Differential anatomical responses to elevated CO2 in saplings of four hardwood species

To determine whether an elevated carbon dioxide concentration (CO₂]) can induce changes in the wood structure and stem radial growth in forest trees, we investigated the anatomical features of conduit cells and cambial activity in 4‐year‐old saplings of four deciduous broadleaved tree species – two ring‐porous (Quercus mongolica and Kalopanax septemlobus) and two diffuse‐porous species (Betula maximowicziana and Acer mono) – grown for three growing seasons in a free‐air CO₂ enrichment system. Elevated CO₂] had no effects on vessels, growth and physiological traits of Q. mongolica, whereas tree height, photosynthesis and vessel area tended to increase in K. septemlobus. No effects of CO₂] on growth, physiological traits and vessels were seen in the two diffuse‐porous woods. Elevated CO₂] increased larger vessels in all species, except B. maximowicziana and number of cambial cells in two ring‐porous species. Our results showed that the vessel anatomy and radial stem growth of Q. mongolica, B. maximowicziana and A. mono were not affected by elevated CO₂], although vessel size frequency and cambial activity in Q. mongolica were altered. In contrast, changes in vessel anatomy and cambial activity were induced by elevated CO₂] in K. septemlobus. The different responses to elevated CO₂] suggest that the sensitivity of forest trees to CO₂ is species dependent.