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Coordination of stem and leaf hydraulic conductance in southern California shrubs: a test of the hydraulic segmentation hypothesis

Pivovaroff, Alexandria L., Sack, Lawren, Santiago, Louis S.
new phytologist 2014 v.203 no.3 pp. 842-850
correlation, ecosystems, hydraulic conductivity, leaf area, leaves, margin of safety, plant organs, sapwood, shoots, shrubs, stems, water potential, wood density, woody plants, California
Coordination of water movement among plant organs is important for understanding plant water use strategies. The hydraulic segmentation hypothesis (HSH) proposes that hydraulic conductance in shorter lived, ‘expendable’ organs such as leaves and longer lived, more ‘expensive’ organs such as stems may be decoupled, with resistance in leaves acting as a bottleneck or ‘safety valve’. We tested the HSH in woody species from a Mediterranean‐type ecosystem by measuring leaf hydraulic conductance (Kₗₑₐf) and stem hydraulic conductivity (KS). We also investigated whether leaves function as safety valves by relating Kₗₑₐf and the hydraulic safety margin (stem water potential minus the water potential at which 50% of conductivity is lost (Ψₛₜₑₘ − Ψ₅₀)). We also examined related plant traits including the operating range of water potentials, wood density, leaf mass per area, and leaf area to sapwood area ratio to provide insight into whole‐plant water use strategies. For hydrated shoots, Kₗₑₐf was negatively correlated with KS, supporting the HSH. Additionally, Kₗₑₐf was positively correlated with the hydraulic safety margin and negatively correlated with the leaf area to sapwood area ratio. Consistent with the HSH, our data indicate that leaves may act as control valves for species with high KS, or a low safety margin. This critical role of leaves appears to contribute importantly to plant ecological specialization in a drought‐prone environment.