Plant responses to decadal scale increments in atmospheric CO2 concentration: comparing two stomatal conductance sampling methods

Main conclusion Our study demonstrated that the species respond non-linearly to increases in CO 2 concentration when exposed to decadal changes in CO 2 , representing the year 1987, 2025, 2051, and 2070, respectively. There are several lines of evidence suggesting that the vast majority of C3 plants respond to elevated atmospheric CO 2 by decreasing their stomatal conductance ( g s ). However, in the majority of CO 2 enrichment studies, the response to elevated CO 2 are tested between plants grown under ambient (380–420 ppm) and high (538–680 ppm) CO 2 concentrations and measured usually at single time points in a diurnal cycle. We investigated g s responses to simulated decadal increments in CO 2 predicted over the next 4 decades and tested how measurements of g s may differ when two alternative sampling methods are employed (infrared gas analyzer [IRGA] vs. leaf porometer). We exposed Populus tremula , Popolus tremuloides and Sambucus racemosa to four different CO 2 concentrations over 126 days in experimental growth chambers at 350, 420, 490 and 560 ppm CO 2 ; representing the years 1987, 2025, 2051, and 2070, respectively (RCP4.5 scenario). Our study demonstrated that the species respond non-linearly to increases in CO 2 concentration when exposed to decadal changes in CO 2 . Under natural conditions, maximum operational g s is often reached in the late morning to early afternoon, with a mid-day depression around noon. However, we showed that the daily maximum g s can, in some species, shift later into the day when plants are exposed to only small increases (70 ppm) in CO 2 . A non-linear decreases in g s and a shifting diurnal stomatal behavior under elevated CO 2 , could affect the long-term daily water and carbon budget of many plants in the future, and therefore alter soil–plant–atmospheric processes.