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.