4.1 Growth forms and intertidal gradients affect leaf traits

We found that most leaf structural traits of mangrove species differed significantly along intertidal gradients. Specifically, the LCC, LA, LT, LV, LFM, LSM, LDM, and LMA increased significantly, and the WSD decreased with elevation. These results differed from those of a previous study of mangrove plants along an intertidal gradient in mangrove wetlands in Hainan, China, which reported that LMA and LT decreased significantly from low to high intertidal zones (Yu et al. , 2023). Salinity and pH are recognized as the principal sediment characteristics influencing the functional traits of mangrove leaves (Reddy et al. , 2021). Elevated salinity levels have been shown to impede mangrove tree growth (Ahmed et al. , 2022). Concurrently, sediment pH indirectly influences the functional traits of mangrove leaves by modulating soil nutrient availability and salinity levels (Hartemink and Barrow, 2023). Several studies have shown that the LDMC, LT, and LMA of mangroves increase with increasing salinity and decreasing pH (del Campo et al. , 2022), indicating greater conservation of plants in highly stressed soils (Wright et al. , 2004; Díaz et al., 2016). Yu et al.(2023) reported that sediment pH increased significantly and salinity decreased significantly from low to high intertidal zones in the Hainan Dongzhaigang Reserve. In principle, LDMC, LT, and LMA could decrease along elevational gradients. Conversely, we found that LDMC, LT, and LMA increased significantly along the elevational gradient. The possible reason for this difference is that LCC and WSD, in addition to conventional traits related to LES, are important physiological parameters that determine the survival and growth of mangrove plants (Biber, 2006). Our findings that trees had greater LA, LV, LFM, LSM, and LDM and lower LMA than shrubs were partially consistent with a study by Wanget al. (2019), who reported that LA, LDM, and LMA were greater in trees than in shrubs. LA reflects a plant’s light capture potential (Strauss et al. , 2020). Tree canopies are typically exposed to high irradiance, while understory shrubs may face constraints in terms of the availability of light resources (Kenzo et al. , 2015; He and Yan, 2018). Therefore, higher LA in trees may be an adaptation to high light intensity to maintain greater photosynthetic capacity and productivity. Moreover, higher LV, LFM, LSM, and LDM can enhance photosynthetic capacity under high irradiance. This is achieved by increasing the nitrogen content and expanding the photosynthetic machinery volume per unit leaf area (Oguchi et al. , 2005; Liu et al., 2019), which may explain the greater LV, LFM, LSM, and LDM in trees. LMAs are the primary driving factors of drought tolerance (Fletcher et al. , 2018). Our findings indicate that mangrove shrubs may experience more limited access to water resources than trees, as evidenced by the greater WSD observed for shrubs. Additionally, a high LMA suggests a reduction in intercellular space and increased resistance to gas diffusion within the mesophyll (Peguero-Pina et al. , 2017). This characteristic could be advantageous by enhancing plant tolerance to cell collapse, a consequence of drought stress (Bussotti and Pollastrini, 2015; Evans, 2021). The diffusion resistance of shrubs with a high LMA may increase to decrease leaf transpiration. Hence, determining the variation in leaf traits between different growth forms is essential for elucidating the mechanisms that underpin the ecological strategies of plant species. These strategies are crucial for successful adaptation and occupancy of diverse habitats (Wang et al. , 2022; Islam et al., 2024). Exploring the effects of intertidal gradients on leaf functional traits between plant growth forms is helpful for understanding species diversity maintenance in forests (del Campo et al. , 2022; Yu et al., 2023). Our study revealed that the responses of leaf structural traits to growth form vary across intertidal zones. Leaf functional trait variation with growth form effectively reflects a plant’s adaptation strategy, which shapes differences in their demand and utilization of resources such as light, precipitation, temperature, and nutrients (Islam et al. , 2024). Thus, leaf traits differ among growth forms in response to intertidal elevation gradients with changes in moisture, temperature, salinity, and wave energy (Wang et al. , 2019). Our study has shed light on the different effects of intertidal gradients on leaf traits between different plant growth forms. This enhanced understanding is expected to deepen our insight into plant adaptive strategies and the evolutionary dynamics of plant traits as they adapt to the mosaic of environmental conditions. In addition, our findings reveal a major mechanism maintaining plant diversity in mangrove forests.