2.2 Sampling measurement protocols
Two
plots (20 m
×
60 m and 20 m × 50 m) were established on a flat forest with the same
slope (14°) and aspect (336°; 0 to
360° measured in degrees from
north). All trunks with diameters >1 cm at breast height
(1.3 m) were tagged and measured for trunk diameter at breast height
(D to the nearest 1 mm) through a measuring tape, tree height
(H ) through a telescopic measuring pole. Meanwhile, the dead
trees with tip were also measured. Tree performance was grouped into
three classes: “inferior”, “moderate”, and “superior” using the
criteria described by Simard: (1) “moribund”, near death, little or no
visible shoot growth; (2) “poor,” little or possibly etiolated shoot
growth, few and/or short needles; (3) “moderate”, moderate shoot
growth, leaf area, needle length; and (4) “good”,
vigorous
shoot growth, high leaf area, long needles, deep green color (Simard
1993). We merged “moribund” and “poor” into the “inferior” class
because these trees have little or no chance for survival. Subsequent
measurements of aboveground biomass and growth rates were consistent
with visual identifications (Table 1). Biomass and growth rate are
commonly used to quantify tree competitiveness, and to further determine
tree performance in competition (Zhang et al. 2019b; Wanget al. 2021). The significant and dramatic decrease in biomass
and growth rate as tree performance deteriorated reinforced the premises
of our research (p <.01, Table 1). Before using the
criteria revised from Simard to assess tree performance in plots, a
pre-experiment was conducted next to the sample plot, in which 15 trees
(5 repetitions × 3 classes) were cut, showing the biomass and growth
rate were significant different between performance levels. The tree
density in our study plot is 8264 individuals per hectare, when we
surveyed in August 2018. Though we acknowledge there are a few
indigenous P . massoniana before seeding and few
recruitments after sowing, most trees are in the same age, 12 yr
(determined by the rings from our destructively sampled trees). While
few Toxicodendron succedaneum and Rhus chinensis scattered
in this forest and were measured, only P . massoniana was
analysed due to the negligible number of other species. Statistical
analysis was only performed on trees not shorter than 3 m (H ≥ 3
m) in this study for the following two reasons, the first was that trees
shorter than 3 m were recruitments and evidently younger than other
trees, and the second was that trees shorter than 3 m were completely
suppressed and had very little influence on the taller trees. With this
criterion, a total of 1739 pine trees (1642 living and 97 dead) were
used to ascertain how H -D scaling relationship related to
tree performance.
Seventeen representative trees were destructively sampled from each tree
performance classes (N = 17 trees
×
3 classes = 51 trees, except for dead trees), for measuring biomass and
diameter growth and exploring branch length-diameter (L -d )
scaling relationships. For each tree, healthy and mature needles were
collected in the four cardinal directions at one quarter, half-way, and
three-quarters of the crown spread (Fig 1A). Branch segments
(~ 5cm) were collected from representative intact
branches in the four cardinal directions acropetally outward from the
base to the tip. Three segments were cut from one quarter, half-way, and
three-quarters of the total branch length (Fig 1B). A 5 cm thick
transverse section was cut at every meter along the length (height) and
the breast-height (1.3m) of each trunk (Fig 1C). All samples were placed
into sealed plastic bags to prevent the loss of water, and stored in an
icebox to prevent decomposition until they were transported to the
laboratory.
Branch diameter at 3 cm from the base was measured to record branch
diameter (d ) for each branch; the distance from branch base to
the apex was measured as length (L ). The distance of the base of
each branch from the apex of each tree was also measured. The total
fresh weight of the trunk, branches, and leaves of each tree was
determined using a hanging scale to the nearest 5 g.
The fresh weight of each sample was measured with an electronic balance
to the nearest 0.01 g on the day of collection. The dry mass of all
samples was obtained after drying samples at 105 ◦C for 30 min (stem
samples for 1 h) and then continued at 65 ◦C until a constant weight was
reached. All dried disks were scanned into bitmap images at a 600 dpi
resolution (Epson Perfection V800 Photo scanner) after being polished
with 120 mesh sanding paper. The width of rings was measured with
WinDENDRO (V.6.1d). The average D increase over three years was
used as a proxy for growth rate. Along the crown length, the crown of
each tree was divided into three equal vertical layers: upper,
intermediate and lower, and the corresponding branches were also
classified into these three layers (Coble, Fogel & Parker 2017).