The dose makes the poison: The longer the heat lasts, the lower the temperature for functional impairment and damage

Climate change increases the intensity and duration of heatwaves. Heat limits for plants are commonly determined by a 30-minute test. This neglects the effect of heat-dose (intensity x exposure-duration) on heat limits, which has been poorly studied. Heat limits for dysfunction (PSII efficiency) and damage were measured after exposure to various heat-doses (34-64 degrees C x 1-512 min) for five alpine species. The ecological significance of heat-dose was tested based on measured natural heat episodes. With increasing exposure-duration heat limits for 5% damage decreased by 11.2-17.5 K. The same was found for PSII dysfunction, but on average at 7.4 K lower temperatures. Exposure-duration and LT50 followed a slightly species-specific, but logarithmic relationship. This seems very useful for modelling. Natural heat episodes lasted longer than 30 min which questions the 30-minute test. Comparison of natural heat load with the dose-dependent heat limits highlighted that adult trees are safe, but small plants are at high risk for prolonged PSII dysfunction, and even heat damage. Heat responses are not triggered by a single threshold temperature but dose-dependent. The logarithmic relationship found makes it possible to shorten test procedures, extrapolate the response for fully hardened plants and overall more accurately model their heat damage risk in future.

Automated summary

Climate change increases the severity and duration of heatwaves, which has a significant impact on plants. The current method of determining heat limits for plants is based on a 30-minute test, which fails to consider the effect of heat-dose (intensity x exposure-duration) on heat limits. This study measured heat limits for dysfunction and damage in alpine species after exposure to various heat-doses, and found that exposure-duration had a significant impact on these heat limits. The findings suggest the need for a more comprehensive approach to assessing plants' heat tolerance.