Seedling traits and community dynamics data in Ailaoshan Forest Dynamic Plot

Study areaWe conducted the study in the Ailaoshan 20 ha (500 m × 400 m) forest dynamics plot, located in the Ailaoshan National Nature Reserve, Yunnan, China (Fig. S1). This plot was established in 2014 following standard protocols (Condit, 1998), where all free-standing woody plant stems with a diameter at breast height (DBH) ≥ 1 cm are tagged, measured, identified to species and mapped (Han-Dong et al., 2018). Ailaoshan experiences a strongly seasonal climate (Wu, 1983) with precipitation of about 1530 mm in the wet season (May to October) and 250 mm in the dry season (November to April). The mean monthly temperature is about 16 ℃ in the wet season and 10 ℃ in the dry season (Fig. S2). Data collectionSeedling censusThe 20-ha plot is divided into 500 20 x 20 m quadrats. In the centre of each quadrat, we established a 2 x 2 m seedling plot in February 2015 (total of 500 seedling plots). In each seedling plot, we tagged all free-standing woody seedlings (i.e., plants < 1 cm in diameter), identified them to species, measured their stem height, and counted the number of leaves. Species were identified by local botanists using nomenclature consistent with the Flora of China (http://foc.eflora.cn/). In subsequent years, we censused seedlings in May and November each year. We define the November census as the wet season and the May census as the dry season. During each census, we tagged, identified and measured all new seedlings. In this study, we analysed seedling dynamics for all individuals recorded from November 2015 to November 2022. Functional trait measurements on seedlingsWe quantified several leaf functional traits using standard methodologies for plant trait measurements (Poorter & Markesteijn, 2008; Perez-Harguindeguy et al., 2013). We measured resource acquisition traits: leaf chlorophyll content (Chl; SPAD), leaf area (LA; cm2), specific leaf area (SLA; leaf area/unit dry leaf mass; cm2/g), leaf dry matter content (LDMC; %), stable isotope of nitrogen (δN15; ‰), and leaf total carbon and nitrogen concentrations (mg/g). For drought tolerance traits, we measured leaf turgor loss point (πtlp; MPa) and stable isotope of carbon (δC13; ‰). Finally, we measured a suite of traits related to plant defense: concentrations of tannins (mg/g), flavonoids (mg/g), non-structural carbon (NSC; mg/g), cellulose (mg/g), and lignin (mg/g). We provided detailed methods by which these traits were sampled and measured in the Supplemental Information (Appendix 1).Seedling relative growth rate measurementsThe relative growth rate of each individual in each season was calculated as ln(Ht + Δt/Ht) / Δt, where H indicates height at successive time steps and Δt indicates the time interval (six months) between measurements (Hunt, 1982). Negative relative growth rates were recorded because of the herbivory or dieback that caused by either disease or environmental stress. Neighbor density measurementsTo model the impact of neighboring plants on seedling growth, we first measured four variables describing the neighborhood of each seedling. Specifically, we calculated the density of conspecific (ConS) and heterospecific (HetS) seedling neighbors within the 4-m2 seedling plots for each individual in each census. For focal individuals in our analysis, we only included seedlings that emerged as new recruits after the plot was established. Because we did not know the age of seedlings that were recorded in the first census, we treated these individuals as seedling neighbors in the analyses but did not use them as focal individuals.We calculated the total basal area of conspecific (ConT) and heterospecific (HetT) trees that were found within a 20 m radius of each seedling plot. We chose a 20-m radius based on a previous study in the same plot (Song et al., 2021).