Influence of arbuscular mycorrhizal fungi and simulated acid rain on the growth and coexistence of the grasses Calamagrostis villosa and Deschampsia flexuosa

Two grass species — Calamagrostis villosa (Chaix) J.F. Gmelin and Deschampsia flexuosa (L.) Trin. — are expanding in mountain Norway spruce (Picea abies L. Karst.) forests of Central Europe damaged by anthropogenic pollution constituted particularly of acid rain. This invasion of grasses may be caused by the higher irradiance reaching the forest floor after the pollution-induced tree defoliation. The relative abundance of the two grass species is changing during the process of forest decline. Our study investigated the effects of arbuscular mycorrhizal fungi (AMF) on the growth and coexistence of both species under simulated acid rain (SAR) and two levels of irradiance. Three microcosm experiments were conducted to investigate how both grasses are influenced by the AMF when grown separately or together interacting via extraradical mycelium (ERM). A positive growth response to inoculation with Glomus mosseae BEG 25 was found for both grass species when cultivated separately and the mycorrhizal dependence and the growth benefit for D. flexuosa was greater than for C. villosa. However, when both grass species were grown together in the rhizoboxes with separated root and hyphal compartments, the growth effect of the AMF was the opposite, i.e. C. villosa benefited more. The plants did not benefit from the AMF inoculation under the SAR treatment compared with dH2O treatment. The SAR also negatively influenced root length colonised by AMF, length of the ERM, alkaline phosphatase and NADH diaphorase activities of the ERM. The role of the ERM in transporting phosphorus between these grasses was verified by applying the radioisotope ³²P. There was a greater transport of isotopic ³²P between inoculated plants C. villosa and D. flexuosa grown in separated root compartments, as compared to non-inoculated plants. The amount of transported ³²P was low: a maximum of 3% of applied ³²P was detected in the shoots of receiver plants. Mechanical disturbance of the ERM significantly decreased the ³² P transport between plants. The ³²P transport between mycorrhizal plants was higher in the D. flexuosa to C. villosa direction than in the opposite one. Neither the SAR nor the low level of irradiance influenced the amount of transported ³²P. We discuss the role of ERM links between root systems in the coexistence of both grass species. This revised version was published online in June 2006 with corrections to the Cover Date.