**3. Results**

This study explained spatial patterns of ecosystem CUE at different temporal scales in a semi-humid and semi-arid transitional area. We identified that the variations of CUE in SNP were obvious at both seasonal and monthly scales. The CUE of GRA in the southwest and DBF in the east showed an upward trend. Monthly and seasonal CUE varied with ecosystem types. The earlier SOS, later EOS

and longer LOS might encourage higher CUE. Spatially, CUE changes were positively correlated with precipitation and temperature in most of the SNP.

#### *3.1. Monthly Change of CUE*

The CUE value of natural ecosystems in the SNP started from May and continued to November, the CUE changed significantly within the year (Figure 3). The lowest CUE values occurred in May. After July, CUE increased, and exceeded 0.8. The highest CUE (over 0.9) values were mainly observed in October. The growing season started in May in SNP with low NPP and carbon sequestration capacity. In contrast, the proportion of GPP increased in October after self-consumption through the growing season. The CUE of the natural ecosystems was higher after July, along with the accumulation of more NPP, which meant that the natural ecosystem protection capacity may be stronger.

There were three abnormally low CUE values in the SNP during the past 15 years (Figure 3). These were November 2002, May 2012 and October 2014. According to field-based meteorological measurements, the average temperature in November 2002 was the lowest in 15 years, which might be the reason that led to decrease in both GPP and NPP. In May 2012 and October 2014, the low CUE values might be associated with lower GPP and NPP due to reduced rainfall in those months.

The monthly CUE varied among different ecosystems in the SNP (Figure 4). Except for May and October, the CUE of DBF was generally lower than the regional monthly average, while the GRA was the opposite. GRA started with a green-up in May and gradually entered a senescence period in October. GRA might produce more net productivity in those two months. The CUE of MF was always higher than the regional CUE average. WET CUE in May, June and August were greater than the CUE mean value. WET CUE was the highest in August, indicating that the proportion of GPP kept by WET ecosystems after self-consumption was the greatest.

The CUE of MF was the highest from May to November except for August. Compared to other ecosystems, MF may have stronger ecological protection effects. Due to higher temperature in August, CUE of GRA may be restricted, while the area covered by humid MF could produce a higher CUE. From June to August, CUE of DBF was the lowest, because the accumulated NPP was relatively lower than other types.

**Figure 4.** Monthly mean CUE of different ecosystems from 2001 to 2015.

#### *3.2. Seasonal Changes in CUE*

Considering the dormancy of vegetation in the SNP in the winter months from December to the following February, we calculated CUE in spring (March to May), summer (June to August) and autumn (September to November), respectively. Figure 5 shows the seasonal variation of ecosystem CUE. The regional average CUE was 0.236, 0.835 and 0.854 in spring, summer and autumn, respectively. The highest and lowest CUEs in spring were observed in the year of 2013 (0.299) and 2012 (0.075), respectively. The lowest value of CUE in spring 2012 may be due to the drought of that year [26]. The maximum summer CUE (0.916) was observed in 2004, whereas the minimum value (0.75) occurred in 2011. CUE in autumn reached the peak (0.974) in 2011, while the lowest value was found in 2014 (0.686).

**Figure 5.** Average CUE variations in spring, summer and autumn from 2001 to 2015.

In most years, average CUE was the lowest in spring. The average CUE values in summer of 2002, 2009, and 2014 were greater than those in autumn, which was related to successive drought from summer to autumn. It was found that the degree of CUE decrease depends not only on the intensity of the drought, but also the duration of the drought intensity and the time of occurrence [27].

Spatially, spring CUE in the southwest of the SNP was higher than the east during the 15 years (Figure 6). It ranged from 0 to 0.4, with an average value of 0.24 (Figure 6a). In summer, vegetation grew vigorously, and the carbon sequestration capacity of vegetation increased (Figure 6b). Similar spatial distribution pattern was observed in summer with an average CUE of 0.83. With the arrival of autumn, the CUE in most regions also increased and ranged from 0.8 to 1.0 with an average value of 0.88 (Figure 6c). In summer and autumn, the carbon sequestration capacity of natural ecosystems was better, indicating the relatively stronger ecological protection function.

**Figure 6.** Spatial distribution of seasonal average CUE of the SNP from 2001 to 2015. (**a**) Spring; (**b**) summer; (**c**) autumn.

In terms of spatial distribution, the pixels showing an upward trend in three seasons were mainly found in grasslands in the southwest and deciduous broadleaf forest in the eastern fringe. According to the slope analysis, about two-thirds of the study area showed an upward trend of CUE in spring (Figure 7a). CUE in summer tended to increase in 53.7% of the study area (Figure 7b), while the CUE showed increasing trend in the area of 56.7% in autumn (Figure 7c). This increasing trend suggested that the carbon sequestration capacity of natural ecosystem in the SNP could be improving. More NPP accumulated in natural ecosystem may make their ecological protection function stronger. The change trends of CUE (over 90% pixels) passed the significance level test at *p* < 0.05.

**Figure 7.** Spatial trend of average CUE in each season of SNP from 2001 to 2015. (**a**) Spring; (**b**) summer; (**c**) autumn.

The seasonal changes of CUE for different ecosystems CUE were also obviously changing in the SNP (Figure 8). In spring, all types of vegetation had low CUE values (Figure 8a). Relatively good hydrothermal conditions in summer were more favorable to vegetation growth and the CUE of each vegetation type was generally increasing (Figure 8b). In autumn, CUE of MF, DBF and GRA continued to rise, whereas WET CUE declined slightly (Figure 8c). Among different ecosystems, CUE of MF had been the highest (spring: 0.288; summer: 0.902; autumn: 0.928).

**Figure 8.** CUE of different ecosystems and seasons from 2001 to 2015. (**a**) Spring; (**b**) summer; (**c**) autumn.

#### *3.3. The Mean Spatial Distribution of LSP*

Spatial distributions of LSP parameters, i.e., SOS, LOS and EOS, in the SNP from 2001 to 2015 are illustrated in Figure 9. The SOS of the natural ecosystem mainly occurred at day of year (DOY) between 100 and 150. The earliest SOS was found in the eastern parts of the SNP region, while the southwestern region had the latest SOS (Figure 9a). The growing season of DBF and MF started from mid-March, and GRA and WET had later start of the growing season (early April). SOS began in March and April, the vegetation began to accumulate GPP, but the CUE value was in a very small range, almost neglected, so we began to record CUE from May.

The distribution of EOS dates showed similar pattern to that of SOS, gradually increasing from west to east, mainly in late October and November (290–330 DOY) (Figure 9b). The end dates of the growing season of DBF and MF occurred in early November. GRA and WET ended their growing seasons about ten days earlier than the forestland. During 2001–2015, the average LOS of natural ecosystems in the SNP was about 192 days, showing similar spatial distribution to SOS and EOS (Figure 9c). The average LOS of MF and GRA was 213 days and 176 days, respectively. LOS dates of DBF were about 5 days shorter than those of MF, and the growing season of WET was about 4 days longer than that of GRA.

**Figure 9.** Spatial distribution of land surface phenology metrics in the SNP from 2001 to 2015: (**a**) average SOS; (**b**) average EOS; (**c**) average LOS.

#### *3.4. Response of CUE to LSP Variation*

After analyzing the correlation between CUE and LSP in the growing seasons, it was found that CUE was negatively correlated with SOS in about 70% of the study area (Figures 10a and 11). This indicated that earlier SOS would encourage higher CUE. In 72% of areas covered by GRA, CUE was negatively correlated with SOS. In 67% of the SNP, the later EOS would result in higher CUE. In 80% of areas covered by DBF, late EOS dates might have the positive effect on the increase of CUE (Figure 10b). CUE was positively correlated with LOS in more than 70% of areas covered by DBF, GRA and WET. The average CUE of MF with the longest growing season was highest (0.529). GRA with the second longest growing season (0.482). Although the LOS of GRA was the shortest, its average CUE (0.482) was greater than that of DBF (0.477). This would be because GRA in cold and dry regions consumed less energy to maintain growth. The area proportions of correlation coefficients after significant test for all the pixels were obtained (Figure 11).

**Figure 10.** Spatial distribution of correlation coefficients (R) between CUE, SOS, EOS and LOS in the SNP during 2001–2015. (**a**) CUE and SOS; (**b**) CUE and EOS; (**c**) CUE and LOS.

**Figure 11.** The area percentages of correlation coefficients between CUE, SOS, EOS and LOS. (Significant Positive Correlation (*r* > 0, *p* < 0.05): No Significant Positive Correlation (*r* > 0, *p* > 0.05), No Significant Negative Correlation (*r* < 0, *p* > 0.05), Significant Negative Correlation (*r* < 0, *p* < 0.05)).

#### *3.5. Direct E*ff*ects of Local Climate Factors on CUE Change*

This study revealed that a partial correlation existed between mean temperature and total precipitation and CUE in the growing season. CUE was negatively correlated with precipitation accounting for about 46.8% of the total pixels (Figure 12a). Among those, 0.98% had significant negative correlation, mainly distributed in the eastern and southwestern fringe areas of SNP. The area showing positive correlation between CUE of DBF and precipitation occupied 61.8% of the total area. About 60% of CUE values of GRA and WET were positively related to precipitation. CUE was positively affected by temperature in more than 90% of the region, of which 14.85% showed a significant positive correlation. Only in the northern and southern margins, CUE decreased with increasing temperature (Figure 12b).

**Figure 12.** Partial correlation coefficients between CUE and major climatic factors in the growing season: (**a**) precipitation; (**b**) temperature.

At monthly scale, the responses of ecosystem CUE to climate drivers were also significantly different. Figures 13 and 14 showed the spatial pattern of correlation coefficients between monthly CUE, precipitation and temperature from 2001 to 2015. Overall, the pixels with a positive correlation coefficient took up higher area proportions of the study area. Except for November, increased precipitation could contribute to higher CUE for the corresponding months in more than 60% of naturally vegetated area in the SNP (Figure 13 and Table 2). From June to August, CUE in more than 50% of pixels in the natural ecosystem had a positive correlation with temperature. On the other hand, as temperature increased, plant ecosystem might suffer higher ecosystem respiration cost and lower net productivity. In May and September, the pixels showing negative correlation coefficient between temperature and CUE occupied most of the SNP (Figure 14 and Table 3).

**Figure 14.** Correlation coefficients between CUE and temperature at monthly scale during 2001–2015.


**Table 2.** The number of pixels and their proportions of correlation coefficients between monthly precipitation and CUE.


