1. Saltmarsh-mangrove ecotones occur at the boundary of the natural geographical distribution of mangroves and salt marshes. Climate warming and species invasion can also drive the formation of saltmarsh-mangrove mixing communities. How these coastal species live together in a “new” mixed community is important in predicting the dynamic of saltmarsh-mangrove ecosystems as affected by ongoing climate change or human activities. To date, the understanding of species interactions has been rare on adult species in these ecotones.

2. Two typical coastal wetlands were selected as cases to understand how mangrove and saltmarsh species living together in the ecotones. The leaves of seven species were sampled from these coastal wetlands based on their distribution patterns (living alone or coexisting) in the high tidal zone and seven commonly used functional traits of these species were analyzed.

3. We found niche separation between saltmarsh and mangrove species, which is probably due to the different adaptive strategies they adopted to deal with intertidal environments.

4. Weak interactions between coexisting species were dominated in the high tidal zone of the two saltmarsh-mangrove communities, and which could be driven by both niche differentiation and neutral theory.

5. Synthesis. Our field study implies a potential opportunity to establish a multispecies community in the high tidal zone of saltmarsh-mangrove ecotones, where the sediment was characterized by low salinity and high nitrogen.

Sampling

The full opened and healthy green leaves were selected and, in each subplot, 30 leaves in total were collected from 15 individuals randomly. Three sediment samples (top 15 cm) for each subplot were collected on the same days with leaves sampling. The three sediments of each subplot were then mixed evenly for chemical analysis. All the samples were packed in sealed-bags and brought to the laboratory immediately after field collection.

Analysis of leaf traits

The leaf area was measured with image analysis software (Image P), then leaves were dried for 48 h at 65 °C, weighed for determination of SLA, defined as the area of one side of a fresh leaf divided by its oven-dried mass and expressed in cm²·g⁻¹ (Cardinale et al., 2012). The leaf biomass was considered as leaf dry weight. The leaf C and leaf N were determined with the elemental analyser (Vario MAX, Vario MACRO, Germany Elementar) after the leaf samples were dried in the oven for 72 hours at 65^oC, ground, and then sieved through 60 mesh sieves. Leaf P was analysed with ICP-OES (Inductively Coupled Plasma Optical Emission Spectrometer, Optimal 7000DV, PerkinElmer USA). The measurement of stem volume, SST, and TDMC followed the protocol by Cornelissen et al. (2003).

    Analysis of sediment properties

  Total C, total sulphur (S), and total N were determined by the elemental analyser (Vario MAX, Vario MACRO, Germany Elementar) after the sediment samples were ground through 60 mesh sieves, weighed 200 mg in white ceramic tubes. For total P, digestion was performed with nitric and perchloric acid; quantification was performed with ICP-OES (Inductively Coupled Plasma Optical Emission Spectrometer, Optimal 7000DV, PerkinElmer USA). Active pH was determined using deionized water (1: 2). Then, EC (electricity conductance) was determined using deionized water (1: 5) with electrical conductivity meter and then converted to commonly used salinity unit ‰.