2.7 Dendrometers
Linear variable displacement transducers were used as high-precision
point dendrometers (ZN11, Zweifel Consulting, Hombrechtikon,
Switzerland). Technical information about the sensors and their mounting
to tree stems can be found at Zweifel et al. (2014). Pairs of
dendrometer units were installed at 60 cm above ground (termed ‘base’)
and at a position where stem diameter was 3 cm (termed ‘top’), resulting
in a total of 24 mounted units. Installation height of dendrometers at
top varied among trees (Table 1), but distance between base and top was
always more than 2.5 m. Pistons of the dendrometers were positioned
directly on bark and sapwood. For the latter, we cut a small bark window
(1 cm2) using a chisel. Bark windows were flushed with
deionised water after carefully scraping cambial cells from the outer
sapwood. Once cleaned and padded dry, the tip of a dendrometer plunger
was positioned directly onto sapwood and bark windows were sealed using
silicone grease to prevent drying of exposed stem tissues.
Measurements recorded on sapwood were used to calculate the amplitude of
radial movement in xylem (D x). SubtractingD x from measurements collected on bark isolates
the bark signal representing radial movement generated by reversible
(cambium and phloem) and irreversible (formation of wood) radial
movement. In a second step, we separated radial movement related to the
two co-occurring processes. First, we calculated the difference between
maximal and minimal dimensions during a single diurnal cycle to
represent the reversible, bark-driven proportion of the total movement
(D b) only. Next, we assessed the difference
between two consecutive minimum expansions measured on bark to determine
irreversible cambial growth
(D Gempir). These are the
simplest mathematical procedures when analysing data from simultaneous
measurements on sapwood and bark using point dendrometers (Mencucciniet al. 2017). We note that the termD Gempir is a composite measure
that contains hydraulic and osmotic signals that can mask true
irreversible cambial growth. See section ‘Stem Hydraulic Model’below for further details.
To assess the effect of girdling on dimensional changes of live bark and
xylem, we zeroed dendrometer readings against the measurement recorded
at midnight of 5 May 2014 (pre-girdling) and 19 May (post-girdling).
Time-series data (base and top positions of all six trees) from 6-10 May
was used as baseline data, measurements form 20-24 May were used to
assess how girdling affected D x,D b andD Gempir. The prefix ‘d’ will be
used to signify ‘change in’ (e.g., dD x is the
dimensional change in radial direction in xylem).