## 4 Discussion
### Climatic Influences on Vegetation Asymmetries  
Vegetation asymmetries are not traditional phenological parameters, but, nevertheless, they are characteristic of these parameters, and, in some regions, their interannual variability is undoubtedly influenced by climatic changes. Different asymmetric patterns may reflect adaptations of vegetation communities to specific climate conditions. In this study, a relative importance (RI) analysis suggests that precipitation plays a role equivalent to that of temperature on productivity. This finding differsslightly from other reports that state that northern temperate ecosystems are primarily influenced by temperature and solar radiation [52; 53; 54; 55]. In this case, the joint controlling effect of temperature and precipitation on variations in plant asymmetries may be attributed to their synchronous changes over time and space (see Figure 11). That joint control could be reflected in the similar spatial pattern of relative importance of AsyTemp and AsyPrcp to AsyV and AsyR (Figure 10). However, due to their low contributions, it is apparent that temperature and precipitation are not adequate constraints for asymmetry in growing season length (AsyL). For example, the inherent gene types of vegetation may limit the changing ranges of phenological events (e.g., SOS, EOS) and further affect changes in AsyL. Based on the inter-comparison of averaged annual indices, a negative AsyTemp indicates a warmer spring. The growth rate asymmetry (AsyR) is generally positive (red and purple patches in Figure 12c), while AsyV tends to be negative (brown and light blue patches in Figure 12b). The reverse is true when AsyTemp is positive. This phenomenon reflects warming springs that can rapidly stimulate plant growth, resulting in a shorter growing season during green up and further reducing average greenness. When AsyPrcp is negative, AsyR tends to be positive and AsyV tends to be negative, which accounts for a large proportion of pixels. In this situation, sufficient water is available to improve the rate of plant activity (i.e., photosynthesis) across the NEC. As with the effect of AsyTemp on AsyV, this rapid plant growth leads to a smaller mean greenness during green up than during senescence. The effects of AsyTemp and AsyPrcp on AsyL show a relatively complex pattern, especially in the southern and eastern forest, where AsyTemp and AsyPrcp with positive values likely correspond to a positive AsyL, but with a considerable number of negative values (green grid cells). The influence of warmer springs may affect subsequent seasons through the development of larger leaves or increased foliar nitrogen [53], which will result in enhanced carbon assimilation in the following summer and autumn. The temporal fluctuation of the magnitude and sign of vegetation asymmetries may affect the allocation of production during different seasons and even change ecological traits, such as mobility or reproduction, that are tightly linked to ecosystem stability and food security [22; 56]. Shifts in vegetation growth asymmetry are driven predominantly by climate change and variability, yet these shifts will also have an impact on future climate change through feedbacks to the atmospheric system. It is noteworthy that crop growth cycles may be uniquely independent of a direct link the influence of natural forces. A variety of human-induced activities, such as planting time, irrigation and harvest time, can lead to different growth pattern for specific crops. Variability and changes in vegetation growth asymmetries for these crops could change land surface properties, such as LAI and albedo, resulting in local climatic changes [57]. The changing vegetation growth asymmetries may also affect carbon uptake process and further enhance seasonal CO\({}_{2}\) exchange in northern ecosystems [58]  
Figure 12: Spatial patterns of the influence of temperature and precipitation on vegetation asymmetries. This is an inter-comparison between average annual asymmetries of AsyL (AsyV, AsyR), AsyTemp and AsyPrcp.