4. Discussion
Here we have assessed the two most critical transport systems in trees,
xylem in sapwood and phloem in bark, as one tightly interconnected
system. This approach is supported by previous experimental work that
highlighted the role of wood rays as a radial link between the two
transport systems (Pfautsch et al. 2015b; Mencuccini et
al. 2017). By refining our Stem Hydraulic Model (Mencuccini et
al. 2017) through incorporation of terms for viscoelastic changes
driven by pressure potential of the xylem and osmotic potential exerted
by live bark, we were able to accurately isolate diel patterns of
irreversible radial growth (D G+) from
asynchronous dimensional changes in bark and sapwood. The resulting
patterns of D G+ showed high rates of wood
growth during the day, which stand in contrast to the paradigm that
radial growth of stems mostly occurs during the night (Steppe et
al. 2015). Moreover, by girdling trees, we were able to untangle the
effects of hydraulic and osmotic pressure gradients on radial movement
of bark and sapwood. Our observed and modelled dimensional changes of
tree stems progress recent theoretical work on source-sink relationships
(Sellier & Mammeri 2019). Specifically, we were able to quantify
dimensional radial expansion and contraction of tissues driven by
diurnal oscillations of osmotic pressure potential in live bark and
xylem. Further, we identified that D G+ close to
the source of loading (treetop) followed a pronounced diel pattern of
high growth during the day and low growth during the night, compared to
continuous increase of D G+ further away from
the source of loading (base). These growth patterns have now been shown
to co-occur simultaneously in eucalypt trees, providing the first
empirical evidence for the suggested pathway effects of phloem loading
with a stabilising function of NSC supply to distant sinks (Sellier &
Mammeri 2019).