Phenology is a key indicator of climate change, and phenological change is the consequence of climate change (Richardson et al., 2013). We found that the annual average temperature was increased significantly over the past 58years, which directly led to the germination of P. euphratica was advanced, and the leaf yellowing delayed. Changes in phenology is an important trait of plant which determine the length of growth period. The growth season of P. euphratica was significantly prolonged after 1990 due to the timing of the first dates of germination advanced and the last dates of leaf fall delayed. This is consistent with the studies that advanced germination and delayed leaf fall by climate warming extended the growing season (Linderholm 2006; Chung et al., 2013; Prevéy et al., 2019). Moreover, a growing number of studies has indicated that germination is becoming earlier throughout the temperate regions and caused a great impact on plant (Root et al. 2003, Wolkovich et al. 2012, Prevéy et al. 2017). However, some studies found that although spring phenology advanced, the effect is maybe counteracted by decreased chilling (Ma et al., 2018).
The dominant factor affecting the phenological change still remain debate. Several studies found that temperature is a major environmental factor affecting phenological change (Xie et al., 2015). This is in consist with the results of our study. The field experiments and satellite data also proved that temperature played decisive role in phenological change (Fu et al., 2018). Besides temperature, precipitation has also been reported to effect phenological change (Liu et al., 2016). Some studies found that precipitation was a more dominant factor than temperature on phenology (Ren et al., 2017). Such as precipitation reduce may advance deciduous and delay spring phenology (Misson et al., 2011). Moreover, previous studies found that although the decrease of precipitation leads to the earlier spring phenology (Shinoda et al., 2007), the increase of precipitation had no effect on phenology (Sherry et al., 2007). There also studies show that precipitation has a weaker effect on the change of phenology (Chang,2018). Several studies also found phenology was not changed with the decrease or increase of precipitation (Morin et al. 2010). The reason may be that precipitation not reach the threshold for changing phenology. Other research has shown that temperature and precipitation all effect flowering time (Rafferty et al., 2019). In addition to non-biological, biological factors also affect phenological change (Zu et al., 2018). For example, research has found that grazing would delay the growth phenology (Li et al., 2018).
Phenological changes have an important effect on frost days. Our study showed that the frost days of P. euphratica increasing from 1990 to 2019, and the frost days was increased with the growth season length. Consistent with this, previous studies have shown that the warming-induced lengthening of the growing season may induce more frost days (Liu et al.,2018). Frost events increasing can disturb nutrient cycling (Estiarte et al., 2015), inhibit plant growth (beck et al., 2007), reduce carbon uptake (Hufkens et al., 2012), effect species composition of ecosystem (Yang et al., 2010), thus affect the function of ecosystems. Inouye et al. (2008) found that frost have a serious effect on floral abundance. Autumn frost may also result crop yield decrease in late summer (Gallinat et al., 2015). Apart from plant growth, frost events also have a great influence on economy. For example, the 2007 spring freeze in United States was caused over $2 billion in economic losses (Wolf, 2008). Thus, when assessing the effect of phenological changes, the days of frost and time of frost occurred, and the effect of frost events must not be neglected.
Phenological change also affect socioeconomic (Cleland et al., 2012), especially ornamental plants. The P. euphratica in Ejina Banner is an intoxicating wonder in the depths of the desert. It is not only a paradise for photographers, but also a paradise for countless travelers. Our results showed that P. euphratica experienced delayed leaf yellowing in the most recent twenty years (2000–2019) compared with the earliest subperiods (1960–1969). The delay of leaf yellow period will inevitably lead to the delay of the best viewing period. Thus, Tourists should reasonably arrange travel time due to National Day golden week may not be the best viewing period for P. euphratica. In addition, the leaf fall of P. euphratica delayed may be extend viewing period, which will increasing tourism revenue. Further research should accurately predict the best viewing period according to climate change, so as to provide basis for tourists' travel and government decision-making.