Red alder (Alnus rubra Bong.) seedlings fertilized with NH₄NO₃ or inoculated with a pure culture Frankia and inoculated with live or dead spores of the mycorrhizal fungus Alpoaa diplophloeus (Zeller & Dodge) Trappe & Smith were grown in a growth chamber or in a greenhouse for six months. Frankia inoculation and subsequent nodulation and N₂-fixation increased seedling growth more than N-fertilization. Alpoaa inoculation significantly increased seedling growth in some parameters by increments of 6 to 16% but only when the seedlings were also inoculated with Frankia. Water stress significantly decreased nodule and Alpova ectomycorrhiza development, N₂-fixation, growth and photosynthesis of red alder seedlings. Alpova inoculation did not improve water relations of red alder seedlings. Heavy N-fertilization with 5m1 of 50mM NH₄NO₃ per seedling three times a week significantly increased mycorrhiza formation and N and Y concentration in leaves but decreased N₂-fixation, shoot growth and P concentration in nodules. P Yertilization with 5ml of 5mM KH₂PO₄ per seedling: three times a week significantly increased total N-fixation, Light intensities below photosynthetic photen flux density of 220 μmol/㎡/s significantly decreased N₂-fixation, total plant growth and photosynthesis, but increased leaf area, shoot to root ratio and N and P concentrations in plant tissues. Reduced light significantly decreased Alpoca mycorrhiza formation after three-weeks shading in Experiment 1 but not after ten-weeks shading of Experiment 2. During the long shade exposure of Experiment 2, alder seedlings adapted morphologically to low light intensity, thus moderating negative effects on mycorrhiza formation plants were unable to do this in the 3-week shade period of Experiment 1. Results suggest that Frankia is more important for Alpova mycorrhiza formation and growth of red alder seedlings than Alpoua is for nodule formation and growth. Alder seedlings apparently adapt to a certain point of light stress by increasing leaf area and shoot growth and maintain balanced symbiont development and growth N-fertilization does not affect modulated alder plant growth. The importance of P fertilization and mycorrhizae to plant growth increases with plant size and age.