## 4 Discussion
### Reasons for the Variations in Precipitation Frequency and GPP Values
#### 4.1.3 Relationship between GPP Values and Precipitation Frequency  
Seasonal precipitation in environments where rainfall is scarce and varies in intensity and frequency may have major effects on plant physiological and morphological traits, regulating vegetation responses and community interactions [66]. The highest vegetation growth rate is generally observed between June and September in the Northern Hemisphere, when roots are more able to absorb minerals from soil water. However, the rapid warming of the pan-Arctic has led to the melting of sea ice and increased precipitation. Clouds,fog, and water vapor have delayed the rise in surface temperature and hindered the photosynthesis of surface vegetation [67], which may have led to the negative correlation between the frequency of pan-Arctic precipitation and vegetation GPP. This study showed that precipitation in spring and summer did not affect GPP in cropland, possibly due to human activity (such as irrigation) in the primary growth period of crops. When crops were harvested at the end of summer, GPP would not be affected by precipitation frequency. Precipitation in winter ensures the presence of enough soil water content, which hardly evaporates because of the low temperatures, facilitating crop growth the following year.  
In forests, the GPP values showed a negative correlation with the frequency of trace precipitation. This may be partially explained by that fact that heavy precipitation penetrates through the soil layer and improves the soil water potential while light precipitation reaches mainly the canopy and quickly evaporates. The same pattern was observed in grassland. Additionally, trace precipitation can infiltrate the surface soil and dissolve soil minerals and organic matter. When the rate of evaporation surpasses precipitation, soil moisture rapidly evaporates, carrying salts upward from the soil bottom. This phenomenon results in soil salinization [68; 69]. The higher the frequency of trace precipitation, the more pronounced the degree of soil salinization becomes. The tundra GPP is not highly correlated with the precipitation frequency in winter; because Arctic tundra growth is limited by low temperatures, changes in precipitation no longer impact GPP in winter.  
A time lag between GPP value and precipitation frequency was detected. Shrubs, the typical woody plants found in arid and semi-arid areas, are characterized by special water use strategies; they present different physiological and morphological features to cope with the harsh conditions and are resilient to drought [70]. The ability of shrubs to convert precipitation into phytomass can be greatly improved [71; 72]. There was a high correlation between GPP and moderate precipitation in the spring. The precipitation conditions in the spring could promote seed germination and germination in the following spring. In addition, reduced precipitation does not necessarily lead to lower vegetation productivity, as it still promotes the absorption of minerals and soil water [73], partially explaining the negative correlation between precipitation frequency and GPP value.