Fungal / ectomycorrhizal microbiome in forest soils near clumps of young trees

Forest soils are rich in fungi, including ectomycorrhizae that form mutualistic relationships with trees. In recent years, more young trees planted in Belgian forests have struggled to survive. To understand these shifting dynamics, soil samples were taken around these trees before planting and at five time points afterwards, over a period of ca. 4 years. Four tree species growing in different forests were studied: Quercus robur, Quercus petraea, Tilia cordata and Pinus sylvestris. Prior to planting, trees received one of four options: two treatments aimed at improving survival and vitality (myco and gel+myco) or two controls (gel and no treatment). The myco treatments consisted of a preparation presumed to contain ectomycorrhiza. Gel was intended to improve contact between ectomycorrhiza and tree roots when combined with myco, but was also tested without myco as a control, since the gel can influence water retention. The roots of the plantlets were dipped in the gel solution prior to planting in case of the treatments including gel; while for the myco treatment, the myco solution was mixed with the planting soil. These treatment solutions, as well as the soil in which the trees were planted, were sampled before application to assess their fungal communities using DNA metabarcoding targeting ITS2. After four years, survival rate, height, and annual shoot length of the young trees were measured. For comparison, soil around mature trees was also sampled at different timepoints and analysed using metabarcoding. Our findings suggest that treatments only benefited Pinus sylvestris, which could be related not to tree species but to forest age, which was relatively young in this case. Moreover, there seems to be no superior treatment: even the gel control without myco resulted in higher survival and vitality. There are probably multiple explanations for this observation. The gel may have influenced the water availability around the roots, which could explain why all treatments had a positive effect compared to the control (no treatment). The myco preparations contained little or no ectomycorrhiza, hence we can hypothesize that this was the reason that their added value was minute. Higher ectomycorrhizal abundance in soil coincided with greater survival and vitality for Pinus sylvestris. The other tree species planted in older forests did not benefit from treatments or higher ectomycorrhizal abundance, possibly a consequence of our observation that ectomycorrhiza were relatively more abundant before planting than in the young forest. In addition to the experimental setup with young plantlets, soil surrounding mature trees was sampled to explore the ectomycorrhizal community structure. In general, the relative amount of ectomycorrhizae in the fungal community was much larger in soils of older forests versus younger forests. Soils surrounding mature trees had approximately the same relative abundance of ectomycorrhiza as soils near young trees in older forests, which was on average 4 times higher than in the soil near the young trees in the young forest. Furthermore, ectomycorrhizal percentages varied over time, even around the same tree, due to the heterogeneous distribution of soil fungi and environmental circumstances (different seasons).