Changes in biomass allocation buffer low CO₂ effects on tree growth during the last glaciation

Area of Science:

• Paleobotany
• Plant physiology
• Climate science

Background:

• Atmospheric carbon dioxide (CO2) levels were significantly lower (~180 ppm) during the last glacial period.
• C3 plants, like junipers, have a CO2 compensation point below which photosynthesis is not sustainable.
• Understanding plant responses to past CO2 fluctuations is crucial for predicting future climate change impacts.

Purpose of the Study:

• To investigate how junipers maintained growth rates under low atmospheric CO2 during the last glacial period.
• To model the physiological and allocation strategies enabling stable growth.
• To assess the impact of glacial climate conditions on juniper carbon assimilation and growth.

Main Methods:

• Isotopic measurements of juniper tissues from southern California.
• Utilized a coupled light-use efficiency and tree growth model.
• Analyzed glacial climate data (temperature, vapor pressure deficit) for La Brea.

Main Results:

• Leaf-internal CO2 (ci) approached the compensation point, yet stem growth rates were comparable to modern levels.
• Stable ci/ca ratios were maintained due to decreased temperature and vapor pressure deficit.
• Reduced photorespiration and a ~27% decrease in fine root to leaf area ratio were necessary for sustained growth.
• Reduced drought stress under glacial conditions facilitated shifts in resource allocation.

Conclusions:

• Junipers exhibited physiological and allocation plasticity to maintain radial growth under past low CO2 conditions.
• Environmental factors like lower temperature and humidity compensated for low ambient CO2.
• Plant adaptation strategies observed in the past offer insights into responses to ongoing global CO2 increases.