The linkages between stomatal physiological traits and rapid expansion of exotic mangrove species (Laguncularia racemosa) in new territories

The fast-growing exotic mangrove species (Laguncularia racemosa) has been widely introduced in new territories such as China to restore mangrove ecosystems. However, the invasiveness, as well as the mechanisms for the rapid expansion after the introduction are still not well studied. Here, we try to reveal possible micro-mechanisms for the fast expansion of L. racemosa, using the data on leaf stomata straits, gas-exchange parameters, stable isotope ratios, carbon-nitrogen allocation from L. racemosa and the adjacent native mangroves (Avicennia marina, Aegiceras corniculatum, Bruguiera gymnorhiza, Kandelia obovata) in Hainan Island, China. We found that the higher density but smaller size stoma of L. racemosa enhanced stomatal conductance and shorten the diffusion path of carbon dioxide, thereby increasing the photosynthetic rate. Moreover, the higher stomatal density of L. racemosa exerts a significant positive effect on transpiration, which thus accelerated the water transport and nutrient uptake to meet the advanced need for nutrients and water for fast-growing. The evidence from leaf δ¹³C and carbon-nitrogen allocation further proved that L. racemosa has a lower intrinsic water use efficiency but a higher rate of photosynthesis than native mangrove species. Our results suggest that stomatal morphological and physiological traits could strongly influence the growth of L. racemosa compared to the adjacent native mangroves, which provides a new perspective for the fast expansion of exotic mangrove species in China. These findings also suggest that L. racemosa has an invasive potential in native mangrove habitats, thereby the mangrove reforestation projects by introducing L. racemosa should be treated with caution.

Introduction

Mangrove forests, one of the most carbon-rich ecosystems, provide numerous ecological functions such as breeding habitats, purifying water quality, protecting coastlines, and sequestrating CO₂ (Duke et al., 2007; Lee et al., 2014; Lovelock and Duarte, 2019). However, one-third of the total world mangrove area has been lost during the last half-century due to human activity or climate change (Kuenzer et al., 2011; Hamilton and Casey, 2016). The mangrove in Hainan Island is considered a hotspot for mangrove research and conservation due to the highest mangrove diversity and the highest carbon stock of all mangrove forests in China (Li and Lee, 1997; Lin, 1999). From 1950 to 2010, the mangrove area of Hainan sharply reduced by 60% (Liao and Zhang, 2014). In order to increase mangrove habitats, reforestation projects have been conducted on Hainan Island. However, in these restoration projects, exotic mangroves, especially fast-growing species were introduced into new territories. Furthermore, the invasiveness risk and the potential expansion mechanism of exotic mangroves were neglected and still need to be systematically studied.

Laguncularia racemosa (white mangrove) is one mangrove species, natively distributed on the coasts of western Africa and eastern America, which was introduced to Hainan, China from the coast of LaPaz, Mexico in 1999 (Liao et al., 2006). L. racemosa was the most widely used exotic mangrove species for reforestation due to its high environmental adaptability and fast-growing character (Wang et al., 2018). Previous studies have indicated that L. racemosa is a quality afforestation species that could be widely introduced to mangrove habitats (Zhong et al., 2011; Chen et al., 2013). However, more field observations have shown that L. racemosa grew faster than native mangrove species and was even more aggressive than Sonneratia apetala (another fast-growing mangrove species) (Li et al., 2020). Previous studies of L. racemosa mainly focused on the reforestation effect and planting technology (Zhong et al., 2011; Chen et al., 2014), physiological character (Han et al., 2010), salt tolerance (Sobrado, 2005; Rodríguez et al., 2017). Few studies estimated the invasiveness and invasive mechanism of L. racemosa. The study by Gu et al. (2019) has reported that the combined effects of salinity and light exerted a significant influence on the growth of L. racemosa. Li et al. (2020) believed that L. racemosa was growing faster because of the lower leaf construction cost. Although those studies suggested that L. racemosa has invasive potential, there was no proposed physiological mechanism to explain why L. racemosa grows faster and has a higher salt tolerance and higher photosynthesis rate.

Exotic fast-growing species often have advantages over native species on growth-related traits, such as photosynthetic rate (A) and resource use efficiency (Funk and Vitousek, 2007; Matzek, 2011). Stomata, the pores on the leaf surface, exert a major influence on plant growth and productivity through regulating stomatal conductance (g_s) and transpiration (E) by stoma opening or closure, which determine the carbon, water, nutrient cycling between plant and ambient environment (Lin et al., 2015; Henry et al., 2019). The g_s to CO₂ and water are ultimately driven by the combined effect of size (taken here as guard cell length, S) and density (number per unit area, D) of stomata in the epidermis (Franks et al., 2009; Doheny-Adams et al., 2012). Generally, the relationship between S and D is well described by a negative power function but the slope is different across species (Hetherington and Woodward, 2003). Previous studies have indicated that stomatal density imposed a positive effect on the conductance therefore enhance leaf photosynthesis capacity (Tanaka et al., 2013; Pathare et al., 2020). Furthermore, plants increase water-use efficiency (WUE) by reducing stomatal conductance (g_s) via altered stomatal density (D) and size (S) (Franks et al., 2015; Dunn et al., 2019), which is in parallel with the evidence from stable isotope studies (Guerrieri et al., 2019). Meanwhile, in terms of resource use efficiency, plants can adjust the water transport and nutrient uptake by altering transpiration and thereby maintain foliar nutrient supply and water balance (Lu et al., 2018; Lanning et al., 2019). Although the stomatal effect has been intensively investigated in many terrestrial plant species, the stomatal traits and effect of mangrove species have remained unclear, especially in exotic mangrove species. It is therefore essential to elucidate the relationship between stomatal traits and gas exchange in native and exotic mangrove species.

This study aims to unveil the physiological mechanisms by comparing the stomatal effect and other physiological characteristics between native and exotic mangrove species. We collected data on mangrove stomata traits (guard cell length, stomatal density), gas exchange parameters, stable isotope fractionation, carbon and nitrogen contents to examine the linkages between stomatal traits and plant growth features of L. racemosa. Our study was designed to address two questions: (a) what are the differences in stomatal traits and gas-exchange characters between exotic and the adjacent native mangrove species? (b) how do the stomatal traits affect the physiological process of exotic mangroves thereby resulting in faster growth than native mangroves? Those results will strengthen the understanding of the potential expansion risk of exotic mangroves and have important implications for mangrove restoration and conservation.

Materials and methods

Study sites

Field measurements were conducted in two study sites on Hainan Island in the south of China, which was considered a hotspot for mangrove research and conservation due to the highest mangrove diversity and the highest carbon stock of all mangrove forests in China (Li and Lee, 1997; Lin, 1999). Haikou (HK) and Sanya (SY), located in the Northern and Southern of Hainan Island, were selected as two main sites for this study. Haikou Dongzhaigang Mangrove Nature Reserve (113°57’E, 22°35’N) was established as the first National Mangrove Nature Reserve of China in 1986 and was then listed as one of the International Important Wetlands in 1992 (Qiu et al., 2011). Dongzhaigang Mangrove Nature Reserve is the first place where two exotic mangrove species (Sonneratia apetala, Laguncularia racemosa) were introduced and planted. The Laguncularia racemosa was introduced from Mexico in 1999 (Liao et al., 2006). Sanya Qingmeigang Mangrove Nature Reserve (109°36’E, 18°13’N) has a mangrove area of 64 ha^-1, which is the largest and best-preserved mangrove region in Sanya City. With a tropical monsoon climate, the mean annual temperature, mean annual precipitation and salinity were 23.8°C, 1685 mm, and 27‰ for the Haikou site, and 25.5°C, 1280 mm, 33‰ for the Sanya site, respectively.

Leaf gas exchange measurements

Gas exchange parameters of leaves were measured using an LI-6400 portable photosynthesis system (Li-COR Inc., Lincoln, NE, USA). The exotic mangrove Laguncularia racemosa and the adjacent native mangroves (Avicennia marina, Aegiceras corniculatum, Bruguiera gymnorhiza, Kandelia obovata) with the same tree age (≈ two years) were chosen to measure the gas exchange parameters. All measurements were conducted between 09:00 and 15:00, with the ambient temperature ranging from 26 to 35°C in Summer. Conditions in the cuvette were maintained around 400 ppm CO₂ concentration, approximate local ambient CO₂ concentration, and photosynthetic active radiation was set at 1000 μmol photons m^-2 s^-1, with an airflow rate of 500 μmol s^-1. The assimilation rate (A), stomatal conductance (g_s), transpiration rate (E), and CO₂ concentration (Ci) were recorded after stabilizing control conditions (≈ 30 mins) with a leaf inside the leaf cuvette. Instantaneous water use efficiency (WUEi) was calculated according to the ratio A/E. For each leaf, five consecutive measurements were averaged as one replicate value, and three to five replicates of individual plants were gathered for each mangrove species.

Stomatal traits

The mature and intact leaves of different species under the same environments were chosen to measure stomatal traits. For each leaf, a razor blade was used to scrape the leaf vein and mesophyll, leaving the leaf epidermis, which was then rinsed clean with deionized water. Small sections of the epidermis were cut, and drip stained on glass microscope slides. Under a fluorescence microscope (Nikon Eclipse Ci-L, Tokyo, Japan) at ×400 magnification, guard cell lengths were measured, and stomatal density was calculated as the number of stomata per field of view. Three fields were sampled for each leaf, and three to five leaves were measured for each species. Both sides of the leaf epidermis were imaged, and the stomatal density of both sides was calculated.

Stable isotope analyses

After measuring leaf gas exchange, we collected the plant samples from the exotic and the adjacent native mangroves. The leaf, stem, and root of each plant were separated and dried by oven at 70°C for 48 h until constant weight, then ground to powder using a grinding mill (Jingxin, JXFSTPRP-32, China), and passed through a 60-mesh sieve. The δ¹³C and δ¹⁵N were measured with an isotope ratio mass spectrometer (IRMS) (Delta V Advantage, Thermo Fisher Scientific, Inc., USA), the C and N content were measured by the elemental analyzer (Flash 2000 EA-HT, Thermo Finnigan, Bremen, Germany). The isotope composition of plant tissue (‰, in parts per thousand) was calculated as:

$\begin{array}{ll}\delta =\left(\frac{R_{sample}}{R_{standard}}-1\right)\times 1000& (1)\end{array}$

Where R_sample and R_standard are the isotope ratios of the plant tissue relative to PDB standard for δ¹³C and N₂-atm standard for δ¹⁵N, respectively. The δ¹³C and WUE were demonstrated as (Farquhar and Richards, 1984; Farquhar et al., 1989).

$\begin{array}{ll}\text{WUE}=\frac{A}{E}=C_a\left(1-\frac{C_i}{C_a}\right)/1.6v& (2)\end{array}$

Where A is the assimilation rate of the plant, E is the transpiration rate. c_a and c_i are the partial pressures of CO₂ in the atmosphere and intercellular, respectively. v is the vapor pressure difference between the intercellular and the atmosphere.

$\begin{array}{ll}{\delta }^{13}{\text{C}}_{plant}={\delta }^{13}{\text{C}}_{air}-a-(b-a)\frac{C_i}{C_a}& (3)\end{array}$

$\begin{array}{ll}\text{WUE}=C_a\left(1-\frac{{\delta }^{13}{\text{C}}_{air}-a-{\delta }^{13}{\text{C}}_{plant}}{(b-a)}\right)/1.6v& (4)\end{array}$

$\begin{array}{ll}\Delta \text{leaf}=\frac{{\delta }^{13}{\text{C}}_{air}-a-{\delta }^{13}{\text{C}}_{plant}}{1+{\delta }^{13}{\text{C}}_{plant}/1000}& (5)\end{array}$

where a is the fractionation caused by CO₂ diffusion in the air (4.4‰), b is the fractionation due to carboxylation (27‰), and δ¹³C_plant and δ¹³C_airare the carbon isotope composition of the plant and CO₂ in the atmosphere, respectively (Farquhar et al., 1982).

Statistical analysis

We applied linear and non-linear regressions to test the significance of correlations (p<0.05) between stomatal density and stomatal size. One-way analysis of variance (ANOVA) was used to evaluate the differences in gas exchange parameters among different mangrove species means. A Tukey’s HSD multiple comparison was used to test significance at α=0.05 level. Mean and standard error (S.E.) values of replicates were calculated for each variable. A multi-linear regression model and spearman correlation heat-matrix were created to explore the relationships among stomatal traits, photosynthesis characters by using the “ggplot2” and “ggcor” package in R 4.0.5. The conceptual diagram was made to summarize the physiological link of mangrove stomatal traits and other factors by using Adobe illustrator CC2018 (Adobe Systems Inc., San Jose, CA, USA). The statistical analyses and figures were carried out using Origin software 2019b. (OriginLab Corp., Northampton, MA, USA).

Results

Relationships between stomatal density and stomatal size in mangroves

For the native mangrove K. obovata, the stomata were only found on the lower epidermis (Figures 1A, C). However, stomata are distributed on both the lower and upper epidermis of leaves in L. racemosa (Figures 1B, D). Furthermore, the stomatal density of the upper epidermis was higher than those of the lower epidermis in L. racemosa. Stomatal density tended to increase with the stomatal guard cell length decline, the negative relationship was best estimated (R^2 = 0.66, p<0.001) by a power function (Figure 1E). The stomatal density of exotic mangrove species is significantly higher than that of native mangrove species. However, the stomatal guard cell length of exotic mangroves is lower than that native mangrove species (Figure 1E).

FIGURE 1

Figure 1 Comparisons in leaf stoma distribution and density between a native mangrove K. obovate and an exotic mangrove L. racemosa, the upper epidermis of K. obovate (A) and L. racemosa (B), the lower epidermis of K. obovate (C) and L. racemosa (D), the relationship between stomatal density and stomatal length pooled for different mangrove species (E).The error bars represent Mean±SE (n=5), and the species name abbreviations are as follows: Rs, Rhizophora stylosa; Ct, Ceriops tagal; Ko, Kandelia obovata; Bg, Bruguiera gymnorhiza; Ac, Aegiceras corniculatum; Ll, Lumnitzera racemosa; Am, Avicennia marina; Sa, Sonneratia apetala; Lr, Laguncularia racemosa.

Photosynthetic gas exchange parameters

Gas exchange parameters varied significantly between species. Specifically, the leaf CO₂ assimilation rate was significantly higher in exotic mangroves (Lr) compared with other native mangrove species (Figure 2A). Consistently, the stomatal conductance and transpiration of L. racemosa were significantly higher than those of native species (Figures 2B, C). Instantaneous water use efficiency in L. racemosa showed no significant difference when compared with B. gymnorhiza but was lower than other native mangrove species (Figure 2D). Remarkably, L. racemosa has the largest stomata density but the smallest stomata size (Figures 2E, F).

FIGURE 2

Figure 2 Comparison of gas exchange parameters and stomata traits. (A) photosynthesis (A), (B) transpiration (E), (C) conductance (g_s), (D) Instantaneous water use efficiency (WUEi), (E) stomata guard cell length (L), and (F) stomata density (D) between exotic mangrove species (red bar) and native mangrove species (green bar), the error bars represent Mean ± SE(n>10), the letters above the bar indicate different significantly (p<0.05). See Figure 1 for species abbreviations explanations.

Stable isotope compositions

The δ¹³C of leaf, stem, root in L. racemosa were all significantly lower than native mangrove both in the Haikou site and Sanya site (Figures 3A, B). In addition, the ⊿leaf of L. racemosa was significantly higher than those of native mangroves across two sites (Figures 3C, D). For the Haikou site, the value of δ¹³C in L.racemosa increased in the order of root>stem>leaf, whereas the value of δ¹³C in K. obovata was leaf >stem>root. the largest difference (≈3 ‰) in the carbon isotope discrimination between L. racemosa and K. obovata was observed in roots. The δ¹⁵N of L. racemosa was not significantly different from those of native mangroves (Figures 3A, B). For Sanya site, the δ¹³C of L. racemosa were more negative than the natives species, but had a different pattern compared with the δ¹³C of L. racemosa in the Haikou site, the value of δ¹³C increased in the order of root >stem>leaf (Figure 3B). Furthermore, the range of δ¹⁵N (root-stem-leaf) discrimination in L. racemosa (4.2‰-6.7‰) was wider than those in C. tagal (4.5‰-5.5‰) but lower than those of L. racemosa in Haikou site (6 ‰-10 ‰). And the leaf δ¹⁵N of L. racemosa was lower than those of all native mangrove species. Generally, the δ¹³C of exotic mangrove (L. racemosa) was more negative than native mangrove. Meanwhile, the δ¹³C discrimination of mangroves in Haikou was lower than mangroves in Sanya, whereas the δ¹⁵N discrimination of mangroves in Haikou was higher than mangroves in Sanya.

FIGURE 3

Figure 3 The value of δ¹³C, δ¹⁵N, and ⊿leaf in native and exotic mangroves. Comparing mangrove δ¹³C and δ¹⁵N values of mangrove in HK(Haikou) site (A) and SY(Sanya) site (B). The⊿ leaf of mangrove in HK(Haikou) site (C) and SY(Sanya) site (D). Exotic mangrove (Lr-Laguncularia racemosa) and native mangrove (Ko-Kandelia obovata, Am-Avicennia marina, Rs-Rhizophora stylosa, Ct-Ceriops tagal, Bg-Bruguiera gymnorhiza, Ac-Aegiceras corniculatum,Ra- Rhizophora apiculata), the error bars represent Mean ± SE (n=5). Red dots represent exotic mangroves, green dots represent native mangroves. The letters above the bar indicate different significantly (p<0.05).L, Leaf; S, Stem; R, Root.

Relationship between stomatal traits and gas exchange parameters

The spearman correlation analyses show that stomata size negatively related to stomata density (R=-0.78). This pattern was similarly observed in WUE (R=-0.56) (Figure 4A). Generally, the stomata density was significantly positively correlated with A, E, Gs, whereas the negative coefficients were observed between stomata size and A, E, Gs (Figure 4A). However, the relationships among stomata size, stomata density and A, E, Gs were nonlinear (Figure 4B, Figure S1). For instance, the WUE decreased slowly with low density but sharply declined with high density, which differed from the trend of stomata size (Figure 4B). With the increase of stomata density, conductance increased rapidly with low density and then stably rose with high density, thus leading to an increase in photosynthesis rate and transpiration (Figure 4B). However, the transpiration increased more in high-density samples compared to the trend of stomatal conductance and photosynthesis rate (Figure 4B).

FIGURE 4

Figure 4 Spearman’s correlation coefficient heat map (A), and a conceptual figure (B) showing the general relationships among stomatal size (L), water use efficiency inferred from carbon isotope ratio (δ¹³C WUE), photosynthesis (A), transpiration (E), stomatal conductance (g_s) to stomatal density (D) in mangroves.

Discussion

Stomatal traits of native and exotic mangrove species

The relationships between stomata size and stomata density exert a significant influence on plant growth and productivity through regulating stomatal conductance and transpiration, which control the carbon, water, nutrient cycling between plant and environment. We found that the relationship between stomata size and stomata density in mangrove was best described by a negative power function (Figure 1E), which was consistent with the previous studies (Franks and Beerling, 2009). Besides, we observed that stomata can be found on both the lower and upper epidermis of the leaves of L. racemosa, whereas stomata exist only on the lower epidermis in most native mangrove species (Figure 1). Also, the exotic mangrove species tend to have higher stomata density and smaller stomata size than the native mangrove species. The negative correlation between stomata size and stomata density of mangrove may be explained by the physical constraints and energy strategy (Franks et al., 2009). On one hand, the limited space of leaves requires the allocation of stomata with the sufficient density and size to achieve the optimized conductance for photosynthesis. On the other hand, the plants balance photosynthetic productivity (energy gain) and respiration (energy loss) through the evolution of stomatal properties.

Contrasting gas exchange parameters between native and exotic mangroves

The higher stomata density corresponding to a higher photosynthetic rate, and stomatal conductance was observed in the L. racemosa (Figures 2A, B, F). The same trend can be found in other mangrove species, namely, that higher stomata density tends to increase stomatal conductance and photosynthetic rate, and that lower stomata density corresponds with a down-regulation of conductance (Figure 4). This trend is consistent with the previous study by (Franks et al., 2009), which showed an enhancement of photosynthetic rate and conductance limited by the relationship between stomatal size and density. This can be explained by the negative relationship between stomata size and density, the allocation traits of stoma constrained by the space-optimized rule. In order to enhance stomatal conductance, plants tend to allocate the small size but high density stoma to optimize the space (Dow et al., 2014). With the higher conductance and photosynthesis of L. racemosa, the transpiration increase results in a decline in instantaneous water use efficiency (WUEi) compared with native mangroves. Although increasing conductance supplies additional CO₂ and water for carboxylation, more water is lost through the more numerous stomata. Several previous studies have verified that conductance plays an important role in regulating the WUE, leading to a negative relationship between conductance and WUE (Figure 4) (Franks et al., 2015; Dunn et al., 2019).

Environmental factors affected stomatal traits and gas exchange of mangroves

However, it should be pointed out that mangroves are restricted by salinity due to the saline environment imposing a negative effect on photosynthesis and conductance through regulating the opening and closure of stoma (Ball and Farquhar, 1984; Parida et al., 2004). This suggests that stomata traits and gas exchange parameters of mangrove will shift with environmental changes (Sperry et al., 2016; Dong et al., 2020). Mangroves, therefore, have evolved salt-tolerant function such as salt exclusion in the root, salt secretion by salt glands, osmolyte accumulation to adapt to the high salinity of seawater (Parida and Jha, 2010; Jiang et al., 2017; Cheng et al., 2020). Even so, only a few mangrove species, such as Aegiceras corniculatum, Avicennia marina, have specialized salt glands. Interestingly, we found that L. racemosa has a good salt secretion function owing to abundant salt glands and aqueous tissues distributed in the leaves (Chen et al., 2018; Li and Lin, 2006). Thus, we concluded that despite the lower WUE, there is no strong limit on water supply for L. racemosa compared to native mangroves. These results may explain why L. racemosa possesses a high salt tolerance.

The evidence from the isotope also confirmed that L. racemosa has a lower WUE (more negative δ¹³C) but higher photosynthetic capacity compared with other native mangrove species (Figure 3). The higher nitrogen (N) content and lower carbon-nitrogen ratio (C:N) of L. racemosa suggest a higher nitrogen uptake efficiency and photosynthetic nitrogen use efficiency (NUE) compared to native species (Figure S2). Moreover, we found that a spatial pattern exists in the δ¹³C and δ¹⁵N values, namely, the δ¹³C discrimination of mangrove in Haikou site (low salinity) were lower than mangroves in Sanya site (high salinity), whereas the δ¹⁵N discrimination of mangroves in Haikou were higher than mangroves in Sanya (Figure 3). The lower foliar δ¹³C of mangrove reflect a lower WUE in Haikou sites due to the lower salt stress. Foliar δ¹⁵N values serve as an indicator of the supply of N relative to plant N demand, and the lower δ¹⁵N values represents a greater N limitation (Craine et al., 2018). The results of δ¹⁵N shows that the mangroves in Haikou exhibited a lower N limitation compared to the mangroves in Sanya, which was caused by lower salt stress and higher N availability in Haikou mangrove habitats (Bai et al., 2021).

The potential mechanism for rapid expansion of exotic mangrove in new territory

Previous studies have reported that L. racemosa was more invasive than other mangrove species due to fast growth and high salinity tolerance (Gu et al., 2019). However, the potential mechanism of fast growth is still unclear. As Figure 5 shows, the foliar stomata density of L. racemosa is higher than that of native mangrove species, particularly on the upper epidermis, which yields two important physiological functions for its fast growth. On one hand, high stomata density enhances the conductance thus supplies more CO₂ and H₂O to support the carboxylation. On the other hand, having numerous stomata on the upper epidermis (chloroplasts distribution) shortens the diffusion path of CO₂ and H₂O. Consequently, more CO₂ diffuses into the chloroplasts thus enhancing the rate of photosynthesis. Meanwhile, the transpiration increases with the conductance increasing, which results in more water loss thus decline of WUE (Figures 2D, 3). This is consistent with the study by Tian et al. (2016). The study shows that stomatal density was positively correlated with vein density thereby balancing the leaf-level water transpiration and supply. As mentioned above, however, the lower WUE has no strong restriction on water supply due to the efficient salt secretion function and aqueous tissues of L. racemosa. Furthermore, the increasing transpiration alters the water potential through the plant and accelerates the water transport and nutrient uptake in L. racemosa. It thereby maintains foliar nutrient supply and water balance (Cernusak et al., 2011; Ocheltree et al., 2016; Lu et al., 2018). As a result, the N content of the L. racemosa is higher than those of native mangroves but the C:N ratio is significantly lower than native species, especially for foliar N content (Figure S2), which provides further evidence to verify the advantages of water and nutrient transport and allocation in L. racemosa.

FIGURE 5

Figure 5 Conceptional diagram for linking stomatal traits to foliar gas exchange performance in native mangroves (right) and exotic mangrove (left) N-nitrogen ability, E-transpiration, g_s- conductance, δ¹³C-the stable carbon isotope of mangrove leaf.

Implications for mangrove conservation under environment change

The ongoing rising atmospheric CO₂ (C_a), is expected to affect the stomatal traits of plants, and thus regulate the gas-exchange and energy balance, such as carbon, water, and nutrient cycling of plants (Voelker et al., 2016). Studies from experiments have reported that the water use efficiency tends to increase with a rise in atmospheric CO₂ by decreasing the stomatal conductance, thus enhancing assimilation rates (Keenan et al., 2013; Franks et al., 2015). The trend was further corroborated by the increase of ¹³C/¹²C isotopic discrimination of plant photosynthesis (Keeling et al., 2017). Whether this trend exists for mangrove plants is not clear. Several studies indicated that elevated CO₂ imposes a positive effect on mangrove productivity (Farnsworth et al., 1996; Ball et al., 2010; Reef and Lovelock, 2015). Furthermore, the above- and belowground production of mangroves have different responses to elevated CO₂ in combination with other environmental factors (e.g. N availability, salinity, temperature) (McKee and Rooth, 2008; Reef et al., 2016). Normally, the optimized assimilation rate is constrained by the balance of maximum C gain and minimum water loss. However, L. racemosa shows higher photosynthesis but lower WUE due to higher conductance. To some degree, there is minimal water limitation of the L. racemosa. Hence, we hypothesize that the rising atmospheric CO₂ is expected to significantly enhance the assimilation rate of L. racemosa and thus facilitate the expansion of L. racemosa into native mangrove habitats. Certainly, more evidence is needed to further test this hypothesis.

Furthermore, more nutrient input could enhance the mangrove productivity in a short term. Aquaculture is a major threat to mangrove forest degradation (Rahman et al., 2013; Richards and Friess, 2016). Aquaculture not only directly reduces the mangrove cover but also increases nutrient input into the neighboring mangroves. Especially in China, the pollution and deforestation caused by aquaculture are the main threats leading to over 50% of mangroves loss (Dan et al., 2016). The increased nutrient loading might also strengthen the CO₂–nutrient fertilized effect of exotic mangroves because of the higher assimilation rate and nutrient use efficiency compared to the native mangroves. Those results suggest that climate change and human activity also impose widespread effects and synergistic interactions to accelerate the expansion of exotic mangroves.

Conclusions

This study provides a new perspective showing the linkages between stomatal physiological traits and the rapid growth of the exotic mangrove L. racemosa in new territories. Stomata density and stomata size were major traits that enhance mangrove photosynthetic capacity by increasing conductance, transpiration, and decreasing water use efficiency. Additionally, the evidences from isotopes shows that the stomata traits and gas exchange of mangroves change with ambient environment change. Our results indicate that leaf stomatal and physiological traits strongly affect the rapid expansion of exotic mangroves (L. racemosa) by modulating the gas exchange process. These findings also imply that L. racemosa exhibited an invasive potential in native mangrove habitats and should be cautiously introduced into native mangroves habitats for reforestation.

Data availability statement

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Author contributions

JB, GL, and XZ designed the research. JB, YM, RG, and ZD performed the field survey, and JB analyzed the data and wrote the manuscript. All authors especially GL and XZ either supported the field surveys and/or contributed to the revisions of manuscript. All authors contributed to the article and approved the submitted version.

Funding

This study was supported by the Ministry of Science and Technology of China (2017FY100703; 2019YEA0606604), the National Natural Science Foundation of China (41877352), and the Guangdong MEPP fund (GDOE [2019] A06).

Acknowledgments

We thank the staff of Hainan University for help during our field plot surveys. We would also like to thank the staff at the Dongzhaigang National Mangrove Nature Reserve, the Qinglangang Provincial Mangrove Nature Reserve, and other county mangrove nature reserves in Hainan for their logistical support during our fieldwork.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fmars.2023.1136443/full#supplementary-material

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