Decline of carpophores of mycorrhizal fungi in stands of Pinus sylvestris

The carpophores of mycorrhizal fungi have declined drastically during this century in the Netherlands and in other European countries. In contrast, saprophytic and pathogenic fungi did not show a significant change. In this thesis, the possible causes of the decline of mycorrhizal mycoflora have been examined. The hypothesis was put forward that the functioning of mycorrhiza was hampered, either through a decrease of tree vitality or by changes in soil chemistry, both resulting from air pollution. Pinus sylvestris was chosen as study object, because in the Netherlands (1) its vitality has decreased considerably, (2) mycorrhizal mycoflora of coniferous tree species decreased more strongly than that of deciduous species, (3) it is the only native conifer which possesses ectomycorrhizas and (4) plantations of P. sylvestris of the same age and on the same soil type can be found throughout the country.In a pot experiment, mycorrhizas of Paxillus involutus appeared to be sensitive to SO 2 fumigation alone, or in combination with NH 3 pollution, in contrast to mycorrhizas of Laccaria proxima . Photosynthesis, measured on P. involutus -inoculated seedlings, was inhibited by SO 2 fumigation. However, effects on plant growth were negligible (Chapter 2).Nitrate and ammonium salts in a pot experiment had a significant negative effect on the mycorrhizas ( Paxillus involutus and Suillus bovinus ), and a significant positive effect on plant growth. Ammonium treatments affected the seedlings more positively and the mycorrhizas more negatively than nitrate. The N content of seedling needles fertilized with ammonium was higher than those treated with nitrate. It was suggested that a high N uptake by the plant decreased the carbohydrate availability for the mycorrhizal fungi (Chapter 3).Ammonium and nitrate fertilization at rates of 0, 30 and 60 kg N ha -1yr -1in two young stands of P. sylvestris during three years had a similar, significantly negative effect on the number and total dry weight of carpophores and on the number of fruiting species. However, the number of carpophores of Laccaria proxima increased due to the fertilization treatments in one stand. Mycorrhizal frequency and number of mycorrhizas were not affected (Chapter 4).Field observations revealed that the mycorrhizal mycoflora of young stands of P. sylvestris included species which have become rare during this century in the Netherlands (Chapter 5). Especially in first rotation young stands on drift sands many of these species were found, several of them in large numbers. This seems to be related to the fact that first rotation stands have become rare in the Netherlands. There was a considerable difference in the mycorrhizal mycoflora of first rotation young stands compared to that of second rotation young stands and of old stands. However, a literature survey showed that the fungal species which appeared to occur specifically in the first rotation young stands were also common in humus-rich and mature stands in Poland, Finland and Russia. Possible explanations for this difference are discussed.In 50 to 80-year-old stands of P. sylvestris , the number of mycorrhizal fruiting species as well as the number and total dry weight of their carpophores had highly negative correlations with the NH 3 deposition and ambient NO x concentration, and to a lesser extent with ambient SO 2 concentration (Chapter 6). The mycorrhizal mycoflora showed insignificant positive correlations with tree vitality, expressed as the needle occupation of the trees. The mycorrhizal mycoflora was very poor in most old plots.Young stands had a much richer mycoflora than old stands. Over the three years of field observations, more species (factor 3) and more carpophores (factor 13) were found in the young plots.The mycorrhizal mycoflora in 5 to 10-year-old stands was negatively influenced by infection of trees by Lophodermium seditiosum. High positive correlations were found with ambient NO x concentrations and could be partially ascribed to infection by L.seditiosum, which occurred in the less polluted areas.The mycorrhizal frequency exceeded 95% in all but one of the old plots and in all young plots, indicating that the decrease of carpophores preceedes that of mycorrhizas.The following conclusions were drawn:(1) No decline of carpophores of mycorrhizal fungi could be detected in young stands of P.sylvestris, this in contrast to the situation in old stands.(2) The carpophores of mycorrhizal fungi in young stands are negatively affected by (artificial) nitrogen fertilization. The mycorrhizas of seedlings can be negatively affected by nitrogen fertilization and by SO 2 fumigation.(3) The nitrogen effect on the mycorrhizas is likely to be a result of decreased supply of carbohydrates by the plant, caused by the increased uptake of nitrogen compounds.(4) The effects of nitrogen deposition and SO 2 pollution on P.sylvestris and mycorrhiza depend on the fungal species involved.(5) The age of forest soils determines to a great extent the mycoflora. The ageing of Dutch forests contributes to the decrease of mycorrhizal mycoflora. It is not clear to what extent air pollution influences the succession of mycorrhizal fungi.(6) Nitrogen pollution is the major factor explaining the decrease of mycorrhizal mycoflora. The absence of effects of nitrogen pollution in young plots is explained by the high nitrogen losses due to clear-cutting and soil ploughing, decreased interception of air pollutants by smaller trees and the higher need for external nitrogen of young trees.(7) Carpophores of mycorrhizal fungi are more sensitive to nitrogen pollution than mycorrhizas.(8) It is proposed that the decline in mycorrhizal mycoflora during stand development might not occur if air pollution, particularly nitrogen pollution, is drastically diminished, and if the excess nitrogen in the ecosystem is removed.