Dominik Merges (PhD candidate, Senckenberg Biodiversity and Climate Research Centre) has written a blog post about his recently published Journal of Ecology article about the spatial patterns of plant-associated fungi. 

When you think of Swiss alpine valleys, you might think of gentle mountain slopes covered with conifers (see picture 1). These tree line forests present in alpine slopes such as these, perform important ecosystem services, such as protection against soil erosion and avalanches. Under climate warming, many plants at the tree line are expected to shift their distribution uphill in order to track their climatic niches. Such upwards shifts can, however, be limited by the presence of antagonistic interaction partners or the absence of mutualistic interaction partners.

Picture 1: Swiss stone pine (Pinus cembra) trees forming the tree line in the Swiss Alps. Mortality is high in Swiss stone pine seedlings, often caused by snow fungi that attack pine needles during the long winter when seedlings are covered by snow (Photo: D. Merges)

In the central European Alps, tree line forests are often dominated by Swiss stone pine (Pinus cembra). The establishment and survival of Swiss stone pine closely depends on mutualisms with mycorrhizal fungi, which provide pines with nutrients and protect against pathogen infection. In turn, pathogenic fungi, e.g. snow fungi, can hinder pine survival (picture 1). Yet, it is unclear whether or not fungal mutualists and pathogens occur across the pine’s elevational distribution and beyond its current range. Understanding the distribution patterns of pathogens and mutualists could help projecting potential upwards movements of Swiss stone pine under global warming.

In our study, we used DNA metabarcoding, a rapid method for assessing biodiversity using high throughput sequencing of environmental DNA from soil samples, to investigate pine-associated fungal communities in the Swiss Alps. We analyzed the effects of elevation, abiotic (i.e. temperature, pH, soil moisture) and biotic factors (i.e. distance to host, vegetation cover) on fungal mutualists and pathogens associated with Swiss stone pine. As host specialist fungi are likely to have stronger effects on hosts than generalists, we additionally examined different levels of host specialization.

Picture 2: We studied pine-associated fungal communities across and beyond the elevational distribution of Swiss stone pine on transects ranging from 1850 to 2250 m a.s.l. (Photo: D. Merges)

We found that fungal mutualists and pathogens are widely distributed within and beyond the current elevation range of their host plant. Generalist fungal associates were not influenced by abiotic or biotic factors, whereas host specialist fungi were mainly determined by the distance to their host. The observed patterns in pathogens were consistent with the Janzen-Connell hypothesis, which predicts accumulation of pathogens close to conspecific adult host plants. Interestingly, we found the same pattern for fungal mutualists, suggesting that positive feedbacks by mutualists could potentially cancel out negative feedbacks of pathogens.

Our results suggests that a potential upwards shift of Swiss stone pine forests to higher elevations under warming climates could be limited by the presence of pathogenic fungi, but might not be limited by the absence of mutualistic fungi. Our study emphasizes that DNA metabarcoding approaches are a valuable tool to analyze spatial occurrence pattern of fungi across environmental gradients.

Dominik Merges, Miklós Balint, Imke Schmitt, Katrin Böhning-Gaese and Eike Lena Neuschulz

Read the full paper: Spatial patterns of pathogenic and mutualistic fungi across the elevational range of a host plant