Plant life at the temperature limit?

When it comes to life living on the edges of what’s biologically plausible or possible one understandably thinks of those microbes known as extremophiles. Living under conditions of, for example, extreme heat, cold or pressure, it’s easy to see why they get the popular vote as Earth’s extremists par excellence. However, although prevalent, that microbomyopic view of extremophilia is erroneous and needs to be corrected; extremophiles are found amongst eukaryotic fungi, animals and plants as well as the prokaryotic microbes. And to add to that catalogue of ‘extremobiota’ we now have multicellular, eukaryotic plants inhabiting areas heated by subterranean geothermal energy.

Mark Smale et al. studied the vegetation clothing the geothermal fields in the Taupō Volcanic Zone, central North Island, New Zealand. They identified 16 vegetation associations, almost all of which were dominated by species indigenous to the area, and unique to geothermal fields. Although flowering plants were few, the vegetation types reflect the broad mix of land plant groups and included moss fields (dominated by non-vascular mosses), fernlands, and one treefernland (typified by vascular, non-seed-bearing ferns and fern-allies), one grassland, shrublands, several scrub areas, a forest, and a treeland (exemplified by vascular, seed-bearing plants).

In an attempt to determine the environmental variables influencing the vegetation, Smale et al. examined soil factors such as temperature, pH, and metal content. Of the abiotic factors measured, subsurface soil temperature – which ranged from ambient (7°C) to near-boiling (98.5°C) – was the main factor controlling vegetation composition. The dwarf swan-neck moss (Campylopus pyriformis) was the most heat-tolerant plant being found in soil where temperatures reached 72°C. But, where the soil was only marginally cooler – a mere 68°C – kanuka (Kunzea robusta), a New Zealand endemic shrub – i.e. a flowering plant – was found As impressive as these heat-defying phytological feats may seem, a note of caution must be introduced at this point. Soil temperatures recorded are subsoil, which means 10 cm below ground level, so plant tissues probably don’t actually experience that temperature. Indeed, an adaptation to life in that thermally-challenging environment is the eminently sensible horizontal spreading of roots, or possession of shallow-penetrating root-like structures in the case of mosses, ferns, and fern-allies. Thus, their owners are shallow-rooted, and thereby avoid the extremes of temperature at greater depth. Nevertheless, such thermophilic behaviour is a pretty impressive feat for the multicellular, complex life forms of Kingdom Plantae. And, considering the temperatures in excess of 60°C that might be encountered, maybe some of these plants contain water that behaves in the odd way we reported previously?

And talking of plants living in ‘challenging’ environments, Kenneth Wood and Warren Wagner have news of a fern that literally clings to survival. Newly described Athyrium haleakalae appears to be an obligate rheophyte, preferring sites of fast moving water along concave walls of streams and waterfalls. And if that’s not a precarious enough habitat, its specific epithet refers to its home in Haleakalā, East Maui (Hawaii), a large, dormant shield volcano(!). Needless to say, this plant is critically endangered, which designation means that the species faces the highest risk of extinction in the wild, an unenviable status it shares with animal taxa such as the black rhino, eastern lowland gorilla, and hawksbill turtle.

[Ed. – if you aren’t able to get hold of the Journal of the Royal Society of New Zealand article, a similar study, entitled A Classification of the Geothermal Vegetation of the Taupo Volcanic Zone, New Zealand and authored by Mark Smale and Susan Wiser (the first two authors of the JRSocNZ article), is freely-available. And for those who’d like even more details of the vegetation types identified in the NZ geothermal area, a technical report, entitled Geothermal vegetation types of the Taupō Volcanic Zone by Mark Smale and Neil Fitzgerald (first and last authors of the JRSocNZ article), is downloadable.]