## 2 Study Area, Data and Methods
### Definition of Phenological Asymmetry Indices  
In the context of global warming, the seasonal cycle of vegetation growth has shown a pronounced shift in both spatial and temporal patterns in northern terrestrial ecosystems. Here, given vegetation growth for a single growing season in the NEC, three growth asymmetry indices were defined based on the difference between key phenological characteristics for the green up and senescence phases. The green up phase corresponds to the first half of the growing season spanning spring to early summer, followed by the second phase, senescence, spanning early summer to late autumn. The two phases were separated by the position of peak growth (Figure 2). Indices consisted of asymmetries for growing season length (AsyL), green up rate and senescence rate (AsyR) and average NDVI (AsyV) for the two stages of growth. Owing to the distinct sign of these parameters (positive RSP and negative RAU), each asymmetry index was calculated as the difference of absolute values for growth parameters in spring and autumn (Equation (3)). LSP and LAU represent growing season length in spring and autumn, respectively. Similarly, MSP and MAU correspond to the mean value of vegetation greenness index (NDVI) during green up (predominantly spring) and senescence (predominantly autumn). RSP  
Figure 2: Conceptual illustration of vegetation growth asymmetries (**a**) symmetrical type; (**b**) rapid in and slow out; (**c**) slow in and rapid out. and RAU indicate the maximum growth rates during green up and maximum declining rate during senescence, respectively. As a result, the sign of the asymmetry index, either positive or negative, can be an indicator of skewness in vegetation growth morphology during spring and autumn. Meanwhile, the magnitude of the asymmetry index represents different levels of vegetation activity during the two stages. \[ \begin{split}\text{AsyL}&=|\text{LAU}|-|\text{ LSP}|\\ \text{AsyR}&=|\text{RAU}|-|\text{RSP}|\\ \text{AsyV}&=|\text{MAU}|-|\text{MSP}|\end{split}  \tag{3}\]  
Based on the skewness of plant growth asymmetry indices and the condition of vegetation for a single growing season across NEC, plant growth asymmetric features in one year are classified into three common forms: (1) symmetrical pattern for green up and senescence phases (weak asymmetry, Figure 2a); (2) 'rapid growing and slow declining' pattern (Figure 2b), reflecting the rapid growth in spring and slow senescence in autumn; (3)'slow growing and rapid declining' pattern (Figure 2c), indicating slowly gradual green up and rapid senescence. The symmetrical growth pattern is reflected in small values of asymmetric indices. In other words, a smaller value of an asymmetric index represents a more pronounced symmetrical pattern in vegetation growth. Given a relatively fixed growing season controlled by internal plant mechanisms for the type 2 growth cycle, the AsyL index will show a positive value due to the earlier arrival of peak growth. In contrast, the AsyL index has a negative value. The signs of AsyV and AsyR cannot be easily determined without additional information. Their long-term trends and variability were investigated at pixel and vegetation cover regional levels as described in the following section. These growth types may not only be attributed to the genotype of plants but may also be influenced by environmental factors such as temperature, precipitation, and soil water stress. On one hand, using these asymmetric indices, the inherent property of vegetation growth rhythm could help to identify different vegetation cover types. On the other hand, the variability of changing climate can alter the distinct growth status in spring and autumn. Optimal combinations of natural factors will enhance vegetation activity; alternately, less desirable environmental conditions disturb the regular pace of vegetation growth. Changes in natural factors, such as extreme events in rainfall or temperature, may occur at different times during the growing season.