Mycorrhizal fungal diversity in willow forests of different age along the river Waal, The Netherlands

In The Netherlands, riparian edge forests are often dominated by Salix species because they are well adapted to anaerobic soil conditions. Willows play therefore an important role in rehabilitation of natural riparian edge forests. They associate with both arbuscular mycorrhizal (AM) and ectomycorrhizal (EM) fungi which are essential for successful restoration of natural ecosystems, but it is still unknown what communities of mycorrhizal fungi develop on Salix species in this environment. Salix alba L. (white willow) forests of three different ages (10, 20 and 60 years old) under restoration management along the river Waal were studied by molecular techniques to describe species composition of mycorrhizal fungi in roots and soil surrounding the trees. Presence of AM colonization was not determined in the roots of S. alba and only EM fungi were observed. Frequencies of EM root tips were below 9% at all sites. Twelve types of mycorrhizat fungi were distinguished in the roots. The mycorrhizal below-ground communities were dominated by Tuber species and to our best knowledge, this is the first molecular indication that Tuber spp. are associated with roots of Salix species. The basidiomycetous fungi including Hebeloma and thelephoroid spp. were found mainly in the 20-year-old forest with the lowest soil nutrient concentrations and were apparently able to withstand flooding. A tendency was observed for higher frequencies of mycorrhizal roots with lower nutrient concentrations and higher soil pH among the sites of different age. The results demonstrate that only a limited number of mycorrhizal fungi can resist the harsh conditions caused by flooding and drought.