Water sources and controls on water-loss rates of epigeous ectomycorrhizal fungal sporocarps during summer drought.

Access to deeper soil water and water-conserving traits should reduce water stress for ectomycorrhizal fungi, permitting function during drought. Here, we explored whether epigeous fruiting of ectomycorrhizal fungi during drought was facilitated by access to deep soil water, how much water was lost from sporocarps, and how sporocarp surface to volume ratios affected water-loss rates. We used oxygen stable isotope analysis of water combined with modeling of water sources used by ectomycorrhizal fungi; measured sporocarp water loss using a transient porometer, and related water loss to vapor pressure deficit (VPD) and sporocarp morphology. In drier soils sporocarps likely derived a significant portion (25-80%) of their water from deep (> 30 cm) or hydraulically lifted water. Amanita muscaria had water-loss rates over twice those of Suillus sp., Boletus edulis, Tricholoma spp. and Russula albonigra. Vapor pressure deficit was an excellent predictor of water-loss rates for individual mushrooms. Sporocarp surface to volume ratios explained much of the variation among mushrooms in the slope of VPD-water loss relationships. Access to deeper soil water might be a significant driver of ectomycorrhizal symbiotic function, sporocarp distribution, fruiting habit and morphology. Sporocarp morphology can affect water-loss rates and hence influences fungal ability to fruit during summer drought.