Distinct Seasonal Dynamics of Responses to Elevated CO in Two Understorey Grass Species Differing in Shade-tolerance

Understorey plant communities are crucial to maintain species diversity and ecosystem processes including nutrient cycling and regeneration of overstorey trees. Most studies exploring effects of elevated CO concentration ([CO]) in forests have, however, been done on overstorey trees, while understorey communities received only limited attention.The hypothesis that understorey grass species differ in shade-tolerance and development dynamics, and temporally exploit different niches under elevated [CO], was tested during the fourth year of [CO] treatment. We assumed stimulated carbon gain by elevated [CO] even at low light conditions in strongly shade-tolerant , while its stimulation under elevated [CO] in less shade-tolerant was expected only in early spring when the tree canopy is not fully developed.We found evidence supporting this hypothesis. While elevated [CO] stimulated photosynthesis in mainly in the peak of the growing season (by 55%-57% in July and August), even at low light intensities (50 µmol m s), stimulatory effect of [CO] in was found mainly under high light intensities (200 µmol m s) at the beginning of the growing season (increase by 171% in May) and gradually declined during the season. Elevated [CO] also substantially stimulated leaf mass area and root-to-shoot ratio in , while only insignificant increases were observed in .Our physiological and morphological analyses indicate that understorey species, differing in shade-tolerance, under elevated [CO] exploit distinct niches in light environment given by the dynamics of the tree canopy.