3 Results
3.1 Effects of warming and N addition on the plant flowering phenology and community dynamics
Warming advanced the flowering time of C3 plants by 4.3 ± 0.1 days (P = 0.01) and of C4 plants by 2.8 ± 0.1 days, respectively (P = 0.01; Table1; Figure1). In five of six species, we observed earlier flowering times under warming conditions. Specifically, we observed advances of 5.6 ± 0.1 days forS. breviflora (P = 0.03), 4.5 ± 0.2 days for A. frigida (P = 0.001), 3.3 ± 0.1 days for C. ammannii(P = 0.01), 3.7 ± 0.1 days for A. tenuissimum (P=0.01) and 3.6 days for C. songorica (P = 0.02) , (TableS1; FigureS2). Warming had no effect on the flowering time of K. prostrata . In contrast, while nitrogen addition advanced the flowering time ofS. breviflora by 5.6 ± 0.1 days (P= 0.04 ; TableS1; FigureS2), it did not alter that of other studied species.
Warming reduced the duration of flowering of C3 plants by 1.8 ± 0.1 days (P = 0.001), and nitrogen addition extended the duration of flowering of C4 plants by 3.4 ± 0.2 days (P = 0.03), (Table1; Figure1). Warming significantly shortened the duration of flowering by 3.0 ± 0.6 days for S. breviflora(P = 0.01), 2.3 ± 0.2 days for C. ammannii(P = 0.04), 2.1 ± 0.1 days for A. tenuissimum (P = 0.01 ); (Table S1; Figure S3). However, it had no impact on that of other studied species. Nitrogen addition prolonged the duration of flowering by 3.9 ± 1.0 days for C. songorica (P = 0.04) and 3.0 ± 0.3 days for K. prostrata (P = 0.02), (TableS1; FigureS3), but did not influence the duration of flowering of other studied species.
The dominance of C3 plant functional group decreased by 4.1% ± 0.3% (P = 0.02) under warming and 6.0% ± 0.06% (P = 0.04) under N addition, respectively (Table1; Figure 1). On the contrary, the dominance of C4 plants, C. ammanniiand K. prostrata , increased by 5.0% ± 0.4% (P = 0.02) under warming and 4.1% ± 0.01% (P = 0.01) under N addition, respectively (Table1; Figure 1). Combined warming and N addition led to an increase of 7.0% ± 0.5% (P = 0.003) in C4plants and a decrease of 6.0% ± 0.01% (P = 0.03) in C3 plants (Table1; Figure 1). The effects of warming and N addition on plant dominance varied between years (P <0.05;Table S1; Figure S4).
3.2 Effects of warming and N addition onenvironmental factors
Experimental warming increased soil temperature by 1.0 ± 0.1℃ at 0-10 cm depth (P = 0.002), 1.2 ± 0.2℃ at 10-20 cm depth (P = 0.01) and 1.4 ± 0.1℃ at 20-30 cm depth (P = 0.0003; Table 2; Figure 2). Combined warming and N addition led to an increase of soil temperature by 1.0 ± 0.2℃ at 0-10cm depth, 0.9 ± 0.2℃ at 10-20 cm depth, and 1.2± 0.1℃ at 20-30 cm depth (P <0.05; Table 2; Figure 2).
Warming did not alter soil moisture at any depth (Table 2; Figure 2). Nitrogen addition significantly increased soil moisture by 4.2% at 10-20 cm depth (P = 0.02) and 2.3% at the 20-30 cm depth (P = 0.03; Table2; Figure2), but did not affect soil moisture at 0-10 cm depth. Combined warming and N addition significantly enhanced soil moisture by 6.1% at 10-20cm depth (P = 0.03) and 3.2% at 20-30cm depth (P = 0.04; Table2; Figure 2).