Study region and sampling sites
The study site was located in the Yellow River Delta (36°55′–38°16′N, 117°31′–119°18′E) in Dongying, Shandong Province, northern China. The Yellow River Delta is one of the fastest-growing estuarine wetlands in the world (Cui et al., 2011). The mean annual precipitation in the study area is approximately 628.6 mm and the mean annual temperature is about 11.9°C (Zhou et al., 2020). The region is simultaneously influenced by river water, ground water, and seawater, and is characterized by strong temporal and spatial heterogeneity of soil water and salinity gradients (Zhou et al., 2020). To describe and characterize the patterns and processes influencing the P. australis communities, 96 quadrats (1 ×1 m) selected at random were investigated along the coastal-inland regions (37°43′–38°05′N, 118°41′–119°13′E) under different gradients of soil water content and electrical conductivity in the Yellow River Delta in 2013. Samples were chosen in terms of typical P. australis communities with little human disturbance, but with significant differences in soil water content and salinity gradient (Figure 1).
Figure 1. Distribution patterns of soil water content and electrical conductivity for 96 different samples.
Measurements of soil properties and functional traits
Soil layer samples were obtained at a depth of 0–20 cm to measure soil water content (SWC) and electrical conductivity (EC). SWC was measured by drying soil samples at 105°C to constant weight.
Soil water content (SWC) = (wet soil weight - dry soil weight) ×100% / dry weight of soil
EC was measured by taking 10 g grated dried soil sample and placing it in an Erlenmeyer flask. Water and soil were mixed in a volume ratio of 1: 2.5 and a magnetic stirrer was used to dissolve the salt for about 40 min. A conductivity meter (DDS12A, LIDA, China) was used to measure the EC in the upper suspension.
For each sample, 10 functional traits were measured including average height, number of plants, soil plant analysis development (SPAD) values, leaf water content, specific leaf area, leaf area, leaf thickness, pitch number, internode height, and shoot basal diameter. Leaf thickness was measured by a micrometer caliper at the widest point while avoiding the midrib. The base stem diameter was determined using a vernier caliper. SPAD values were determined using a chlorophyll meter (SPAD-502PLUS, Konica Minolta, Japan). The fresh mass of leaves (LFM) was weighed and then oven-dried at 80°C for 48 h, after which the dry mass of leaves (LDM) was weighed. Leaf area (LA) was analyzed using a broadleaf analysis system (WinFolia pro LM2400P, Regent) through scanning with a Canon scanner (Canon 2000). Specific leaf area was calculated using the following equation:
SLA=LA/LDM, (1)
where SLA (cm2/g) was the specific leaf area, LA was the leaf area (cm2), and LDM (g) was the dry mass of the leaf. Leaf water content was calculated using the following equation:
LWC= (LFM-LDM)/LFM=1-(LDM/LFM), (2)
where LWC (g/g) was leaf water content, LFM (g) was the fresh mass of the leaf, and LDM (g) was the dry mass of the leaf.
We used LA, LDMC, and SLA to assign combinations of leaf traits to CSR strategies under different salt conditions (Pierce et al., 2017).