4 Discussion
Increasing studies have showed
the importance of ITV on the ecological process. However, the patterns
and sources of ITV in themselves are still unclear (Cope et al., 2022).
There are large ITVs in mangroves due to the complex environment but are
less concerned on propagule which can determine the recruitment or/and
distribution (Feller et al., 2010; Petersan & Bell, 2012). It is urgent
to study the ITV of mangrove propagule at the background of global
changes. We analyzed the relationship between biotic and abiotic factors
and hypocotyl traits on a large scale for a typical viviparous mangrove
species. We found that PC1 (FW, FL, TDmax and
TDmin) was on the establishment dimension, and ITV of
PC1 was mainly structured between populations which was directly shaped
by climate. While RTD was mainly structured between hypocotyls, which
was constrained by fitness tradeoff between dispersal and retention. Our
study provides insights to the ITV mechanism of hypocotyl traits in
mangrove plants.
4.1 Structured
traits variation of mangrove hypocotyl
Pervious study showed that ITV in
propagule size has been observed,
especially in the genera Rhizophora , Bruguiera ,Kandeli and Ceriops (Tomlinson, 2016). Our results showed
that variation of PC1 was significantly negatively correlated with
latitude, similar to the results of a previous study (Yang et al.,
2020). But there was an inverse relationship in a black mangrove study.
The propagule of Aegiceras
corniculatum was smallest in the northern (low latitude) population,
while the largest propagule was in the southern population in Australian
(Saenger & West, 2018). The opposite pattern of structured ITV was
possibly due to the different strategies for improving fitness between
true viviparous and crypto viviparous mangrove species. Moreover, the
genetic analysis also suggested that differences evident in populations
of A. corniculatum at the extremes of its range, are
explained as evolutionary adaptations in response to the local physical,
climatic or biological characteristics (Maguire et al., 2000).
Therefore, ITV of PC1 sources from the local biotic and abiotic factors
at different sites.
We found that climate (especially temperature) was the main driver of
intra-specific variation in PC1. In general, temperature and
precipitation have a negative relation with latitude (Hijmans et al.,
2005), while oceanic factors such as salinity and tides have little to
do with latitude. Obviously, higher temperature means that more
materials can be accumulated in hypocotyl. MAP had little effect on PC1,
probably because mangrove plants have adapted to tidal environments and
are able to obtain fresh water from seawater (Parida & Jha, 2010).
Although oceanic factors were not found to be the main factors affecting
the variation of hypocotyl traits, salinity did have a significant
effect on PC1 variation. Salinity has long been considered an important
factor limiting the growth and distribution of mangroves (Chen & Ye,
2014; Richards et al., 2021), and study also found that length and
weight of propagules of mangrove was negatively correlated with the
salinity (Alam et al., 2018). In addition, salt-stress is also a factor
in the evolution of vivipary in mangrove plants. The hypocotyl of
viviparous seeds contains a large amount of nutrients and water, which
are needed for the growth of newly colonized seedlings, thus avoiding
the vulnerable life stage (Joshi, 1933; Zhou et al., 2016), that also
can keep its vitality even floating in the sea for server months. When
salinity is too high, maternal tree will invest more energy to against
with salt stress (Alam et al., 2018; Parida et al., 2004), which may
reduce the investment on the hypocotyl and eventually lead to variation
in its traits. Tide is an important factor to mangrove growth and
hypocotyl dispersal process (Clarke et al., 2001; Duke et al., 1998;
Van der Stocken et al., 2015;
Zhang et al., 2021), however, the effect of tides on the variation of
hypocotyl traits was marginal in our study. The possible reason is that
previous studies have only considered the influence of tidal effects,
rather than the combination of multi biotic and abiotic effects. When
count on the climate, the effect of
tides may be marginal. In addition, we suggest that PC1 is the
variability of traits on the establishment axis (Saatkamp et al., 2019;
Moles & Westoby, 2004), which is greatly affected by resource-based
factors (such as temperature and nutrients etc. ), while tides may
not be an important factor affecting hypocotyl mass.
4.2 Unstructured
traits variation of mangrove hypocotyl
We noted that the shape index RTD did not vary with latitude, and it was
also less affected by abiotic and biotic factors. There are three
possible reasons for ITV of RTD was structured between individuals
(hypocotyls): 1) those traits may be constrained by limited genetic
variation (Bradshaw 1991). Studies have shown that the genetic diversity
of mangrove populations is low
(Richards et al., 2021; Ruan et al., 2013) and the complex matrix also
limits gene flow between mangrove populations (De Ryck et al., 2012; Van
der Stocken et al., 2015); 2) traits are related to plant fitness
tradeoffs. Such as costs induce tradeoff, they give rise to covariation
between dispersal and other life-history traits at different scales of
organismal organization (Bonte et al., 2012). Dispersal event through
investments in special morphologies, are facing the risk and opportunity
for transfer and settlement; 3) low ITV may also be caused by very sharp
contrasts in conditions in space and time (Holt & Gomulkiewicz, 2004),
which induce the plant stress. This is exactly what happens in coastal
ecosystems where mangroves grow. Plants evolving in stressful
environments have less plasticity in making use of resources upon their
increased availability (Quadros et al., 2021). However, even the low
genetic diversity and stressful environments, there were still large ITV
of mangroves (Tomlinson, 2016), or such as the ITV of PC1. Consequently,
RTD was related to plant fitness tradeoffs.
Specifically, we inferred that RTD was correlated with
dispersal/retention tradeoff, because the center of gravity was
important to determine the position relative to the water surface while
floating (Van der Stocken et al., 2019; Figs. 1B, 1C). Although
dispersal can avoid competition with conspecific species, it also faces
the risk of colonization or/and establishment in an unfavorable
environment. On the contrary, retention means colonizing in a similar
environment to the parent tree, but cannot avoid the between conspecific
species and natural enemies (Stump & Comita, 2020). Not all mangrove
species recolonize proportionately with the previous generation at the
same site (Kamruzzaman et al., 2017). Therefore, mangroves may regulate
the RTD to determine more dispersal or retention.K. obovata is the most
widely distributed species of mangrove (Sheue et al., 2003; Wang et al.,
2011), the RTD of K. obovata may be relatively small which is
facilitated to dispersal. While other Rhizophoraceae spp., likeBruguiera sexangular , B. gymnorrhiza , Rhizophora
stylosa etc. are thermophilic species (Wu et al., 2018), the RTD
of those species may relatively large which can ensure the
settlement/establishment of offspring (propagule) in a suitable place.
Based on that strategy of tradeoff, mangroves can maintain a high-level
fitness.
Many studies have emphasized the importance of maternal effects in the
variation of offspring traits (Alam et al., 2018; Cochrane et al., 2015;
Galloway, 2005), but the strength of maternal effect on ITV was minor in
this study. As we only recorded the height and DBH of maternal plants,
which reflecting the size and age, that may have less to do with
morphology than with yield (Hangelbroek & Santamaria, 2004). Rather,
heredity may be the major contributor to maternal effects. In general,
genetic diversity is low in the natural mangrove population (Hodel et
al., 2018; Richards et al., 2021), but researchers discovered that high
epigenetic diversity can maintain the high plasticity of traits in
mangrove (Mounger et al., 2021). That was possibly why the ITV of RTD
was mainly contributed from within individuals. Due to the lack of
dormant seed periods, true mangrove species do not have seed banks and
are difficult to regenerate after anthropogenic disturbances or natural
disasters (Nettel & Dodd, 2007). Therefore, the collection of germplasm
resources and artificial cultivation of true-viviparous mangrove species
are particularly important, especially under global climate change.
Study showed that K.
obovata had higher regenerative capacity than other
viviparous species (Zhang et al.,
2021), because K. obovata had higher dispersal capacity.
We recommend that the RTD of
hypocotyl should be considered in germplasm collection and restoration,
thus conducive to the regeneration of the population. More studies
should be combined those biotic and abiotic factors to help us to have a
comprehensive understanding of the trait variation of mangroves and to
better protect the mangroves.