1 Introduction
Despite being recognized as a
foundation for the theory of evolution by natural selection, the
importance of intraspecific trait
variation (ITV) has been neglected over time in ecology (Bolnick et al.,
2011). Recently, there has been a resurgence of ecological interest in
ITV, stimulated by the proliferating studies that underline the
tremendous effects of ITV on community assembly and ecosystem
functioning (Violle et al., 2012; Cardou et al., 2022). However, the
patterns and sources of ITV in themselves are still unclear (Cope et
al., 2022). For example, ITV can be structured at various spatial scales
in relation to different drivers. Between-population ITV tends to be
shaped by large-scale environmental gradients, whereas within-population
ITV is more likely the consequence of heritable differences and/or
plastic responses to the local environment (Albert et al. 2010; Martin
et al., 2017). Alternatively, ITV can be unstructured if the trait is
mainly determined by stochastic processes or
constrained by fitness tradeoffs
(Kendall & Fox, 2003; Moran et al., 2016). Elucidating ITV distribution
across multiple scales is yet crucial for understanding and predicting
ecological responses to global changes (Cochrane et al., 2015; Moran et
al., 2016).
Trait-based plant studies to date have largely focused on vegetative
traits (e.g., leaf and root functional traits), with less attention
placed on seed traits despite the significance of seed traits for plant
regeneration and long-term persistence (Saatkamp et al., 2019).
Knowledge of seed traits is particularly urgent for mangroves, which
once covered over 200,000 km2 of sheltered tropical
and subtropical coastlines but nowadays has been disappearing worldwide
for decades at an extremely high rate (Duke et al., 2007; Friess et al.,
2019; Giri et al., 2015). Due to the property of low species richness
and redundancy, mangrove degradation is always followed by pronounced
losses of ecological multifunctionality (Arifanti et al., 2022; Donato
et al., 2011; Feller et al., 2010). Such situation emphasizes mangrove
recruitment and thereby calls for characterizing seed/propagule traits
that represent key dimensions of the ‘regeneration niche’ (sensuGrubb, 1977) in mangroves (Feller et al., 2010; Peterson & Bell, 2012).
Many mangroves (e.g.,
Rhizophoraceae, Avicenniaceae)
have evolved a special reproductive strategy of viviparous seeds
(germinating precociously while attached to the maternal plant),
probably reflecting an adaptation to the salty and flooding intertidal
environments (Feller et al., 2010). Due to the lack of seed dormancy,
mangrove forests destroyed by severe disturbances such as hurricanes and
tsunamis may not have local seed reserves, necessitating recolonization
through long-distance seed dispersal from relatively undisturbed
populations (Nettel & Dodd,
2007). This is particularly the case for Rhizophoraceae forests, as
Rhizophoraceae species lack the capacity of resprouting from damaged
trees as Avicenniaceae species (Baldwin et al., 2001). Therefore,
dispersal/retention is a key dimension of propagule functions where
focal traits should be targeted for mangroves (Van der Stocken et al.,
2019). Whether dispersed or retained, establishment is a prerequisite
for any propagules to contribute to the regeneration, representing
another critical dimension of seed trait (Krauss et al., 2008).
Most ITV studies in mangroves, though few in absolute number, have
investigated propagule traits (e.g., size and weight) in relation to
establishment, rarely considering
the dispersal/retention dimension (Saenger & West, 2018; Yang et al.,
2020; Zhu et al., 2021). These studies showed that temperature and/or
precipitation may have given rise to structured ITV in mangroves across
geographical gradients (Saenger & West, 2018; Yang et al., 2020; Zhu et
al., 2021), a pattern commonly reported in terrestrial forests
(Kumordzi et al., 2019).
Nevertheless, as mangroves are coastal vegetation, their trait variation
may also be shaped by oceanic factors including salinity and tidal
currents (Richards et al., 2021; Sousa et al., 2007). Additionally,
maternal plants can also affect propagule traits, both through
genetically fixed differences and environmentally induced
transgenerational plasticity (Alam et al., 2018; Cochrane et al., 2015).
However, the relative importance of these abiotic and biotic factors in
shaping the distribution of ITV across multiple scales, and whether the
patterns differ between the propagule function dimensions (establishment
vs. dispersal/retention) are still poorly understood.
Here we use Kandelia obovata as the model species to investigate
the issues. Kandelia
obovata (Rhizophoraceae) is the most cold-tolerant true-mangrove
species and has a wide latitudinal distribution along the southeast
coast of China (Sheue et al.,
2003; Wang et al., 2011), providing an ideal system for studying the ITV
of mangroves. Using a stratified sampling design across a 9° latitudinal
gradient, we analyzed the structure of intraspecific variability for
five hypocotyl functional traits in relation to propagule
dispersal/retention and establishment. The stratified sampling design
and contrasting environmental conditions allowed us to address the
following questions: (i) How is ITV structured spatially (between
populations, maternal trees, hypocotyls)? (ii) What is the major driver
(climatic, oceanic, or maternal factors) shaping the distribution of
ITV? Based on the results from previous studies (Saenger & West, 2018;
Zhu et al., 2021), the
variability of traits on the establishment axis is expected to be higher
between populations than within populations and predominantly shaped by
abiotic factors. By contrast, the trait on the dispersal/retention axis,
likely more reflecting a tradeoff between post-disturbance
recolonization and local recruitment (Sousa et al., 2007; Van der
Stocken et al., 2019), may be less structured or even unstructured with
regard to particular environmental gradients.
2 Materials and methods