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