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.