Background It is of great significance to study the changes of plant physiological parameters under climate change for soil and water conservation and vegetation restoration. Phoebe bournei is a unique and rare species in China. It needs to live in a place with better site conditions. Thus, how to adjust the relationship between transpiration and photosynthesis of P. bournei to adapt to changes in soil moisture and CO₂ concentration, under global climate change, is an urgent problem to be solved. In order to understand the underlining mechanisms of effects of CO₂ concentration and soil moisture on leaf stomatal traits and gas exchange parameters of Phoebe bournei, indoor simulated experiments were conducted.

Methods Utilizing artificial incubators, 4 CO₂ concentrations (400, 500, 600, and 800×10^-6) and 2 soil moisture gradients (14%-16% (drought) and 25%-27% (field capacity)), a total of eight water-carbon control conditions were set to cultivate 5-year-old P. bournei for 4 months. The new leaves were collected to measure the indicators, including stomatal density, opening extent of stomatal, stomatal conductance, net photosynthetic rate, transpiration rate and intercellular CO₂ concentration. Meanwhile, water use efficiency was calculated.

Results 1) The response of leaf stomatal density to CO₂ concentration was more sensitive than to soil moisture gradient. High CO₂ concentration resulted in the reduction of stomatal density of leaves in tip, middle and tail. The stomatal density was higher in field capacity than drought. The higher the CO₂ concentration was, the greater the difference of stoma density under different moisture gradient was. 2) The sensitivity of stomata opening extent to soil moisture response was higher than that of CO₂ concentration. Under the same soil moisture gradient, for every 100×10^-6 increase of CO₂ concentration, the proportion of pore opening decreased only about 2%-4%. With different soil moisture gradient, the range of stoma opening ratio was around 15%-50%. 3) CO₂ concentration had strong influences on the net photosynthetic reaction rate, transpiration rate and water use efficiency of Phoebe bournei. They showed exponential relationships with CO₂ concentration. Especially in field capacity, the response relationship was faster than under drought. However, the effects of soil moisture gradient on leaves gas exchange parameters were weaker.

Conclusions The sensitivity of stomatal density and gas exchange parameters of Phoebe bournei to CO₂ concentration was higher than that to soil water. However, stomatal opening extent and conductance responded more strongly to soil moisture.