New research suggests that alpine plants may be more vulnerable to heatwaves than previously thought. A study published in Environmental and Experimental Botany, by Gilbert Neuner and Othmar Buchner at the University of Innsbruck, investigated how heat exposure duration, as well as intensity, affects the temperature thresholds for damage and dysfunction in alpine plant species. Their findings are given away in the paper’s title: “The dose makes the poison: The longer the heat lasts, the lower the temperature for functional impairment and damage.” The results could improve the accuracy of models predicting the ecological impacts of longer heat waves.
The research came about because while the peak temperatures of heatwaves may grab the headlines, While heatwaves Neuner and Buchner were concerned about heatwaves also getting longer. They set out to examine the effect of heat-dose, a measure of both intensity and duration of heat exposure.
To do this, the researchers exposed leaves from tree species at the treeline as well as alpine dwarf shrubs and herbs to a range of temperatures, from 34°C to 64°C, and durations, from 1 minute to 8 hours. They found that with increasing duration, the critical temperature for heat damage decreased substantially by 11-18°C.
In particular, they found that Photosystem II, the part of photosynthesis that splits water into hydrogen ions and oxygen, was impaired at even lower temperatures. Photosystem II could start breaking down at temperatures up to 10°C lower than the critical temperature for heat damage. This disturbance is not a trivial situation for a plant. Many herbicides work by damaging Photosystem II.
The results mean that modelling heat damage to ecosystems is more complicated than taking a single value for temperature. In their article, Neuner and Buchner write:
Our results essentially confirm the early findings of Kappen and Zeidler (1977) on the dose-dependence of heat damage. However, by testing prolonged heat exposure of more than 30 min, we add completely new findings. They are relevant because natural heat episodes lasting longer than 30 min at the leaf level were frequently observed at the studied alpine field sites for small plants. Describing resilience of a species against heat by a single threshold temperature may be definitely helpful for species comparisons (Larcher, 2003, Schulze et al., 2019) or when the intraspecific heat hardening capacity (acquired thermo-tolerance) is assessed (Buchner and Neuner, 2003, Neuner and Buchner, 2012). However, a single threshold may not be sufficient to assess the ecological significance or to model the risk of heat damage in a future warmer climate. To answer these questions, it is imperative to consider the dose effect, i.e., specifically exposure duration above critical thresholds in addition to heat intensity.Neuner and Buchner 2023.
The findings suggest that heatwave intensity, duration, and frequency must all be considered when evaluating heat risk for alpine plants. The researchers say their dose-dependent model provides a more accurate way to predict future ecological impacts of climate warming in alpine regions.
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Neuner, G. and Buchner, O. (2023) “The dose makes the poison: The longer the heat lasts, the lower the temperature for functional impairment and damage,” Environmental and Experimental Botany, 212(105395), p. 105395. Available at: https://doi.org/10.1016/j.envexpbot.2023.105395.