## 3 Results
### Spatiotemporal Pattern of Growth Asymmetric Indices
#### 3.1.2 Long-Term Trends  
Long-term trends in the three asymmetry indices were analyzed using the Mann Kendall trend test (expressed as MK tau values, Figure 5). The spatial heterogeneity of AsyL trends is still pronounced in that most northern coniferous forests show a positive trend, while some broad-leaved forests in the east, grasslands in the west, and croplands in the south display negative trends. Similar to the average annual metrics (Figure 4), spatial patterns of trends in AsyV and AsyR are also significant. A majority of cropland AsyV values have positive trends, indicating that cultivated vegetation activity during green up in the spring tends to have an advantage over senescence in the fall in the long run. Meanwhile the dominant negative trends of AsyV in coniferous forest in northern NEC indicate that vegetation activity during green up would continue to have a greater impact than senescence in autumn. AsyR has a negative trend for most of the cropland, meaning that, in the future, senescence in autumn will become slower than green up in spring. AsyR in natural vegetation has a positive trend, reflecting that the rate during autumn senescence will continue to be stronger than the green up rate in spring.  
Figure 4: Spatial distribution of multi-year averaged asymmetric metrics (**a**–**c**) and statistical distribution (**d**–**f**).  
Trends in the asymmetric metrics for each vegetation cover type demonstrate substantial variability (Figure 6). It is noteworthy that there are more vegetation cover types with negative trends than with positive trends (18 vs. 9). Prevailing negative trends in these asymmetric indices may imply that vegetation growth cover types in NEC will be stable into the future, but trends for the three asymmetric indices do not show a consistent pattern for each vegetation cover type. Positive trends are primarily seen with AsyR and AsyV. Only AsyV shows a positive trend in the grassland, steppe and cultivated (cropland) vegetation areas. AsyR shows a positive trend in meadows, alpine vegetation, broad-leaved forests, coniferous-broadleaved mixed forests, and coniferous forests. AsyL only shows a positive trend in coniferous forests. In terms of the magnitude of change, AsyV and AsyR show the largest positive and negative trends in the grassland biome. The trend for each index in shrublands is relatively small, probably due to its small, discrete coverage that is insufficiently sensitive to the coarse spatial resolution of satellite data. In addition, AsyL and AsyR show negative trends in grassland, steppe and cropland areas, whereas AsyV has a positive trend, indicating that vegetation growth during green up tends to be longer than growth during the senescence period in autumn. Overall vegetation production, however, is still controlled by the senescence period. The multi-year trend of AsyR in meadow, broad-leaved forest and coniferous broad-leaved forest is negative, while AsyL and AsyV in these cover types show an increasing trend.  
Figure 5: Spatial distribution of long-term trends of vegetation asymmetries (mk tau).  
Figure 6: Statistics of asymmetry trends for various vegetation cover areas (averaged value + one standard deviation).