Scorched, not senescent: When hot droughts burn leaves instead of aging them

Hot droughts are becoming increasingly frequent worldwide, causing widespread and abrupt leaf discoloration in temperate forests. Because changes in leaf colour are commonly associated with autumnal senescence, such abrupt discoloration is often interpreted as premature or stress-induced senescence. Nonetheless, under hot droughts, excessive heating and/or hydraulic failure may cause leaf tissue damage, leading to leaf scorching. This process produces visual symptoms similar to senescence but arises from fundamentally different physiological processes. Despite its potential importance, leaf scorching remains poorly studied.

Using climate chambers, we exposed three species (Fagus sylvatica, Quercus Pubescens, and Prunus mahaleb) to four temperature treatments (25°C, 35°C, 40°C, and 45°C) under severe water limitation. Through regular physiological (predawn and midday water potential, chlorophyll content, stomatal conductance, maximum potential quantum efficiency of Photosystem II, leaf embolism) and continuous leaf colour measurements, we aimed to identify the physiological tipping points of leaf scorching and to provide a clearer distinction between the leaf discoloration processes.

 Leaf scorching occurred only under the highest temperature treatments (40°C and 45°C), with its extent varying among species according to their inherent thermotolerance. Notably, in Fagus sylvatica, leaf tissue damage appear to develop prior to leaf embolism, indicating that temperature excess rather than hydraulic dysfunction was the primary trigger of scorching under extreme heat.