Distinct seasonal dynamics of responses to elevated CO2in 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 CO2concentration ([CO2]) 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 [CO2], was tested during the fourth year of [CO2] treatment. We assumed stimulated carbon gain by elevated [CO2] even at low light conditions in strongly shade‐tolerantLuzula sylvatica, while its stimulation under elevated [CO2] in less shade‐tolerantCalamagrostis arundinaceawas expected only in early spring when the tree canopy is not fully developed. We found evidence supporting this hypothesis. While elevated [CO2] stimulated photosynthesis inL. sylvaticamainly in the peak of the growing season (by 55%–57% in July and August), even at low light intensities (50 µmol m−2 s−1), stimulatory effect of [CO2] inC. arundinaceawas found mainly under high light intensities (200 µmol m−2 s−1) at the beginning of the growing season (increase by 171% in May) and gradually declined during the season. Elevated [CO2] also substantially stimulated leaf mass area and root‐to‐shoot ratio inL. sylvatica, while only insignificant increases were observed inC. arundinacea. Our physiological and morphological analyses indicate that understorey species, differing in shade‐tolerance, under elevated [CO2] exploit distinct niches in light environment given by the dynamics of the tree canopy.