Testing the ecological stability of ectomycorrhizal symbiosis: effects of heat, ash and mycorrhizal colonization on Pinus muricata seedling performance

Understanding how abiotic conditions mediate the outcome of biotic interactions is a key question in community ecology. This is particularly interesting in the case of mutualisms because changing environmental conditions may be a source of evolutionary or ecological instability in the relationship between symbiotic partners. For the mycorrhizal symbiosis, elevated nutrient levels may make the carbon cost to plants of supporting mycorrhizal fungi outweigh the benefits of mycorrhizal nutrient acquisition. In this study, we simulated the effects of post-fire nutrient enrichment on the ectomycorrhizal symbiosis between Rhizopogon occidentalis and Pinus muricata by using factorial combinations of soil heating, ash addition and mycorrhizal colonization. In general, the presence of R. occidentalis and the heat and ash treatments increased growth of P. muricata seedlings. However, significant interactions between the treatments indicated that there were context-dependent effects on seedling growth. Soil heating increased soil mineral nitrogen content, but did not decrease the benefit of mycorrhizal colonization relative to unheated controls. This appeared to be driven by a decrease in the proportion of the root system occupied by R. occidentalis, indicating that host-plants can optimize colonization in a way that maximizes growth. Because seedlings in heat-treated soils were larger than unheated controls, reduced percent colonization did not cause any decrease in fungal biomass in heat-treated soils. Since there was no decrease in the performance of either partner, our results show that, in this system, ectomycorrhizal mutualisms are likely robust within the natural degree of environmental variability caused by moderate intensity wildfires.