Spore heat resistance plays an important role in disturbance-mediated assemblage shift of ectomycorrhizal fungi colonizing Pinus muricata seedlings

Abiotic disturbance plays an important role in determining assemblage structure and maintaining species richness in many ecosystems. Disturbances events are complex, often affecting multiple environmental parameters simultaneously and causing selective removal of biomass. For this reason, observational studies often fail to elucidate the primary mechanism by which disturbance modifies assembly structure. This is particularly true of microbial systems where reduction in biomass or removal of competitive dominants is not visually obvious. Fire is a major disturbance in pinaceous forests, which are characterized by ectomycorrhizal fungal root associations. To determine the specific mechanisms through which this disturbance affects ectomycorrhizal communities, we experimentally simulated the effects of fire by growing seedlings in field soil exposed to factorial combinations of soil heating and ash addition. Ectomycorrhizal colonists in each of the treatments were then identified from seedling roots using DNA sequences. Both soil heating and ash addition caused changes to the soil chemical environment and altered the fungal assemblages found on seedling roots. The effect on the ectomycorrhizal assembly was most pronounced from soil heating, which caused an overall simplification and increasing dominance by a few taxa with ruderal life-history strategies. We also conducted a follow-up experiment to see whether selective mortality of ectomycorrhizal spores might contribute to these patterns. Heat exposure had strong negative effects on all species tested. However, spore heat tolerance was highest for the species that showed the greatest increase in abundance on seedlings grown in heated soils. Synthesis. Fire alters below-ground communities through changes in soil chemistry and mortality of resident organisms. Our results show that soil heating and selective mortality are key mechanisms determining post-fire assemblage structure of ectomycorrhizal fungi. These disturbances favour competitively inferior species and may help maintain diversity of ectomycorrhizal assemblages at the landscape scale.