Tree-ring width and basal area increment
In late August 2018, two 5 mm diameter increment cores per tree were
collected at breast height for each studied species using an increment
borer, and tree diameter at breast height (DBH) of each sample tree was
measured. A total of 272 tree cores were sampled from 136 trees, with
one or two cores per tree and the number of cores for each species
ranging from 36 to 40. Tree-ring width data were obtained according to
the standard procedures described by Cook and Kairiukstis (1990).
Samples were air-dried, glued and mounted on wooden staves, and then
polished with 400 and 600 mesh sandpaper in sequence until the tree
rings were clearly visible. Tree-ring measurements were crossdated by
standard dendrochronological techniques
(Schweingruber,
1988) and the quality of crossdated results was validated using the
COFECHA program (Holmes, 1983). To remove the inherent effect on annual
increments caused by tree aging and potential disturbance signals due to
forest stand development, the crossdated tree-ring width measurements
were detrended using a cubic smoothing spline with 50%
frequency-response cutoff equal to two-thirds of the length of each
series, which was performed using the ARSTAN program (Cook, 1985). All
detrended series data for each species were separately averaged to
obtain tree-ring width standard chronology using the bi-weight robust
mean method (Cook & Kairiukstis, 1990).
The
common statistical of tree-ring width standard chronologies including
signal-to-noise ratio (SNR), expressed population signal (EPS) and all
series correlation are shown in Table 1. The reliable periods of the
chronologies were determined by the criterion of EPS surpassing 0.85
which were used for the subsequent climate-growth correlation analyses
(Wigle, Briffa, & Jones, 1984). Mean sensitivity was calculated as a
measure of growth sensitivity to climate based on the method by Speer
(2010). The higher mean sensitivity value reflects greater variability
in the chronologies and indicates a stronger response to inter-annual
climate change (Fritts, 1976; Speer, 2010).
In
order to measure the radial growth rate,
the cumulative basal areas (CBA) at
the same cambial age
(Tognetti,
Cherubini, & Innes, 2000) for each tree radius were calculated as
follows:
\(\text{CBA}_{i}=\pi\times r_{i}^{2}\) (1)
where ri is the cumulative tree-ring width from
the first year to the i th year. Since all species showed a
relatively stable growth rate (Fig. 2), we calculated the CBA at
15-year-old (CBA15) of each species to compare radial
growth rate at the same cambial age among the studied species.