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Proteaceae of the Cape Floristic Region, South Africa, transpire throughout the summer drought, implying access to deep water. Hydraulic redistribution by Protea ‘Sylvia’ [P. susannae E. Phillips × P. exima (Salisb. Ex Knight) Fource; Proteaceae] was investigated in overnight pot and field experiments, where it was hypothesised that (1) Proteaceae replenish water in upper soil layers, (2) hydraulic redistribution facilitates nutrient uptake and (3) shallow-rooted understorey plants ‘parasitise’ water from proteas. Potted Sylvias redistributed ~17% of the tritiated water supplied, equating to 34 ± 1.2 mL plant−1. Shallow-rooted Cyanodon dactylon (L.) Pers. (Poaceae), plants growing in the same pots as Sylvia contained amounts of labelled water similar to those found in Sylvia, indicting water parasitism. In the field, Sylvia plants growing in aeolian sands took up the deuterated water applied at 1.2 m depth as indicated by increased δ2H of plant xylem water from –38 ± 0.8 to 334 ± 157‰. This deuterated water was then redistributed to the upper soil layer (0.2 and 0.4 m), as indicated by increased δ2H of soil water from –24.5 ± 0.7 to –8.0 ± 3.0‰ and soil moisture from 0.48 to 0.89%. Lithium, as a K-analogue, was taken up equally by plants watered with deep water and those not watered, probably since both had access to naturally-occurring deep water. Water in stems of the shallow-rooted understorey shrub, Leysera gnaphalodes (L.) L. (Asteraceae) had similar δ2H values to stems of Sylvia (P = 0.939), again indicating water parasitism was tightly coupled to the protea. We conclude that hydraulic redistribution by Proteaceae plays an important role in soil water replenishment, water supply to shallow-rooted plants, and, thus, ecosystem structure and function during the summer drought of the Cape Floristic Region.

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