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 CO2 concentration
when exposed to decadal changes in CO2, representing the year 1987, 2025, 2051, and 2070, respectively.
Abstract There are several lines of evidence suggesting that the vast majority of C3 plants respond to elevated atmospheric
CO2 by decreasing their stomatal conductance (gs). However, in the majority of CO2 enrichment studies, the response to
elevated CO2 are tested between plants grown under ambient (380–420 ppm) and high (538–680 ppm) CO2 concentrations
and measured usually at single time points in a diurnal cycle. We investigated gs responses to simulated decadal increments
in CO2 predicted over the next 4 decades and tested how measurements of gs may difer 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 diferent CO2 concentrations over 126 days in experimental growth chambers at 350, 420,
490 and 560 ppm CO2; representing the years 1987, 2025, 2051, and 2070, respectively (RCP4.5 scenario). Our study demonstrated that the species respond non-linearly to increases in CO2 concentration when exposed to decadal changes in CO2.
Under natural conditions, maximum operational gs is often reached in the late morning to early afternoon, with a mid-day
depression around noon. However, we showed that the daily maximum gs can, in some species, shift later into the day when
plants are exposed to only small increases (70 ppm) in CO2. A non-linear decreases in gs and a shifting diurnal stomatal
behavior under elevated CO2, could afect the long-term daily water and carbon budget of many plants in the future, and
therefore alter soil–plant–atmospheric processes.