## 4 Discussion  
### Interpretation of Asymmetries for Depicting Vegetation Cycle  
Since vegetation asymmetric indices are not typical variables for characterizing seasonal difference in plant growth, it is necessary to discuss their potential and implications of their use in describing vegetation seasonal cycles. These asymmetric metrics are designed to measure the degree of skewness of the vegetation growth process that typically represents a single growing season. Here, the asymmetry of the vegetation growth curve is expressed by the difference in growth rate (AsyR), growing season length (AsyL), and vegetation activity (i.e., greenness; AsyV) before and after the peak of growing season. The rising segment of the growth curve corresponds to the vegetative green up process. Thus, the green up rate captures information about plant activity, such as leaf development or increasing photosynthesis. In contrast, the declining rate in autumn reflects vegetation browning or senescence.  
Figure 11: Spatial patterns of relative contribution of asymmetries in temperature (the first column) and precipitation (the second column) to vegetation growth asymmetries (in rows) at pixel scale (white grids are insignificant). RI denotes relative importance.  
Differences between the length of active growing season during these two periods are denoted by the asymmetric index AsyL, which is tightly correlated to the start of season (SOS), peak of growing season, and end of season (EOS). Thus, AsyL reflects the dynamics of phenological events, or differences in timing of carbon uptake [45]. Furthermore, given the relatively fixed lifespans of leaves in northern ecosystems [46], the position of peak vegetation activity plays a key role in controlling the asymmetry of vegetation seasonality, which determines the allocation of plant production to green up period and senescence period. Furthermore, the inherent characteristic (e.g., vegetation gene type) of terrestrial biosphere may be a more dominant driver of growing season asymmetry [18]. For AsyV, it is the difference between averaged greenness in the first- and second-half of growing season, so AsyV could reflect the capacity of vegetation photosynthesis or production.  
By constructing these asymmetric indices, information about annual cycles of vegetation development is retrieved. In general, the indices serve as a set of advanced phenological traits for north ecosystem. [PERSON] et al. suggested that plant photosynthetic seasonal cycles could be seen as an extension of plant phenology. Potential uses of these asymmetric indices include examining regulation of phenophase in land surface productivity models and monitoring vegetation response to interannual climatic variability. In the long run, changes in the seasonal cycle of plants and the asymmetrical characteristics of the growing season have many consequences for ecological processes, agriculture, and forestry. For example, thriving plants in spring are able to provide adequate food for birds and insects thus will enhance ecosystem biodiversity, but sparse plants in spring will affect the livelihood of various animals. Furthermore, vegetation status can affect land surface albedo, thus AsyV may influence the temporal distribution of energy between spring and autumn. In this study, long-term AsyV and AsyR show a pronounced cycle that reflects variability in vegetation photosynthesis, which further reveals that there is not a constant trend in which the green up or senescent phase consistently dominates plant production. The fluctuation of vegetation asymmetries also represents the adaptability of plants to environmental changes.