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).