Mycorrhizas are horizontally transmitted symbioses: each sexually produced generation of plants must independently obtain symbiotic partners. The ability of plants to establish may therefore be limited where mycorrhizal fungi are absent. We tested whether a lack of ectomycorrhizal fungal infection might limit the ability of northern pin oak (Quercus ellipsoidalis) and bur oak (Q. macrocarpa) seedlings to establish in old fields and oak savanna openings in Minnesota. Seedlings were planted adjacent to established trees, where mycorrhizal fungi should be abundant, and distant (> 10 m) from trees, where mycorrhizal fungi might be absent. Seedlings planted adjacent to trees had an average of 39% of root tips infected, while seedlings planted distant from trees had only 5% of root tips infected. Preliminary analyses showed higher N concentration and content in seedlings near than seedlings distant from trees. In a second experiment, seedlings planted along transects at variable distances from the edge of old fields had greatly reduced infection with increasing distance from forest edge. Seedlings at 0, 4 and 8 m from established trees had 53-56% of root tips infected, while seedlings at 20 m had < 1% infection. Seedlings at 12 and 16 m had intermediate (19-23%) but highly variable infection rates. RFLP analysis of fungi community composition showed no species-specific associations with particular distances from trees, although the high diversity of fungi present (29 RFLP-species, 19 of which occurred only once) and relatively low replication (8 transects) may have obscured any patterns. The results show that mycorrhizal infection can be low in seedlings germinating distant from established trees, and that this may lead to reduced nutrient uptake. This supports our hypothesis that lack of mycorrhizal fungi can be an important limiting factor in seedling regeneration in old fields and savanna openings.

Key words: ectomycorrhiza, mycorrhiza, Quercus ellipsoidalis, Quercus macrocarpa, savanna, symbiosis