Pre-industrial CO2 levels cause variation in growth response of Schoenoplectus americanus

Background/Question/Methods

Schoenoplectus americanus is a C₃ tidal marsh plant whose seeds from the last 100 years have remained viable in marsh sediments and have been collected, dated, and germinated.  Atmospheric CO₂ has risen from 290ppm to 393ppm in the time since the oldest seeds were set.  This difference in CO₂ has major implications for physiological responses such as carbon balance, water use efficiency, and allocation patterns. The goal of this experiment was to determine the effects of dropping CO₂ to pre-industrial levels (290ppm) on old and modern Schoenoplectus americanus plants and to quantify the responses of various genotypes back through time to low CO₂.  Dropping CO₂ below ambient levels was accomplished in Conviron CO2 chambers with Soda Lime amendment.  CO₂ concentrations remained stable at 290ppm and 400ppm throughout the two-month study. Three populations-1 old, 2 modern- were cloned out and planted into the treatment (290ppm CO₂) and control (400ppm).  Wet biomass, dry biomass, stem heights, shoot numbers, chlorophyll content, stomatal density, C:N ratios, and photosynthesis rates were measured. 

Results/Conclusions

Results show that lowering CO2 significantly decreased the performance of all plants.  The population by treatment interaction on total biomass (p=0.043) and rhizome biomass (p=0.009) was significant.  Additionally, the effect size of genotype was comparable to the effect size of CO2 level on biomass production, suggesting that the population identity matters when measuring responses to CO2.  These results suggest that changes in Schoenoplectus americanus over time have led to differences in response to CO2.