Data for "Rising importance of drought tolerance for biomass stocks in an old-growth subtropical forest"

Plant hydraulic traits

Early in the morning, five to ten healthy, leaf-bearing branches (5–9 mm in diameter) were harvested from three to five individuals of each species. For hydraulic measurements, branch segments measuring 60–100 cm in length were used (Zhu et al., 2019). The maximum vessel length was assessed using an air infiltration technique (Brodribb & Feild 2000). Branch segments that exceeded the maximum vessel length were placed in a pressure sleeve (PMS, Corvallis, OR, USA). According to the method established by Sperry et al. (1988), the maximum hydraulic conductivity of the segment (Kh) was calculated using the equation Kh = FL/DP, where F represents the flow rate (kg s-1), DP denotes the pressure gradient (MPa) across the segment, and L is the segment length (m). The specific conductivity (KS, kg m-1 s-1 MPa-1) was determined by dividing Kh by the average sapwood cross-sectional area at both ends of the branch segment. The leaf area was quantified with a leaf area meter (Li-3000A, Li-Cor, USA), and the leaf area/sapwood area ratio (AL/AS; m2 cm–2) was calculated as the ratio of leaf area to the sapwood cross-sectional area.

Wood density (WD; g cm-3) was assessed using the same branch segments utilized for hydraulic conductivity measurements. The volume of fresh sapwood (excluding bark and pith) was measured using the water displacement method (Poorter et al., 2010), and the dry mass was determined after oven-drying at 70 ºC for 72 hours. WD was then calculated as the ratio of dry mass to fresh volume.

Turgor loss point (TLP; MPa) is an important trait for evaluating drought tolerance in various species and biomes (Bartlett et al., 2012). Leaf-bearing branches from three to five individuals of each species were rehydrated until the leaf water potential exceeded -0.05 MPa. The leaves were initially weighed to obtain the fresh mass and then placed immediately in a pressure chamber to assess the initial water potential. Leaf mass and water potential were measured periodically during gradual desiccation. TLP was determined using a pressure-volume relationship analysis program developed by Schulte and Hinckley (1985).

We selected five individuals of each species and harvested five to six sun-exposed leaves from each individual for photosynthetic measurements. The maximum net CO2 assimilation rate (Amax) and stomatal conductance (gs) were recorded between 9:00 and 11:00 a.m. using a portable photosynthesis system (Li-6400; Li-Cor, Lincoln, NE, USA). The photosynthetic photon flux density was set to 1500 µmol m-2 s-1, a level shown to achieve photosynthetic saturation for the species in previous studies (Zhu et al. 2013; Li et al. 2015). Leaf temperature and chamber CO2 concentration were maintained at 28 °C and 400 ppm, respectively, for 15 minutes to stabilize the photosynthetic parameters before recording. Leaf instantaneous water use efficiency (WUEi) was calculated as the ratio of Amax to gs.

Statistical analyses

The community weighted mean (CWM) value for each hydraulic trait was calculated using the following equation (van der Sande et al., 2018; Da et al., 2023):

Where wi represents the relative basal area and Ti denotes the trait value of species i, with n being the total number of species in the community. We used basal area as a weighting factor because it reflects relative biomass and serves as a more effective indicator of ecosystem functioning than abundance (Prado-Junior, et al., 2016).

To determine aboveground living biomass stocks (AGB; Mg ha-1), we used data from four censuses (2005, 2010, 2015, and 2000). All trees with DBH ≥ 1 cm in the 20 ha plot were considered. We calculated the AGB for each tree using the following equation from Chave et al. (2014):

AGB = exp[ − 1.803 − 0.976 × (E) + 0.976 × ln(WD) + 2.673 × ln(DBH) − 0.0299 × (ln(DBH))2],

Where E represents a measure of environmental stress at the site, which is influenced by temperature seasonality and water deficit, and has a value of 0.670237 at Dinghushan (extracted from http://chave.ups-tlse.fr/pantropical_allometry.htm using the retrieve_raster function in R). DBH denotes the diameter at breast height (cm), and WD refers to wood density (g cm-3). The biomass stock was determined by summing the biomass of all live trees.

The aboveground biomass productivity (ABP; Mg ha-1 year-1) of the forest between two censuses was calculated as the sum of the growth of all woody species present in both censuses, along with the growth of newly recruited species (van der Sande et al., 2018; Da et al., 2023).