Back to the Future: Ancestral plants test assumptions in future growth trajectories

Background/Question/Methods

Plant growth response to elevated [CO₂] is difficult to predict because relationships between plant performance and atmospheric [CO₂] are not static: ecosystem interactions, plant acclimation, and evolution also affect plant performance.  Ecosystem interactions and plant acclimation do alter plant performance under elevated [CO₂], but the role of evolution in responses to rising [CO₂] over a 100-year period is uncertain. We cannot fast-forward a century to investigate the trajectory of growth in elevated [CO₂], but we can go “back in time” to measure plant response to rising [CO2] of the past and the present. Seeds from Schoenoplectus americanus, a dominant C₃ salt marsh sedge of the Mid-Atlantic region, remain viable in sediments for up to 100 years. We germinated these old seeds and cloned out the plants, forming an “ancestral” population not adapted to current [CO₂] levels, genetically and morphologically distinct from modern populations. To quantify the effect of genetic changes on plant growth predictions, we subjected the ancestral and a modern population to past (~290ppm) and current (~400ppm) [CO₂] levels and measured growth throughout the two-month experiment.

Results/Conclusions

Results indicate that if the ancestral Schoenoplectus americanus population were used to predict the trajectory of growth in 400ppm [CO₂] without accounting for genetic shifts in the population, the prediction would overestimate the stimulation of growth in the modern population by about 30%.  Ancestral plants had significantly more aboveground growth and stem density in 400ppm [CO₂] than the modern population (p<0.05).  Modern plants also had reduced phenotypic plasticity in response to [CO₂] compared to the ancestral plants, suggesting genetic accommodation as a mechanism for selection.  This study shows that genetic changes over a relatively short period (100 years) can significantly change the trajectory of growth as [CO₂] continues to rise, which could alter how we think about projections for the future.