4.2 A trade-off between leaf economics and plant
size
Despite a wealth of research into trait relationships in plants (Prietoet al. , 2018; Simpson et al., 2020;
Zhou et al., 2022), our understanding of leaf trait
relationships in mangroves lags behind that of terrestrial species.
Plant height and diameter are important variables for characterizing
plant size. Our study revealed that large mangrove species (tall
and large diameter) tended to have low leaf structural investment (low
LMA, LD, and LDMC). This is contrary to several studies that have shown
positive relationships between LMA and tree size across developmental
stages for conspecific individuals (Nouvellon et al.,
2010; He and Yan, 2018; Liuet al., 2020). There are three possible interpretations for this
difference. First, the size-trait may have different drivers, which may
co-occur within species while being decoupled among species (Zhou et al., 2022). Thus, the negative size-trait
relationships found across species disappeared when the analysis focused
on intraspecific patterns. Second, decreases in LMA are often
interpreted as a strategic adaptation by plants to enhance their
light-harvesting efficiency (del Campo et al., 2022).
The canopies of taller plants experience greater solar irradiance
compared to those of their shorter counterparts (Maynardet al., 2022). These differences in the light exposure of plant
crowns may be instrumental in the observed variability in LMA. Finally,
decreases in LMA may result from decreases in leaf water stress (e.g.,
WSD) with increasing plant size. Our study underscores the importance of
elucidating the underlying mechanisms behind trait-trait relationships,
both within and among species. These mechanisms are essential for
deepening our insight into the intricate adaptive strategies employed by
plants in their quest for survival and success.Our results were also inconsistent with previous research on
global plant size-trait relationships (Díaz et al.,
2016; Joswig et al., 2022; Maynard et al., 2022). A previous study
pinpointed a crucial collection of functional traits that summarize the
spectrum of forms and functions within the plant kingdom, with leaf
economics (e.g., LMA) and plant size (e.g., tree height) being thetwo dominant dimensions underpinning life-history
strategies (Maynard et al. , 2022). The distinct orthogonality of
these two axes implies that they are shaped by different environmental
drivers. For example, a comprehensive global assessment examining 17
traits across an extensive sample of over 20,000 species demonstrated
that variations in size-related traits are strongly associated with
latitudinal gradients, which are indicative of constraints on water or
energy availability (Joswig et al. , 2022). In contrast, economic
traits show a near-exclusive response to soil conditions, highlighting
the unique influence of soil factors on these characteristics.
Therefore, these interspecific size-trait relationships are
confounded by environmental drivers at a global scale.Consistent with previous research (Kenzo et al. , 2006;
Louis et al., 2012), we found that LCC
significantly decreased with plant height. High LCC leaves are related
to low dark respiration rates and light compensation points, permitting
better acclimation to poor light for small trees and shrubs (Guimarãeset al. , 2022). Negative correlations between LCC and plant height
indicate that a high LCC contributes to light-harvesting efficiency at
low irradiances.
Despite substantial differences in leaf traits among growth forms and
intertidal gradients, negative relationships between leaf traits (LMA,
LDMC, LD, LCC, and WSD) and plant size (height and DBH) were detected
within different growth forms and intertidal zones. Our results disagree
with the findings of Li et al. (2021), who utilized leaf trait
networks derived from global data to assert that the interdependence of
leaf economic traits was more pronounced in shrubs than in trees. Plants
in environments with limited resource availability are likely subjected
to more intense selective pressures, leading to a tighter correlation
between traits to ensure efficient resource acquisition and utilization(Flores-Moreno et al., 2019; Liuet al., 2019). For instance, leaf economic and hydraulic traits
are found to be independent in humid regions (Li et al.,
2015) but exhibit strong coupling in arid regions (Yin et
al., 2018). In comparison to terrestrial plants, mangroves
may face more constrained availability of water resources since
water uptake under saline conditions is energetically expensive