Measuring the response of Heard Island plants to global warming

As the effects of global warming intensify, plants that have evolved in the cool environment of Heard Island must cope with rising temperatures. The amount of carbon plants acquire from the air by photosynthesis, or lose to the atmosphere by respiration, strongly affects their growth. Both processes are temperature-dependent and their balance is likely to change in a warming environment.

We can measure the response to an instantaneous change in temperature, but this does not tell us whether and how organisms might acclimate to a long-term, sustained temperature change. If you live in Darwin, for example, you will likely perceive and physiologically react to a cold day differently than someone living in Hobart. If you move from Darwin to Hobart and still freeze after 20 years, it’s probably because there are limits to how well you can acclimate. These limits vary between people, and vary between plant species.

On Heard Island, we need to know whether plants in a future, warmer environment will have an altered physiology compared to those growing today. But how do we compare the plants of today and tomorrow? Over the 2003-04 summer on Heard Island, our approach was to treat the island as a natural laboratory, and to study the amount of carbon gained and lost by plants grown at different altitudes.

The theory is that plants growing at an altitude of say 200 m, currently experience temperatures that coastal plants would have experienced 50 years ago. In our natural laboratory, we worked on a transect from sea level, at Skua Beach, to the escarpment of Scarlet Hill, at an altitude of more than 200 m. Our ‘guinea pig’ was the Kerguelen cabbage, Pringlea antiscorbutica. By measuring the amount of carbon gained and lost through photosynthesis and respiration, we now know that acclimation to warmer temperatures does occur, but that the delicate balance between carbon gain and loss in Pringlea is upset on days warmer than 10-12°C. This likely results in slower growth.

Pringlea and other ‘guinea pigs’ are currently under close observation in Canberra, where they are being treated with cool and warm days in different walk-in growth chambers.

Marcus Schortemeyer
Australian National University