3.4 Effect of girdling on radial dynamics in bark and
xylem
Phloem girdling markedly impacted radial fluctuations in xylem
(dD x) and bark tissues
(dD b +
dD Gempir) below and above the
girdle (Fig. 4). Prior to girdling (6-10 May), average
dD x was 22.6 µm d-1 (± 5.0) at
the base and 22.2 d-1 µm d-1 (± 7.9)
at the top of trees. dD b +
dD Gempir was nearly four times
larger compared to dD x, measuring 76.8 µm
d-1 (± 29.8, ± 1 SD) at the base and 71.8
d-1 µm d-1 (± 13.5) at the top (Fig.
4a, b). Importantly, and independent of girdling, the dynamic changes of
dD x and dD b +
dD Gempir were asynchronous,
whereby dD x exhibited the largest expansion
around 14:00 h when dD b +
dD Gempir reached its minimum.
Following girdling (20-24 May), dD x at the top of
stems (above the girdle) remained unchanged (21.9 µm
d-1 ± 7.9), but significantly declined below the
girdle (13.2 µm d-1 ± 9.7; p <0.001) (Fig.
4c, d). Girdling slightly increased dD b +
dD Gempir above the girdle (82.3
µm d-1 ± 26.0) while it was reduced by more than half
below the girdle (33.5 µm d-1 ± 10.7; p
<0.001). dD Gempir was
similar at both measurement positions (base: 49.5 µm
d-1 ± 29.7; top: 51.6 d-1 µm
d-1 ± 17.6) before trees were girdled. Post-girdling,
dD Gempir ceased at the base
(0.2 µm d-1 ± 8.5) yet slightly increased at the top
(80.1 µm d-1 ± 28.9).
Interpolated cumulative D Gempir(empirical data based on daily minima) as well as modelledD G+ at the base of trees showed a continuous
increase of plastic growth prior to girdling with no distinct diurnal
pattern (Fig. 4e). During daytime (7:00-19:00 h) meanD Gempir was 52% (±4.5) andD G+ was 50.3% (±2.7) of total plastic growth
realised during a 24-hour cycle (i.e., 7:00-7:00 h). As indicated
earlier, plastic growth below the girdle ceased entirely, thus no diel
patterns were identifiable. At the top of trees, plastic growth was also
continuous before the gridle was applied, yet at this positionD Gempir andD G+ showed a distinct diel cycle with high
plastic growth during the day (Fig. 4f). Between 7:00 h and 19:00 h,D Gempir accounted for 77.2%
(±6.8) and D G+ for 77.6% (±1.0) of total
plastic growth within the 24-hour cycle. After trees were girdled, this
proportion declined to 62.5% forD Gempir and 71.9% forD G+, retaining a clear diurnal rhythm of high
plastic growth during the day and lower plastic growth during the night.
Our improved model reproduced the magnitude of plastic growth, stripped
of hydraulic and osmotic signals (D G+), well
for the base and top measurement positions, and also before and after
girdling when compared toD Gempir (Fig. 5). During the
5-day interval before girdling, average cumulativeD Gempir was 258.2 µm at the
base and 256.2 µm at the top. Model predictions for the same time
interval were similar at the base (287.2 µm), but slightly higher at the
top (346.3 µm). After girdling of trees, the improved model matched
empirical measurements well, reflecting the collapse of growth below the
girdle and an increase of D G+ at the top (Fig.
5).
Separating \(\frac{{dD}_{b}^{\text{TOT}}}{d_{t}}\) into components 1, 2
and 3 using empirical measurements from dendrometers, sap flow, leaf
water potential and environmental information allowed us to separate
simultaneously occurring hydraulic (Fig. 6) and osmotic processes (Fig.
7) in live bark, the cambial growth zone and xylem during different
parts of the diel cycle. First, model outputs documented systematic
fluctuations in dD b and
dD x in response to diel changes in hydrostatic
pressure in the xylem (Fig. 6a, b). With the onset of transpiration in
the morning, pressure potential in the xylem
(dD xΨP,x) declined, leading to
a small (approx. 3 µm) but systematic contraction of sapwood. The
increasing negative pressure in xylem vessels was transduced via rays
into the live bark, where after a short lag time capacitive water stores
began to empty water into the xylem, leading to a large contraction of
live bark tissue (dD bΨP,x).
Following peak transpiration during midday (see Fig. S3), negative
pressure in xylem eased, yet capacitive water continued to be drawn from
live bark tissue leading to further contraction of live bark, leaving a
total daytime contraction of up to 60 µm. With the onset of darkness,
transpiration rates were low, allowing
dD xΨP,x to fully recover and
remain largely unchanged during the night. At the same time, capacitive
stores were recharged leading to an increase of
dD bΨP,x. Prior to girdling, the
magnitude of these hydrostatic processes did not vary between the
measurement positions, producing a sine-type oscillation of
dD bΨP,x.
After trees were girdled, the amplitude of
dD xΨP,x and
dD bΨP,x markedly declined (Fig.
6c, d). dD xΨP,x was reduced to
less than 1µm while its distinct transpiration-driven phases of decline,
recovery, and steady-state, remained below and above the girdle. The
principal sine-wave pattern of viscoelastic changes in live bark also
remained, albeit reduced from 60 to around 20 µm after girdling and at
both measurement positions. However, model outputs indicated that
girdling caused dD bΨP,x below
the girdle to deviate from the oscillating pattern around zero to one
that was increasingly positive (Fig. 6c), indicating a ‘swelling’ of
live bark tissue. This was not observed for
dD bΨP,x above the girdle.
Prior to girdling, our model showed a simultaneous decline in
dD bΨP,x (Fig. 6) andD xΨΠ,x (Fig. 7a, b), with
similar dimensional change at the base (up to 60 µm) but not the top of
trees (15-20 µm). Similar to the oscillating sine-pattern of
dD bΨP,x,D xΨΠ,x at the base of trees
recovered overnight, peaking during early morning. However,D xΨΠ,x at the top of trees
showed no recovery during the night, but a sharp increase during the
early morning before and also after trees were girdled. This pattern was
very similar to dendrometer measurements of D xshown in Fig. 4b and 4d. Finally, expansion of live bark as result of
changes in osmotic pressure within this tissue
(D bΨΠ,b) peaked at the end of
every day before girdling at the base and top of trees (Fig. 7c, d).
After girdling, these peaks disappeared below the girdle andD bΨΠ,b steadily declined,
showing that expansion of D b was increasingly
less driven by osmotic pressure gradients. The opposite trend, a
constant increase in D bΨΠ,b was
observed above the girdle. At both positions, the typical late afternoon
peak observed prior to girdling had disappeared post-girdling.
Our mixed effect model, that accounted for more than one distinct source
of variability, was able to explain a large proportion of uncertainty
(conditional R2 was 0.47 before and 0.40 after
girdling; marginal R2 was 0.51 before girdling and
0.47 after girdling; Table 3). The observed dimensional changes inD x and D b resulting from
viscoelastic changes in live bark and phloem, as well as changes in
patterns of D G+ are in agreement with effects
associated with reduced transpiration (Fig. S3) and changes in the
concentration of NSC that affect physiological and osmotic dynamics
(Fig. 1), below and above the girdle.