Mature oaks have deep roots. Understory plants — seedlings, cherry trees, balsam — have shallow roots. During drought, the shallow soil dries out first. The deep soil stays wet longer. The conventional expectation: deep-rooted trees redistribute water to shallow soil at night, when their own transpiration stops and the gradient reverses.
Dluhosch and colleagues at TU Munich injected deuterium-labeled water at 50-70 cm depth around mature oaks during a drought and tracked where it went. It went into the understory plants. Oak seedlings drew 16% of their root water from redistributed deep water. Black cherry got 13%. Small balsam, 8%. After 60 days, all species converged at roughly 20%.
The surprise wasn't the redistribution — that's well documented. The surprise was the timing. Redistributed water didn't peak at night, as conventional models predict. It peaked at midday, when transpiration was highest. The understory plants got 29% of their daily transpired water from the oaks' redistribution during the hours of maximum stress.
The mechanism doesn't require generosity. At midday, the shallow soil is driest. The gradient between deep-water roots and dry shallow soil is steepest. Water flows down the gradient regardless of whether the oak benefits. The oak is simultaneously pulling water up through its trunk to transpire and leaking it laterally through root-soil interfaces to its neighbors. The gift is a physical consequence of the gradient, not a choice.
Two pathways deliver the water: direct root-to-soil contact at millimeter scale, and mycorrhizal fungal networks at centimeter scale. The oak seedlings get first access — they germinate in the parent's root zone, plugged into the same fungal network. Other species catch up eventually, probably through shared mycorrhizal connections.
The through-line: the cost to the giver and the benefit to the receiver peak at the same moment, because both are driven by the same gradient. The drought that stresses the understory is the drought that steepens the redistribution. The system doesn't time the gift for the receiver's convenience — it physically can't deliver it any other way.