2.1 Study site and plant materials
Field sampling was carried out in a species-rich subtropical old-growth evergreen forest located in Dinghushan Forest Ecosystem Research Station, Central Guangdong Province, South China (23°09′21″ N to 23°11′30″ N, 112°30′39″ E to 112°33′41″ E). The mean annual precipitation is 1900 mm, nearly 80% falls in the wet season from April to September, and the mean annual temperature is 22.3 ℃ (Zhou et al ., 2011). The most dominant plant families in this study site includeFagaceae , Lauraceae and Myrtaceae (Li et al ., 2015), and the most dominant tree species are Castanopsischinensis , Schima superba , Cryptocarya chinensis ,Machilus chinensis and Syzygium levinei (Zhu et al ., 2013).
Ten evergreen woody species were selected from this subtropical forest. Characteristics of all the selected species, including diameter at breast height (DBH) (range: 14.5-46.5 cm), were reported in supporting information (Table S2). Three mature individuals per species (totaln = 30) were selected. Branch and coarse root samples were collected in September, 2021. At least three branch and root segments were measured per individual. To collect branches, the top of the canopy samples approximately 80 cm in length and of 6 mm (range: 4.1-9.2 mm) in diameter were cut. Roots (soil depth: 5-25 cm) were excavated, and traced from their parent tree to identify the species and then clipped (approximately 80 to 90 cm in length and 4 to 9 mm in diameter). Branch and root samples were immediately placed in black plastic bags with wet paper towels, put into a cooler, transported back to the laboratory within 60 minutes. Prior to hydraulic trait quantifications, for open vessels were tested following the method described in Ewers et al ., (1989). Only segments with no open vessels were used for subsequent measurements.
We also collected data of root and branch P 50 andK S along with the measurements of anatomical traits, including vessel wall reinforcement (t 2/b 2) and hydraulically weighted diameters (D h) (Table S1). The data were derived from previous literature published between 1994 and 2021 using Web of Science (http://isiknowledge.com), Google Scholar (https://scholar.google.com), and China National Knowledge Infrastructure (http://www.cnki.net). The keywords used to search the literature included “root/branch hydraulic vulnerability”, “xylem cavitation”, “xylem embolism”, and “branch/stem hydraulic traits”. Based on information in source publications and online floras (e.g., http://frps.eflora.cn), sites were assigned to biomes: tropical seasonal forest (TRS), temperate seasonal forest (TMS), woodland and shrubland (WDS), and desert (DES). To avoid systematic error, we selected data that met the following criteria: (a) hydraulic and anatomical trait measurements of sample segments came from the same plant, (b) experiment was conducted in natural habitats, and (c) measurements were carried out with were mature plants, not the saplings and seedlings. We took the average value of the same species from the same site for data analyses.