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Radially transmitted changes in hydraulic and osmotic pressures help explain reversible and irreversible patterns of tree stem expansion
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  • Sebastian Pfautsch,
  • John Drake,
  • Mike Aspinwall,
  • Victor Resco de Dios,
  • Craig Barton,
  • Patrick Meir,
  • Mark Tjoelker,
  • David Tissue,
  • Maurizio Mencuccini
Sebastian Pfautsch
Western Sydney University
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John Drake
SUNY College of Environmental Science and Forestry
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Mike Aspinwall
Auburn University
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Victor Resco de Dios
Southwest University of Science and Technology
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Craig Barton
Western Sydney University
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Patrick Meir
The Australian National University
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Mark Tjoelker
Western Sydney Univeristy
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David Tissue
Western Sydney University
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Maurizio Mencuccini
University of Edinburgh
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Abstract

It is easy to measure annual growth of a tree stem. It is hard to measure its daily growth. The reason for this difficulty is the microscopic scale and the need to separate processes that simultaneously result in reversible and irreversible stem expansion. Here we present a model that separates reversible from irreversible cell expansion. Our model is novel, because it explains reversible expansion as consequence of longitudinally and, importantly, radially transmitted changes of hydraulic and osmotic pressures in xylem and bark. To capture and quantify these changes, we manipulated daily stem growth by applying a phloem girdle to stems of 9-m tall trees. The model was informed by measurements of radial movement in stem tissues and sap flow before and after and positions below and above the girdle. Additional measurements of whole-crown fluxes of H2O and CO2, leaf water potentials, non-structural carbohydrates and respiration were used to document the physiological impacts of girdling. This work sheds new light on the role of radial transport processes underpinning daily growth of tree stems. The model helps explain diel patterns of stem growth in trees.