Eco-evolutionary consequences of living with close relatives: Resistance to herbivores and climatic stress increases with investment into mycorrhiza in adult oaks

1. Mycorrhizas are known to increase plant resistance to herbivores and climatic stress. However, it is unknown if particularly hostile environments select for increased investment of plants into mycorrhiza.

2. We studied hostile biotic environments: phylogenetically proximate neighbourhoods known to increase herbivory and pedoclimatic stress. In a common garden, we studied the resistances, tri-trophic interactions and microclimate of 28-years-old oaks (Quercus petraea), descending from provenances of contrasting phylogenetic neighbourhoods.

 3. We found that oaks descending from phylogenetically proximate neighbourhoods had increased ectomycorrhizal enzymatic activities without affecting mycorrhization rate of root tips. We also found that increased ectomycorrhizal enzymatic activity decreased damages by one group of specialist herbivores (leaf-miners), without affecting another (galls) or generalists (ectophages). Consistently, descendants from phylogenetically proximate neighbourhoods showed decreased damage by leaf miners. We finally found that oaks descending from phylogenetically proximate neighbourhoods were most capable of maintaining leaf chlorophyll under heat-induced drought, reflecting their increased ectomycorrhizal enzymatic activities.

4. Synthesis. Overall, oaks living with closely related neighbours can rapidly evolve increased investment into ectomycorrhizas and thereby resistances to herbivores and climatic stress, ultimately facilitating coexistence among closely related neighbours. While phylogenetically diverse forests remain essential for maintaining biodiversity, the selection regimes in less diverse forests can be used as a source of native genotypes resistant to future climate change, rather than using exotic populations and species as a source pool.