Hydraulic stress limits thermal acclimation in trees under chronic drought
Alyssa T Kullberg1,2, Arianna Milano1,2, Alvaro Poretti1,2
1Plant Ecology Research Laboratory, School of Architecture, Civil and Environmental Engineering, École polytechnique fédérale de Lausanne, Lausanne CH-1015, Switzerland.
Summary
Chronic hot droughts challenge trees. While oaks and beeches can adapt to warming alone, combined heat and drought impair leaf cooling, leading to thermal injury and limiting forest resilience.
Area of Science:
- Forest ecology
- Plant physiology
- Climate change science
Background:
- Tree survival during hot droughts relies on hydraulic safety and leaf thermoregulation.
- The limits of this coordination under chronic stress are not well understood.
Purpose of the Study:
- To investigate how 5 years of soil moisture limitation affects leaf thermoregulation in European beech and downy oak.
- To assess the influence of acclimation on hydraulic safety margins (HSMs), thermal safety margins (TSMs), and leaf scorching under manipulated soil moisture and temperature.
Main Methods:
- A 5-year experiment manipulating soil moisture and air temperature in European beech (Fagus sylvatica) and downy oak (Quercus pubescens).
- Monitoring of leaf thermoregulation, HSMs, TSMs, and leaf scorching.
Main Results:
- Both species acclimated to warming alone, maintaining stable leaf temperature and positive TSMs.
- Chronic soil drought narrowed HSMs and reduced evaporative cooling capacity.
- Co-occurring drought and heat created a feedback loop: impaired water transport led to loss of cooling, breaching thermal thresholds, and causing photosystem II failure and scorching in beech.
Conclusions:
- Temperate trees like oak and beech can acclimate to warming independently but not to simultaneous heat and drought.
- The interaction between heat and drought creates a hydraulic-thermal cascade that exceeds safety margins.
- This interaction fundamentally limits the resilience of temperate forests to future hot droughts.
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