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.