Root symbioses ofAlnus glutinosa (L.) gaertn. and their possible role in alder decline: A preliminary study

During the last few years alder has declined in South Bohemia. The possible role of mycorrhizal and actinorhizal symbioses is reviewed and some of the preliminary results from experiments testing the influence of these symbioses on alder growth and the influence of eutrophication on the development of these symbioses are reported. Seedlings ofAlnus glutinosa were inoculated with arbuscular mycorrhizal (AM) fungi and the actinomyceteFrankia in experiment 1, and with rhizosphere soil collected from field sites with different degrees of alder damage in experiment 2. In both experiments, a solution containing nitrate, ammonia and phosphorus in concentrations simulating eutrophic waters, was applied. Both symbioses markedly promoted the growth of the seedlings in experiment 1. The plants inoculated with the rhizosphere soil microflora in experiment 2 were larger than the control plants. Response of the seedlings to the inoculation with the soil from the rhizosphere of damaged alder trees from six field sites differs, even though no correlation was found relating growth to the health status of the trees. Nutrient treatment did not have any effect on the growth of seedlings in either experiment. The dry weight ofFrankia was greater in mycorrhizal plants compared to nonmycorrhizal plants and mycorrhizal colonization is reduced inFrankia inoculated plants supplemented with phosphorus in experiment 1. Nitrogen enhanced mycorrhizal colonization in nodulated plants which were not supplemented with phosphorus no effect of nitrogen on actinorhiza was observed.