Impact of climate warming on planting suitability of alfalfa with different fall dormancy grades in Xinjiang

Using the data of daily mean temperature, the minimum temperature from 102 meteorological stations in Xinjiang during 1961–2019, based on statistical method and ArcGIS, the fundamental spatial-temporal change characteristic of key climatic factors affecting alfalfa planting, such as ≥ 5 ℃ accumulated temperature (\sum T_{mean ≥ 5 ℃}), and the minimum temperature in winter (T_min) were analyzed. Taking 80% guarantee rate ≥ 5 ℃ accumulated temperature (\sum T_{mean ≥ 5 ℃} of 80% guarantee rate) and 90% guarantee rate minimum temperature (T_min of 90% guarantee rate) as the index factors, the climate suitability changes of different fall dormancy grades in alfalfa planting were studied under the background of climate warming in Xinjiang. The results showed that spatial distribution of \sum T_{mean ≥ 5 ℃} of 80% guarantee rate and T_min of 90% guarantee rate had very obvious differences from place to place in Xinjiang. Generally, they were more in the south than in the north and more in the plain than in the mountainous regions. Under their comprehensive influence, the suitable planting areas for alfalfa off all dormancy grades 4 ~ 6 was mainly in the Tarim, Turpan, and Hami Basins. The suitable areas for fall dormancy grades 2 ~ 3 was in the sloping plains around the Tarim Basin, Turpan Basin, Hami Basin, the Yili Valley, and the low plains in the west and east of the Junggar Basin. Areas for fall dormancy grades 1 ~ 2 was in the north and west of northern Xinjiang, the hinterland of Junggar Basin, and the middle and low mountain belts of Tianshan mountain and Kunlun mountains. Alfalfa was not suitable for planting in the Altai, Tianshan, and Kunlun mountains. The dominant factors affecting the distribution of suitable planting areas for alfalfa of different fall dormancy grades were different. Generally, the suitable planting areas of fall dormancy grades 4 ~ 6 and 2 ~ 3 were mainly affected by \sum T_{mean ≥ 5 ℃} of 80% guarantee rate, but the areas of suitable planting for alfalfa of fall dormancy grades 1 ~ 2 and unsuitable for alfalfa planting were mainly affected by T_min of 90% guarantee rate. In the background of global warming, \sum T_{mean ≥ 5 ℃} and T_min significantly (P < 0.001) increased at the rate of 65.52 (℃·d)·10 a⁻¹ and 0.63 ℃·10 a⁻¹ respectively, during 1961–2019 in Xinjiang. \sum T_{mean ≥ 5 ℃} of 80% guarantee rate and T_min of 90% guarantee rate were increased too, but the changes were different in different periods during 1961–2019. During period Ⅰ (1961–1980), \sum T_{mean ≥ 5 ℃} of 80% guarantee rate and T_min of 90% guarantee rate were the lowest in nearly 59 years. In period Ⅱ (1981–2000), the T_min of 90% guarantee rate was 2.9 ℃ higher than that of period Ⅰ, but the \sum T_{mean ≥ 5 ℃} of 80% guarantee rate had no obvious change. In period Ⅲ (2001–2019), the T_min of 90% guarantee rate was 0.8 ℃ lower than that of period Ⅱ while the \sum T_{mean ≥ 5 ℃} of 80% guarantee rate increased by 147.0 ℃·d. Under their influence, the suitable planting areas for 4 ~ 6 and 2 ~ 3 grades of fall dormancy alfalfa showed an expanding trend, whereas the suitable planting areas of 1 ~ 2 grades and the unsuitable planting areas of alfalfa showed a decreasing trend in Xinjiang. These results showed that climate warming was beneficial to the development of alfalfa planting in Xinjiang.