Field growth of Pinus sylvestris following nursery inoculation with mycorrhizal fungi

Growth of Pinussylvestris L. seedlings inoculated with a number of forest mycorrhizal fungi (Amanitamuscaria (L. ex Fr.) Hooker, Lactariusrufus (Scop.) Fr., Suillusvariegatus (Swartz ex Fr.) O. Kuntze, Tricholomaalbobrunneum (Pers. ex Fr.) Kummer, and an unknown mycorrhizal fungus) in the nursery was studied for 3 years after field planting. At outplanting, 10–40% of the root tips were mycorrhizal, consisting of both the inoculated fungi and an indigenous nursery mycorrhiza, Thelephoraterrestris (Ehrh.) Fr. In the field the target fungi were replaced by several indigenous forest mycorrhizal fungi, which also rapidly colonized the remaining uncolonized root system. At outplanting, some of the inoculated seedlings were substantially smaller than the uninoculated seedlings. However, after 2.5 years seedlings in some of the treatments were up to 50% larger in volume than control seedlings. Even at low initial mycorrhizal colonization rates some mycorrhizal species stimulated seedling growth. The most noticeable response occurred in seedlings inoculated with A. muscaria; although they were about 50% smaller than the control seedlings at outplanting, they grew so rapidly that 18 months later they were about 20% larger than the controls. This observation contradicts previous reports on the importance of seedling size at outplanting. Possible explanations for the observed effects are discussed.