Hydraulic redistribution of water from Pinus ponderosa trees to seedlings: Evidence for an ectomycorrhizal pathway

• While there is strong evidence for hydraulic redistribution (HR) of soil water by trees, it is not known if common mycorrhizal networks (CMN) can facilitate HR from mature trees to seedlings under field conditions. • Ponderosa pine (Pinus ponderosa) seedlings were planted into root-excluding 61-μm mesh barrier chambers buried in an old-growth pine forest. After 2 yr, several mature trees were cut and water enriched in D₂O and acid fuchsin dye was applied to the stumps. • Fine roots and mycorrhizal root tips of source trees became heavily dyed, indicating reverse sap flow in root xylem transported water from stems throughout root systems to the root hyphal mantle that interfaces with CMN. Within 3 d, D₂O was found in mesh-chamber seedling foliage > 1 m from source trees; after 3 wk, eight of 10 mesh-chamber seedling stem samples were significantly enriched above background levels. Average mesh-chamber enrichment was 1.8× greater than that for two seedlings for which the connections to CMN were broken by trenching before D₂O application. • Even small amounts of water provided to mycorrhizas by HR may maintain hyphal viability and facilitate nutrient uptake under drying conditions, which may provide an advantage to seedlings hydraulically linked by CMN to large trees.