Delayed succession from alpine grassland to savannah with upright pine: Limitation by ectomycorrhiza formation?

We investigated causes for the slow re-forestation of grassland surrounded by upright pine forest (Pinus mugo (L.) var. uncinata) in the Swiss National Park. General opinion holds that the large numbers of grazing and trampling wild animals prevent re-forestation. However, pine trees depend on the symbioses with ectomycorrhizal fungi, especially at the study site with its poor, nutrient-limiting soil, as reflected e.g. by the low nitrogen concentration in needles. Here, we show that the ectomycorrhizal symbiosis needs to be considered as well.We investigated young trees of upright pine along four transects from the edge of the forests into the grassland and expected a gradient of mycorrhizal colonization, but all the 47 young trees collected in the four selected transects possessed well-developed ectomycorrhizas independent of their position. Similarly, ergosterol concentration in rhizosphere soil of the young trees i.e. mainly ergosterol from ectomycorrhizal fungi, showed no gradient along the transects. Morphotyping of ectomycorrhizas on the fine roots and fruit body determination revealed that Suillus granulatus was the dominant ectomycorrhizal fungal species at the study site.Young pine trees were randomly distributed along transects and did not develop preferentially at the forest edge, indicating that the common mycorrhizal network (wood wide web) established in the surrounding forest had little importance for re-colonization. Fungal fruit bodies of S. granulatus, in contrast, displayed a clustered distribution and 50% grouped within 1 m of the nearest young tree.Chamois feed on fruit bodies of S. granulatus, as observed at the site, and a model experiment showed colonization of axenic pine seedlings by S. granulatus when inoculated with faecal pellets from chamois. Chamois may profit also from the mushrooms: the concentration of nitrogen in the cap of the fruit bodies was almost 5% (pine needles 1.0%). Thus, grazing animals such as chamois may spread mycorrhizal inoculum and promote, rather than prevent, re-forestation.