Larger trees suffer most during drought in forests worldwide

Area of Science:

• Forest Ecology
• Climate Change Impacts
• Drought Stress Physiology

Background:

• Increasing frequency and severity of global droughts due to climate change.
• Forest structure and function are significantly altered by drought events.
• Large trees are keystone species crucial for ecosystem carbon storage and hydrology.

Purpose of the Study:

• To synthesize global data on tree growth and mortality during drought events.
• To determine if larger trees exhibit greater drought sensitivity across diverse forest ecosystems.
• To investigate factors influencing size-dependent tree mortality during droughts.

Main Methods:

• Meta-analysis of tree growth and mortality data from 40 distinct drought events worldwide.
• Statistical examination of the relationship between tree size and drought impacts.
• Identification of co-occurring factors (e.g., bark beetle presence) associated with tree mortality.

Main Results:

• Droughts consistently exert a more detrimental effect on the growth and mortality rates of larger trees.
• Tree size-dependent mortality increased during 65% of examined droughts, particularly under high community mortality or bark beetle infestation.
• Larger trees show greater sensitivity due to hydraulic stress, radiation/evaporative demand, and preferential bark beetle attack.

Conclusions:

• Larger trees are disproportionately vulnerable to drought impacts, a pattern observed globally.
• Future droughts are predicted to have severe consequences for large tree growth and survival.
• This size-dependent vulnerability may amplify climate change feedbacks through altered forest dynamics.