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).