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

Our study demonstrated that the species respond non-linearly to increases in CO concentration when exposed to decadal changes in CO, 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 by decreasing their stomatal conductance (g). However, in the majority of CO enrichment studies, the response to elevated CO are tested between plants grown under ambient (380-420 ppm) and high (538-680 ppm) CO concentrations and measured usually at single time points in a diurnal cycle. We investigated g responses to simulated decadal increments in CO predicted over the next 4 decades and tested how measurements of g 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 concentrations over 126 days in experimental growth chambers at 350, 420, 490 and 560 ppm CO; 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 concentration when exposed to decadal changes in CO. Under natural conditions, maximum operational g 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 can, in some species, shift later into the day when plants are exposed to only small increases (70 ppm) in CO. A non-linear decreases in g and a shifting diurnal stomatal behavior under elevated CO, could affect the long-term daily water and carbon budget of many plants in the future, and therefore alter soil-plant-atmospheric processes.