System establishment and material processing
In December 2014, seedlings of the six mangrove species were brought back to the greenhouse of the Agricultural Base of Hainan University for simulation of control experiments (hereinafter referred to as control experiments) targeting the three factors of light, salinity, and flooding. Based on field data and previous studies (Liao et al., 2009; Shiau, 2017; Christian, 2005; You et al., 2015), the light intensity, seawater salinity, and flooding time treatment parameters were set as follows:
(1) Treatment settings
Light intensity (G) treatments: G1 (0% shading), G2 (20% shading), G3 (40% shading), and G4 (80% shading).
Seawater salinity (S) treatments: S1 (10‰), S2 (20‰), S3 (30‰), and S4 (40‰).
Flooding time (T) treatments: T1 (4 h/d), T2 (8 h/d), T3 (12 h/d), and T4 (16 h/d).
(2) Establishment of the control experiments:
① Light intensity control experiment: shading nets with different shading effects were used to simulate different light intensities: 3-pin shading nets for 20% shading, 4-pin shading nets for 40% shading, and 6-pin shading nets for 80% shading.
② Seawater salinity control experiment: artificial seawater with four salinity levels was prepared using tap water, sun-dried coarse salt, and Hoagland nutrient solution and was stored in corresponding storage tanks for repeated use. The salinity level was checked weekly, and coarse salt or tap water was added if needed. The sun-dried coarse salt was purchased from Sanjiang Town, Meilan District, Haikou City, Hainan Province.
③ Flooding time control experiment: a small water pump was used to simulate the half-diurnal tidal cycle (twice per day), that is, the flooding times were 2, 4, 6, and 8 h. A small water pump was used to complete the daily water injection and drainage cycle. The pump (Sensen HQB-2000 model) had 24 w power, 1.8 m head, and 1400 L/h flow rate.
(3) Material processing
Thick-walled red plastic buckets with a height of 80 cm and a diameter of 60 cm were selected as the planting containers. In the planting bucket, the red soil and river sand were used as the culture medium. Each planting bucket represented one treatment, with three seedlings of each mangrove species (total of 18 seedings) planted in one bucket. There were a total of 64 treatments involving 1,152 (64 * 18 = 1,152) seedlings in this study.