Light availability and light demand of plants shape the arbuscular mycorrhizal fungal communities in their roots

Woody plant encroachment is influencing many open, grassy ecosystems across the globe, such as savanna, tundra and temperate grassland ecosystems. Drivers of woody plant encroachment are local land use change and global climate change, with shifts in grazing and mowing regimes as important local drivers and elevated CO2 levels, higher temperature and altered precipitation amounts as global drivers. Encroachment of woody species into open, herbaceous ecosystems comes along with substantial shifts in soil conditions, a reduction light availability and ultimately vegetation shifts in the understorey towards species better adapted to the ambient conditions. While vegetation shifts in response to woody plant encroachment in grassy ecosystems have been frequently investigated, e.g. regarding altered plant composition and functional traits related to resource acquisition and dispersal, consequences for biotic interactions have been less studied.

The symbiosis of plant roots with mycorrhizal fungi is one of the most relevant biotic interaction for plants species, with over 90% of all plants forming mycorrhizal symbiosis and arbuscular mycorrhizal symbiosis as the most prominent mycorrhizal type among herbaceous species. Plants involved in the arbuscular mycorrhizal (AM) symbiosis trade photosynthetically derived carbon for fungal-provided soil nutrients. However, little is known about how plant light demand and ambient light conditions influence root-associating AM fungal communities, and thus their response to prominent climate change processes like shrub encroachment.

We conducted a manipulative field experiment to test whether plants’ shade tolerance influences their root AM fungal communities in open and shaded grassland sites. We found that light-dependent shifts in AM fungal community structure were similar for experimental bait plant roots and the surrounding soil. Yet, lower AM fungal beta and gamma diversity for shade-intolerant plants in shade likely reflected preferential carbon allocation to specific AM fungi due to the limited plant carbon available to support symbiotic fungi. We conclude that favourable environmental conditions, including optimal light availability, widen the plant biotic niche, i.e. selectivity for specific AM fungi is reduced, and compatibility with a larger number of AM fungal taxa is promoted. With respect to predicted stronger woody plant encroachment predicted under current climate change scenarios, these results indicate that we might be losing AM fungal diversity and the functions associated with these fungal taxa. This calls for continous investment into conservation efforts and management practices to counteract this trend and keep savanna, tundra and grassland ecosystems open.