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.