 35(1), 26030. https://polarresearch.net/index.php/polar/article/view/3261
• Megaherb. Wikipedia article. https://en.wikipedia.org/wiki/Megaherb. Accessed 5 Nov 2023.
• Mitchell, A. D., Meurk, C. D., & Wagstaff, S. J. (1999). Evolution of Stilbocarpa, a megaherb from New Zealand’s sub‐antarctic islands. New Zealand Journal of Botany, 37(2), 205-211. https://www.tandfonline.com/doi/pdf/10.1080/0028825X.1999.9512628
• Nicholls, V. J., & Rapson, G. L. (1999). Biomass allocation in subantarctic island megaherbs, Pleurophyllum speciosum (Asteraceae) and Anisotome latifolia (Apiaceae). New Zealand Journal of Ecology, 87-93. https://newzealandecology.org/system/files/articles/NZJEcol23_1_87.pdf
• Ross, James Clark.  1847. A voyage of discovery and research in the southern and Antarctic regions, during the years 1839-43.  https://doi.org/10.5962/bhl.title.98449
• Saldivia, P., Wagstaff, S. J., Breitwieser, I., Orlovich, D. A., & Lord, J. M. (2022). A generic taxonomic synopsis of the Pleurophyllum Clade (Asteraceae: Astereae: Celmisiinae) with the recognition of the New Zealand endemic new genus Macrolearia. Systematic Botany, 47(2), 607-634. https://doi.org/10.1600/036364422X16512564801722
• Vennell, Robert. 2017. Megaherbs of the Subantarctic Islands.
• Wagstaff, S., Breitwieser, I., Quinn, C., Ito, M. (2007) Age and origin of enigmatic megaherbs from the subantarctic islands. Nature Precedings https://doi.org/10.1038/npre.2007.1272.1
• Wagstaff, S.J., Breitwieser, I. and Ito, M. (2011), Evolution and biogeography of Pleurophyllum (Astereae, Asteraceae), a small genus of megaherbs endemic to the subantarctic islands. American Journal of Botany, 98: 62-75. https://doi.org/10.3732/ajb.1000238