Study site and cultivation of mangrove seedlings

The study was performed in the Futian Mangrove Nature Reserve (22°31' N, 114°05' E) located in an estuary of the Zhujiang River in Shenzhen, Guangdong Province, China. The study site is characterized by a subtropical monsoonal climate with an annual precipitation of 1927 mm and a rainy season from May to September [37]. The monthly change in precipitation was around 26.3–553.8 mm. The mean annual air temperature is 22°C, with a low mean monthly temperature of 14°C (in January) and a high mean monthly temperature of 28°C (in July) [37]. The monthly changes in air temperature in summer and winter seasons were 27.8–28.9°C and 15.9–18.4°C, respectively. The mean monthly insolation is 450 j·m^-2, with the highest insolation in August (590 j·m^-2) and the lowest in February (322 j·m^-2) [37]. The mean duration of light is 6.1 h·day^-1, with the minimum and maximum hours occurring in February (4.2 h·day^-1) and August (7.8 h·day^-1), respectively [37]. The study site area has the features of non-regular, semi-diurnal tides and the spring tidal range is about 2.8 m [37].

The seedlings of the four native mangrove species were germinated from seeds or propagules harvested from the nursery garden of the Futian Mangrove Nature Reserve prior to the study, while that of the two Sonneratia species were collected from the monoculture forest of the reserve. To ensure that all seedlings had comparable morphological features and sizes, all native species were 12 month-old. However, as the aliens, Sonneratia, grew faster than the native species, the 12 month-old seedlings were too big with extensive roots to be transplanted, leading to a high death rate of the transplanted seedlings. Therefore, younger seedlings of Sonneratia (3- to 4-month old) with morphology and size similar to the native mangrove seedlings (12-month old) were used (Table 1).

Mean and standard error of replicates are shown for the height, diameter, moisture and biomass.

At the beginning of the experiment, uniformly sized seedlings of the same species were transplanted into pots filled with mangrove soil collected from the reserve on 12^th July 2010. Each pot had a dimension of 45 cm in height and approximately 45 cm in diameter and contained one transplanted seedling. Prior to transplanting, the seedling was gently washed with tap water to remove mud and wiped with tissue paper. The height, ground diameter, moisture content, and fresh and dry initial biomass of the seedlings were determined and are summarized in Table 1. All pots with a tray at the bottom of each were placed on the open ground at the landward edge of the mangrove forest in the reserve, and the seedlings were grown for 18 months prior to the first measurement on 12^th January 2012. During high tides, tidal water was flooded to the tray but not to the surface of the pot. The bottom soil could be wetted from below. To have sufficient moisture for plant growth, the pots were watered daily (at 18:00 h) by the tidal water collected from a nearby drain connecting the sea, about 3 litre tidal water per pot, except rainy days, to simulate the natural condition of the mangrove forest. At the end of the 18 months growth period, four consecutive samplings were carried out, i.e., in January, April, July and October, representing the winter, spring, summer and fall seasons, respectively. For each species, a total of 12 pots were prepared, with three pots randomly sampled in each season. In every sampling season, the photosynthesis of each plant in the three pots were measured in situ, and these plants were harvested for the determination of the morphological index, CC and growth traits, with triplicates for each species.