Ecological constraints on cushion population dynamics
To assess effects of temperature, light and water availability on seed germination and subsequent seedling survival, we carried out experiments in three artificial climate chambers with temperatures of 0/5, 10/15 and 20/25 oC in 12 h night (dark)/12 h day (light) cycles. Seeds were collected from the populations in Pujin pasture in late September in 2020 and were mixed since they had no differences in term of seed quality (Chen et al., 2020a). See Text S7 for the details.
Since extreme climate events frequently occur in the study region (Wang, 2006; Ning et al., 2012), a further aim was to elucidate their effects on establishment of the cushion species’ seedlings. For this, we set two kinds of extreme climate events. They were short-term events which were considered to simulate the frequent extreme climate events in the plant growing season, and long-term events which was designed to test and compare the over-wintering capacity of seedlings that had, and had not, experienced climate events during the growing season. Text S8 for the details.
Since allelopathy is one of the important biotic mechanisms that affect seed germination and seedling performance and hence community dynamics (Fenner & Thompson, 2005; Zhang et al., 2015; Pilsbacher et al., 2021). To detect whether allelopathy exists and (if yes) how it affects cushion seed germination and seedling performance, we collected potentially allelopathic materials from PJ1 and PJ3 sites and tested the effects of them on seed germination and seedling survival. See Text S9 for the details.
Additionally, we also assessed seeds’ and seedlings’ performance in the real natural conditions by subjecting them to burial and transplantation treatments in the Pujin populations. See Text S10 for the details.
To assess potential effects of beneficiary plants on the process of cushion population degeneration, we first selected the PJ2 population to test effects of beneficiary plants on the nutrient contents of A. polytrichoides cushions in mid-August 2020. Then, we assessed the physiological status of A. polytrichoides cushions along an elevational/climate gradient and associated series of community successional stages. Specifically, we determined the specific leaf area (SLA, the ratio of fresh leaf area to leaf dry mass in m2 kg-1) and leaf dry matter content (LDMC, the ratio of leaf dry mass to fully rehydrated fresh mass in g kg-1) in three populations in Pujin pasture. See Text S11 for the details.
Finally, a further aim was to elucidate effects of beneficiary plants on cushions’ reproductive outputs. See Text S12 for the details.