Introduction
Leaf functional traits are effective indicators of the ecological
strategies employed by species and their adaptive performance within a
specific environmental context. (Iida et al. , 2014;
Asao et al., 2020; Mueller et al.,
2024). They have the potential to encapsulate plant strategies that
pertain to water-use efficiency, growth dynamics, and nutrient
acquisition (Roskilly et al. , 2019; Visakorpi et
al., 2023). For example, an increase in leaf mass per area (LMA) and
leaf thickness (LT) indicates greater investments in leaf C structures
and a longer leaf lifespan, which, in turn, enhance the mean nutrient
residence time in leaves (Wright et al. , 2004; Díazet al., 2016). Understanding leaf trait variation is crucial for
delineating niche differentiation, elucidating competitive exclusion
dynamics, and interpreting the mechanisms of community assembly
(Valverde-Barrantes et al. , 2017; Bergmann et
al., 2020). Nevertheless, the relationships between leaf functional
traits and plant size remain unclear.
The leaf economics spectrum (LES) represents a well-established
framework within the realm of plant functional ecology that describes
trait covariation relevant to carbon and nutrient economics across plant
species (Wright et al. , 2004; Mueller et al.,
2024). For instance, less costly structural leaf phenotypes, such as
low LMA and low leaf dry matter content (LDMC), are commonly linked to a
suite of traits that enhance rapid growth and resource acquisition,
including elevated leaf nutrient concentrations and increased metabolic
rates (Guimarães et al. , 2022; Yan et al.,
2023). Conversely, the opposite traits (high LMA and LDMC) are
associated with conservative economics, which are slower growth rates,
reduced resource uptake, and decreased tissue turnover (Joswig et
al. , 2022).
Leaf economics and plant size represent two pivotal
dimensions—exemplifying a decoupled correlation—that are fundamental
to life-history strategies across the global spectrum of plant form and
function (Díaz et al. , 2016; He et al., 2024).
However, the findings differ among studies, and the field is far from
resolved. Several studies have related the traits of LES (e.g., leaf
area, LA; specific leaf area, SLA; and leaf nitrogen concentration, LNC)
to growth rates (dos Santos and Ferreira, 2020; Simpsonet al., 2020). In principle, since tree size affects access to
resources and, thereby, growth rates (Piponiot et al. , 2022), it
is expected that tree size is associated with leaf economic traits (Iidaet al. , 2014; He and Yan, 2018). For example, larger
trees tend to preempt light resources to smaller trees that, in turn,
enables faster growth among trees of larger stature (Maynard et
al. , 2022). Previous studies have shown that LA and LNC increased among
larger plants (He and Yan, 2018; Zheng et al., 2022),
which is interpreted as the result of plants adopting acquisitive
economic strategies in response to higher growth rates through
acclimation and plasticity. However, larger trees exhibit heightened
vulnerability to environmental stressors such as drought and higher
solar irradiance (Rozendaal et al. , 2006; Bennettet al., 2015; McGregor et al., 2021).
Consequently, leaf traits often undergo corresponding shifts toward more
conservative economic strategies as plant size increases, as exemplified
by reductions in SLA and LA, along with an increase in LDMC (Kenzoet al. , 2015; Dayrell et al., 2018;
Park et al., 2019). Therefore, what we have learned
about the effects of plant size on leaf economics is not consistent
across studies.
Numerous recent studies have explored size-trait relationships in
terrestrial plants (Park et al. , 2019; Thomas et
al., 2020; Zheng et al., 2022). However, coastal
mangroves have not been well studied. Mangroves constitute an ecological
assemblage of trees and shrubs that have adapted to thrive in the
intertidal zones of tropical and subtropical coastal regions. The
intertidal zone experiences considerable fluctuations in moisture and
temperature between the highest tides, when it is submerged, and the
lowest tides, when it is exposed to air and sun (Weitzman et al. ,
2021). This zone is distinguished by a gradient that ranges from high to
low and is influenced by the continental shelf’s structure, variations
in tidal fluctuations, and the succession of plant communities (Yuet al. , 2023). The interplay of sediment formation matrices,
sedimentation rates, and the extent and duration of tidal waterlogging
among intertidal zones leads to a diverse array of sediment
characteristics, including nutrient composition, salinity, oxygen
levels, and temperature (Hayes et al. , 2017; Maet al., 2020). Considering that salinity and temperature are
paramount environmental factors influencing mangrove functional traits
(Medina-Calderón et al. , 2021; Lang et al.,
2022), mangroves could have specialized structural traits along
intertidal gradients (Yu et al. , 2023) and thus provide a unique
opportunity to improve our understanding of plant size-trait
relationships.
In this study, we examined leaf traits and their relationships with
plant size in a sample of 10 dominant mangrove species in Dongzhaigang,
China. We hypothesize that: (1) leaf functional traits exhibit
significant variation across growth forms and intertidal zones due to
the differential responses of various growth forms to changing
environmental conditions along the intertidal gradient (Islam et
al. , 2024), and (2) smaller species are inclined to adopt increasingly
conservative economic strategies characterized by high LD, LMA, and
LDMC, as they are more susceptible to carbon starvation induced by
shading (McDowell et al. , 2018).