Trees conserve water under high CO2 by narrowing their stomata to limit water vapor loss, but this reduced gas exchange restricts the carbon intake necessary for growth. While this increases water-use efficiency by up to 85%, it prevents accelerated growth, especially as hotter, drier conditions cause trees to restrict stomata further to protect internal plumbing.

Trees conserve water under high 

CO2 by narrowing their stomata to limit water vapor loss, but this reduced gas exchange restricts the carbon intake necessary for growth. While this increases water-use efficiency by up to 85%, it prevents accelerated growth, especially as hotter, drier conditions cause trees to restrict stomata further to protect internal plumbing. 

Key Reasons for Reduced Growth Rates Despite High CO2

Stomatal Constriction:-

 While high CO2 allows plants to take in enough carbon with smaller pore openings, this increased efficiency does not equate to higher total carbon absorption.

Water-Carbon Trade-off: -

Trees face a direct trade-off between maximizing carbon absorption and minimizing water loss, explains Phys.org. In hotter, drier air, trees often close their pores to avoid dehydration, which limits the CO2 intake required for photosynthesis.

Nutrient Limitation: -

Increased CO2 may not lead to faster growth if trees lack sufficient soil nutrients (like nitrogen or phosphorus) to support faster development, a common limitation in many forest ecosystems.

Resource Allocation: -

Instead of increasing growth, studies show that trees might invest the extra absorbed carbon into root development or simply store it rather than using it to build new wood, notes israelagri.com. 

    Ultimately, this increased efficiency allows trees to survive under drought conditions better, but it does not act as a "fertilizer" that significantly speeds up their growth in a warming world, as explained in this article .

Trees "save" water when CO2 is higher, but they don't grow faster .

MJF Lion ER YK Sharma