Arbuscular mycorrhizas (AM) prevail in warm and dry climates and ectomycorrhizas (EM) in cold and humid climates. We suggest that the fungal symbionts benefit their host plants especially in the corresponding conditions. The hypothesis tested was that AM plants are more drought resistant than EM or non-mycorrhizal (NM) plants.Gray alder (Alnus incana) seedlings were inoculated with two species of either AM or EM fungi or none. In one controlled-environment experiment, there was a watering and a drought treatment. Another set of seedlings were not watered until permanent wilting.The AM plants were somewhat smaller than EM and NM, and at the early stage of the drought treatment the soil-moisture content was slightly higher in the AM pots. Shoot water potential was highest in the AM treatment during severe drought, while stomatal conductance and photosynthesis did not show a mycorrhizal effect. In the lethal-drought set the AM maintained their leaves longer than EM and NM plants, and the AM seedlings survived longer than NM seedlings. Foliar phosphorus and sulfur concentrations remained higher in AM plants than EM or NM but potassium, copper and iron increased in EM during drought. The root tannin concentration was lower in AM than EM and drought doubled it.Although the difference to EM plants was not large, the hypothesis was supported by the better performance of AM plants during a severe short-termed drought. Sustained phosphorus nutrition during drought in AM plants was a possible reason for this. Moreover, the higher foliar sulfur and lower metal-nutrient concentrations in AM may reflect differences in nutrient uptake or (re)translocation during drought, which merit further research. The much larger tannin concentrations in EM root systems than AM did not appear to protect the EM plants from drought. The differential tannin accumulation in AM and EM plants needs further attention.
