Experimental design and measurement

Based on the field observation, most seeds of H. ammodendron germinated after rain from April to June. Thus, climate data of four seed sources were analysed to determine experiment conditions. On average, there were 2.00 to 3.71 times rainfall events in the four seed sources in April, 1.71 to 5.00 times in May, and 4.00 to 5.00 times in June (Table 2). Thus, four rain frequencies (10, 6, 3 or 2 times per month) were applied in our experiment, considering historical rainfall data and the decrease of rain frequency resulting from climate change in the future.

Rain frequency (times per month) (mean ± SE, range) during the germination season in the four deserts in Northwest China.

Seed germination tests were conducted from 12 April to 13 May in 2020 in a non-heated greenhouse at the Chinese Academy of Forestry. The substrate for seed germination was river sand passed through a sieve with the diameter of 2 mm. Plastic pots (15.6 cm diameter × 13.2 cm height) were filled with sand to within 1 cm of the top, and 25 seeds were planted uniformly with forceps at 1 cm depth in each pot because the highest seed germination percentage of H. ammodendron was obtained at 1 cm depth (Wang and Zhao 2015). There were four pots (replicates) for each treatment, totalling 64 pots in this experiment (4 rain frequencies × 4 seed sources × 4 replicates). A seedling was considered to be emerged when its first foliage leaf was 5 mm above sand surface. Daily air temperatures varied from 14 °C to 31 °C in the greenhouse during the experiment with the mean minimal value of 20.1 °C and the mean maximal value of 28.1 °C.

There were four watering frequencies (10, 6, 3 or 2 times per month) for H. ammodendron seeds from the four deserts with a monthly total rain of 50 mm, which was equivalent to 88, 147, 293 and 440 mL water each time, respectively. Seedling emergence was recorded daily for each pot, and the experiment was terminated after 30 days, at which time no seedlings had emerged for at least 5 consecutive days. All sand in each pot was passed through a sieve with 2 mm diameter to search for germinated but non-emerged seedlings, and then seed germination percentage was calculated for each treatment.

From April to June, maximum daily temperature in the four deserts varied from 17.62 °C to 21.01 °C, from 22.91 °C to 26.82 °C and from 26.90 °C to 32.44 °C; meanwhile, minimum daily temperature varied from 4.12 °C to 8.51 °C, from 9.33 °C to 14.11 °C and from 15.19 °C to 19.37 °C (Table 3). Thus, based on the air temperature data in the germination season, four constant temperatures (10 °C, 15 °C, 20 °C and 25 °C) and four alternating temperatures (20/10 °C, 25/10 °C, 25/15 °C and 30/15 °C) were used in our experiment.

The air temperature (°C) during the germination season in the four deserts, Northwest China.

T _max: maximal daily temperature; T_min: minimal daily temperature.

Seeds of H. ammodendron from the four deserts were tested for germination in the incubators in light (fluorescent tube with light intensity of 50 μmol m⁻² s⁻¹) at the Plant Ecophysiology Laboratory in Chinese Academy of Forestry. The test using four constant temperatures was conducted from 22 April to 7 May 2020; and the one using four alternating temperatures was conducted from 11 June to 26 June 2020. The period for high and low temperature was 12 h and 12 h in the alternating temperature treatments, respectively. Twenty-five seeds were uniformly placed on two layers of Whatman No. 1 filter paper and 2 mL distilled water in a 9 cm diameter plastic Petri dish using forceps. There were four Petri dishes (replicates) for each treatment and a total of 64 dishes in each experiments (4 temperatures × 4 seed sources × 4 replicates). A seed was considered to be germinated when the radicle appeared (1 mm) from the seed coat. Germination was examined daily, distilled water was added when necessary and the germinated seeds were removed from the Petri dish. The seed germination experiment was terminated after 15 days, at which time no seeds had germinated for at least 5 consecutive days.