Variations in ectomycorrhizal exploration types parallel seedling fine root traits of two temperate tree species under extreme drought and contrasting solar radiation treatments

Summary statement: High solar radiation exacerbated the negative effects of extreme drought on plant growth and fine root traits. Ectomycorrhizae did not compensate for fine roots under drought stress. Fine roots biomass determined the role of ectomycorrhizal fungi, supporting the energy limitation hypothesis. Abstract: Extreme drought can greatly impair forest ecosystem function by limiting root nutrient acquisition. Such effects become more severe when combined with high solar radiation. Two different hypotheses offer explanations of the symbiotic adaptation that trees have evolved with ectomycorrhizal (ECM) fungi to enhance their nutrient exploration potential. ECM fungi obtain carbohydrates from plant partners to produce emanating hyphae that enhance soil exploration potential, but also proves costly. The Functional Compensation Hypothesis suggests that medium/long-distance exploration type (M-L type) will compensate the role of roots due to their extensive hyphae; the Energy-Limited Hypothesis suggests that M-L type will decline due to less carbon allocated to fine roots under stress, thus, this type parallels roots. However, the effect of this relationship on trees’ response to coexisting above- and belowground stresses is not well defined. The present study addressed this question by examining growth and biomass allocation, fine root traits, and ECM fungal communities of seedlings of two temperate tree species [Quercus mongolica (shade-intolerant) and Tilia amurensis (shade-tolerant)]. A two factor randomized block experiment with three watering regimes [60% (control), 40% (medium drought), 20% (extreme drought) of field capacity] and two light intensities (200 and 400 μmol m-2 s-1) was set up in a greenhouse. Smaller variations in plant growth and fine root biomass were consistently found in seedlings of both species under combined extreme drought and high light. Specific root length, specific root surface area and the relative abundance of M-L type declined significantly with increasing drought intensity, suggesting M-L type paralleled fine roots, did not support the functional compensation hypothesis. While there was a positive correlation between fine root biomass and the relative abundance of M-L type, supporting the energy limitation hypothesis. Light had no significant effects on fine root traits, but high light increased the relative of M-L type in control by increasing biomass allocation to fine roots. Regardless of species, ectomycorrhizae increased the resistance of seedlings to drought through actions consistent with the energy limitation hypothesis, reasserting the importance of this symbiotic relationship for resistance under climate change.