## 4 Discussion  
### Similarities between Spruce and Chinese Pine  
Both spruce and Chinese pine were closely related to canopy activity from late February to early March. Previous research on GPP and tree rings have rarely been reported, which cannot support our findings. However, some studies have been conducted on tree rings and NDVI around our study area. For example, [PERSON] et al., 2021 found a high correlation between radial growth of spruce in Bayinbruck and NDVI from June to August [6]. In Qinling, Shaanxi Province, [PERSON] et al., 2021 found a high correlation between radial growth of Chinese pine and NDVI from May to July [33]. In the Huanglong Mountains, [PERSON] et al., 2017 found a high correlation between radial growth of Chinese pine and NDVI from June to July [34]. [PERSON] et al., 2020 found a high correlation between radial growth of Chinese pine in the Helan Mountains and NDVI from May to July [35]. Compared with our results, it can be found that there is a significant correlation between tree rings and GPP in the early growing seasons and even before the growing seasons, while the significant correlation between tree rings and NDVI were found in the mid-growing season (May to August). This may be due to the allocation strategy of organic matter produced by the canopy among different organs. This also indicates that, compared to studying the relationship between tree rings and NDVI, revealing the relationship between tree rings and GPP directly has more physiological significance and can lead to some new conclusions.  
The relationship between tree rings and climate also shows that water supply during this period (late February to early March) is key to tree growth, which is consistent with the results of tree-ring research conducted around areas [36; 37; 38; 39; 40]. This is because, in arid areas, adequate water supply in the early growth season indicates that roots can access water more easily which can ensure more water use in the canopy and increase canopy activity, thus promoting photosynthesis and increasing the accumulation of nutrients, which ultimately facilitates the division of cambium cells and leads to the widening of tree rings [41; 42]. This phenomenon has been widely reported in arid or semi-arid areas. Studies in the Bayinbuluke region of Xinjiang have found a significant correlation between radial growth of spruce and water [6]. [PERSON] et al., 2021 found a significant correlation between radial growth of spruce and water in the Tianshan Mountains [43]. [PERSON] et al., 2022 found a significant correlation between radial growth of spruce and water in the Qilian Mountains [44]. [PERSON] et al., 2020 found a significant correlation between radial growth of spruce and water in low altitude areas of the middle Qilian Mountains [45]. Studies in Arizona, USA, have found a significant correlation between radial growth of ponderosa pine and water [46]. Studies in the Mediterranean region have found a significant correlation between radial growth of trees and moisture conditions [47; 48]. Studies in Spain have found a significant correlation between radial growth and water availability [49].  
Figure 7: Correlation of residual chronology of Chinese pine tree rings with meteorological data. (Max-T, mean monthly maximum temperature; Min-T, mean monthly minimum temperature; Mean-T, mean monthly mean temperature; SPEI, mean monthly standardized precipitation evapotranspiration index; RH, mean monthly relative humidity; SD, mean monthly sunshine duration; Pre, mean monthly precipitation) (\({}^{\star}\), \({}^{\star\star}\) and \({}^{\star\star}\) indicate significance at 0.1, 0.05 and 0.01 level, respectively).