Distinguishing leaf scorching from senescence under climate extremes

Global temperatures have risen rapidly in recent decades, leading to more frequent extreme events such as droughts, heatwaves and their co-occurrence — so-called hot droughts¹. These events have caused widespread and abrupt leaf discolouration in temperate forests, as observed during the 2021 Pacific Northwest heat dome² and the 2018 and 2022 European widespread hot droughts³. Because leaf colour change is commonly associated with autumn senescence, such discolouration is often interpreted as premature or stress-induced senescence. However, not all early discolouration reflects an activated senescence program: thermal damage and hydraulic failure under hot droughts can cause passive leaf scorching. This process visually resembles senescence but arises from fundamentally different physiological mechanisms, impairing nutrient remobilization from leaves, and thereby depleting resources critical for future plant survival and growth. Distinguishing programmed senescence from stress-induced scorching is therefore crucial for accurately attributing phenological shifts and assessing how nutrient depletion and stress damage shape forest ecosystem responses to climate change.

Unlike programmed senescence, stress-induced leaf scorching is abrupt, rapid (that is, can occur within hours²) and unregulated, strongly impairing nutrient resorption (Fig. 1). Scorching commonly begins at leaf tips or margins and spreads inwards⁷, producing dry, crispy or curled tissues, and often uneven colour loss (Fig. 1). Under high temperatures, intense radiation and elevated vapour pressure deficits, water supply becomes insufficient to hydrate and cool leaf tissues, leading to overheating and desiccation. Proposed mechanisms include hydraulic failure, such as reaching 80% loss of hydraulic conductivity in the xylem (P₈₀), and cellular failure associated with the exceedance of foliar thermal thresholds, although both remain unconfirmed in situ (Fig. 1). Analogous damage, sometimes termed leaf firing, is documented in crops such as maize, sorghum and grapevine^8,9, where genotypes with greater membrane stability exhibit higher thermal tolerance and delayed scorching symptoms under hot drought¹⁰. Pathogens such as Xylella fastidiosa may further exacerbate scorching damage by obstructing xylem flow via biofilm formation, a hallmark of Pierce’s disease¹¹.