A recent study led by Dr Owen Atkin, a researcher at Australian National University, together with international scientists, examined trees, shrubs and herbs at sites across a range of latitudes throughout the world.

The research, accepted for review by Global Change Biology, focuses on the leaves of canopy plants, as these are most exposed to temperature extremes.

The tolerance of heatwave (heatwaves in this paper were defined as 12⁰C from the norm) events can profoundly influence the health of organisms, yet previous research has focused on animals.

Presenting an alternative avenue of research, this study shed light on the fact that some plants may also struggle to survive if current climate change scenarios are borne out.

The researchers found that it is the extremes, rather than increasing mean temperatures that are of primary concern, and their results show that the narrowest thermal safety margins (or the highest potential for damage) occur in species growing at latitudes 20-50⁰ (i.e. Bowen to Hobart), coinciding with the global region where the severity of heatwaves is most pronounced.

The region identified is characterised by dry summers, which exacerbate the impacts of heat through water stress. These mid-latitudes also have the highest numbers of species at risk of damage from such events.

“This means that, as heatwaves become more extreme in the future, the safety margin for high temperature tolerance will shrink first and most quickly for plants in the mid-latitudes”, said Professor Mark Tjoelker, a co-author of the study.

Plants are particularly at risk because they cannot move to avoid the effects of sudden increases in temperature. Heat stress can impact plants through the processes of photosynthesis, transpiration and metabolism and repeated exposure to heat stress can cause permanent tissue damage.

The study provides a conservative estimate of the possible effects of future warming on the plants – between five and seven percent of species are likely to exceed their thermal safety margins in current and future climates. However, the authors note that the deleterious effects of warming could be even more severe.

Indeed, Professor Robert Furbank, director of the ARC Centre of Excellence for Translational Photosynthesis at the Australian National University (ANU), described the paper as “pivotal” and that it “paints a gloomy picture for our global flora if the magnitude of extreme weather events increases substantively”.

Chief investigator Professor Justin Borevitz at the ARC Centre of Excellence in Plant Energy Biology’s ANU node underlined the importance of the attempt to delineate heat tolerance for plant species.

“A diversity of plants will struggle physiologically under more extreme temperatures from an unstable climate. Indeed they may suffer even before reaching these limits from droughts, fire and/or flooding that also comes with an unstable climate, if deforestation is avoided,” Borevitz said.

However, more research is needed in this area to examine whether this is the case for a wider sample of species. In addition, more species at different locations grown under common conditions could be examined, as this would provide insights into the role acclimation versus adaptation.

Heat stress is influenced by the ability of the metabolic system to acclimate or change with the conditions. Although the study observed seasonal adjustments in several species that enabled thermal acclimation, this can allow the plant to cope only up to a point.

But heat also affects other plant processes such as growth and reproduction. The long-term impacts and consequences of heatwaves on community and ecological processes are yet to be examined.

“Healthy plants and ecosystems are a big part of reducing CO2,” said Borevitz. “Understanding their temperature limits sets a hard bound nature’s resilience not to be crossed.”