2.1 Species, study sites and sample collection
Kandelia obovata was first reported by Sheue, Liu, & Yong (2003), which was mainly different from the relative species K. candel (L.) Druce in leaf shape and cold tolerance. The two species are well-differentiated sets of geographical populations separated by the South China Sea. In China, K. obovata ranges from Hainan, Guangxi, Guangdong, Fujian, Zhejiang and Taiwan. Mature hypocotyls ofK. obovata were collected during 2020–2021 from 26 sites along the coastline of southern China, spanning from 19°37′N to 28°41′N in latitude and from 108°05′E to 121°24′E in longitude (Fig. 1A). This geographic range provides wide climatic gradients in annual average precipitation (1102–2373 mm, mean 1578 mm), annual mean temperature (14.7–24.7 ℃, mean 21.1 ℃), and surface seawater salinity (13‰–35‰, mean 21.8‰). For detailed site information, please refer to Supplementary Material Table S1. We sampled 1–6 maternal trees from each site (66 trees in total), depending on the population size. To ensure genetic independence, the sampled trees were spaced at least 30 m apart according to Geng et al. (2008). Each maternal tree was thus considered as a single genealogy. We randomly collected 30 hypocotyls from each tree for the study.
2.2 Trait measurement forhypocotyls and maternal trees
We measured traits including fresh weight, fresh length, maximum transverse diameter, and minimum transverse diameter for each hypocotyl (Fig. 1B). We also calculated the shape index of hypocotyl as the ratio of maximum transverse diameter to the minimum (RTD). This index describes the position of barycenter, and is thus a crucial parameter determining the balance between propagule stranding (retention) and buoyancy (dispersal) (Figs. 1B, 1C; Van der Stocken et al., 2019a). We measured height (H) and diameter at breast height (DBH) for maternal trees, and transformed them into aboveground biomass (AGB) using an allometric model of tropical wood tree: AGB = 0.0673 × (ρ × DBH2 × H)0.976 (Chave et al., 2014), where wood density ρ is set as 0.57 g/cm3 (Jiang, 2021). The aboveground biomass is a comprehensive outcome of the interaction between the genotype and the environment experienced by maternal trees, thereby capturing multiple facets of how maternal trees can affect the progeny. We thus used AGB as a synthesis indicator to predict potential effects of maternal performance on hypocotyl traits.