*2.3. Sampling*

Within each circular plot in both PP and MP, three individuals of Chinese fir tree were chosen as reference plants and the rhizosphere and bulk soils, twig and needle litters under these tree canopies, roots of various orders, stems, fresh twigs, and needles varying with age in the trees were sampled. Rhizosphere soils were sampled using a hand shaking method and defined as the soils <4 mm away from the fine roots distributed 0–20 cm in the surface soil layer (>50% of the total Chinese fir fine roots [23]). Bulk soil was collected using soil cores (10 cm diameter) at 0–20 cm depth in the middle locations of forest gaps. Roots (living roots with <4 mm diameter) were extracted by shovel, the Chinese fir roots carefully collected by hand, and divided as three functional orders of absorptive roots (AR, the first three orders), transportive roots (TR, the 4th–5th orders), and storative roots (SR, >5th orders) in the laboratory [25]. Stems were sampled at breast height of the reference trees using an increment borer. Fresh needles and twigs from one first-order branch were collected from all three reference trees. Needles and twigs were divided into the first, second, and third orders of branching based on their ages (one-year old, two-year old, and three-year old needles or twigs) (see [18]). Meanwhile, twig and needle litters were directly cut from the trees using the combined method of people climbing and a tree pruner, since part of new branch litters (including the twig and needle litters) generally remain on the Chinese fir trunks. In our study, the concentrations of nutrients remaining in twig and needle litters were defined as nutrient resorption proficiencies in twig and needle organs, respectively [26]. All same samples were mixed as a sample within a plot. Soil and plant samples were air-dried at room temperature and dried at 60 ◦C for more than 72 h in an oven before nutrient measurement.
