How do fungal associations of trees shift across ranges? Exploring climatic and non-climatic drivers of rhizosphere fungal communities across temperate tree species ranges

Background/Question/Methods

Forests are expected to undergo dramatic shifts in community composition as the climate continues to change. Predictions of future forest composition are made based on mapping current species-climate relationships onto projected future climates. This is possible for tree species in the eastern temperate forests of North America due to our highly resolved information on tree distributions. However, a tree species’ ability to expand into new areas or tolerate novel climates in its current range may be influenced by other species interactions, for instance with rhizosphere fungi that can directly affect root water and nutrient uptake. We have much less understanding of how rhizosphere fungal species are distributed across space, or across climatic and non-climatic environmental gradients. To address this, we have worked with citizen scientists for the last six years to collect root samples from forests across the eastern US, from which we characterize rhizosphere fungal communities using metabarcoding of the ITS2 gene. With this large, and growing, dataset, we test whether tree genera associated with a different diversity and composition of fungi in their range centers vs leading or trailing edges, and the relative influence of climatic gradients, abiotic soil properties, and host traits on rhizosphere fungal community structure.

Results/Conclusions

To date, we have received >800 root samples from >60 distinct locations, ranging from the southeastern US to the Canadian border. Samples have come from 25 tree genera. Roughly half of the samples come from trees that associated with ectomycorrhizal fungi, and half from trees that associate with arbuscular mycorrhizal fungi, mirroring the generally even distribution of these mycorrhizal strategies in temperate forests. Mycorrhizal type was a major factor structuring rhizosphere fungal community composition, not surprisingly. More interestingly, mycorrhizal type determined how fungal community structure responded to climatic gradients. Fungal communities in AM tree species showed a strong compositional response to mean annual temperature, with no significant response to precipitation. Fungal communities in EM tree species showed equally strong, and significant, responses to both temperature and precipitation. This in part reflected the response of different fungal guilds. Composition of EM fungal communities was only significantly associated with precipitation, not temperature, while composition of non-mycorrhizal fungi was associated with both gradients, while AM community composition was not significantly associated with either climatic variable. These results suggest that fungal guilds may respond to changing climates in unique ways, which could act to constrain or accelerate changes in forest structure.