**3. Results**

### *3.1. Size and Storage of Litter Standing Crop in Tibetan Plateau Shrublands*

The litter standing crop ranged from 0.04 kg m<sup>−</sup><sup>2</sup> to 0.48 kg m<sup>−</sup>2. The branch standing crop ranged from 0.02 kg m<sup>−</sup><sup>2</sup> to 0.48 kg m<sup>−</sup>2, and foliage was no more than 0.25 kg m<sup>−</sup>2. The mean litter standing crop was 0.23 kg m<sup>−</sup><sup>2</sup> in the shrublands overall (Table 1), and it was 0.25 kg m<sup>−</sup><sup>2</sup> and 0.19 kg m<sup>−</sup><sup>2</sup> in desert shrublands and alpine shrublands, respectively (Table 1). The mean values for foliage and branch standing crops in the shrublands were 0.04 kg m<sup>−</sup>2, and 0.19 kg m<sup>−</sup>2, respectively (Table 1). Most litter standing crop was stored in the branch standing crop, accounting for 80% of all litter standing crop overall (Figure 2a), and 76% and 87% in alpine shrublands and desert shrublands, respectively (Figure 2b,c).

**Table 1.** Size, storage and carbon storage of foliage, branch, and litter standing crops in the shrublands of the Tibetan Plateau.


**Figure 2.** The proportions of branch and foliage standing crop in the litter standing crop on the Tibetan Plateau, (**a**) all shrublands, (**b**) alpine shrublands, (**c**) desert shrublands.

Overall, the litter standing crop storage in the shrublands was 24.83 Tg; 16.74 Tg and 8.09 Tg in desert shrublands and alpine shrublands, respectively. Storages in the foliage and branch standing crops were 4.05 Tg and 20.78 Tg, respectively (Table 1).

### *3.2. Conversion Coe*ffi*cient, Storage, and Distribution of Litter Standing Crop Carbon*

The conversion coefficient of litter standing crop to litter standing crop carbon was 0.44 in the shrublands of the Tibetan Plateau, which supported our hypothesis that the conversion coefficient was different from that in the shrublands of southern China. The conversion coefficients for foliage and branch standing crop to foliage and branch standing crop carbon were 0.39 and 0.45, respectively. Litter standing crop carbon was 10.93 Tg C in the northeastern Tibetan Plateau shrublands, and they were 7.40 Tg C and 3.53 Tg C in desert shrublands and alpine shrublands, respectively (Table 1). Furthermore, the conversion coefficients of branch standing crop to branch standing crop carbon were 0.42 and 0.46 in desert shrublands and alpine shrublands, respectively. Meanwhile, the conversion coefficients of foliage standing crop to foliage standing crop carbon were 0.27 and 0.40 in desert shrublands and alpine shrublands, respectively.

Spatially, the litter standing crop carbon was stable with increased latitude (Figure 3a), but it decreased with longitude in total shrublands (Figure 3b). Similarly, the significant trend also existed in the alpine shrublands (Figure 3c). In the desert shrublands, the range of longitude was between 94.51◦ and 98.73◦. The litter standing crop carbon has a decreasing trend with an increasing longitude in desert shrublands (Figure 3d). Specifically, the highest litter standing crop carbon was in a low longitude (94.51◦–95.91◦), while the lowest litter standing crop carbon was in high MAT (97.32◦–98.73◦). In medium MAT (95.91◦–97.32◦), the litter standing crop carbon was moderate (Figure 3d).

**Figure 3.** Relationships between litter standing crop carbon and latitude and longitude, (**a**) and (**b**) total shrublands, (**c**) alpine shrublands, (**d**) desert shrublands.

### *3.3. E*ff*ects of MAP and MAT on Litter Standing Crop Carbon*

Litter standing crop carbon significantly decreased with MAP (Figure 4a) (*p* < 0.01). In the alpine shrublands, the range of MAP was between 160.83 mm and 572.50 mm. The litter standing crop carbon showed a negative trend with MAP in alpine shrublands. Specifically, the highest litter standing crop carbon was in low MAP (160.83 mm–298.06 mm), while the lowest litter standing crop carbon in high MAP (435.28–572.50 mm), and in medium MAP (298.06 mm–435.28 mm), the litter standing crop carbon was moderate (Figure 4b). Similarly, and the range of MAP was between 35.83 mm and 265.00 mm in desert shrublands. The highest litter standing crop carbon was in low MAP (35.83 mm–112.22 mm), while the lowest litter standing crop carbon in high MAP (188.61 mm–265.00 mm), and in medium MAP (112.22 mm–188.61 mm), the litter standing crop carbon was moderate (Figure 4c).

Litter standing crop carbon increased with MAT (*p* < 0.01) (Figure 4d). In the alpine shrublands, the range of MAT was between −2.83 ◦C and 3.20 ◦C. The litter standing crop carbon showed a positive trend with MAT in alpine shrublands. Specifically, the highest litter standing crop carbon was in high MAT (1.18 ◦C–3.20 ◦C), while the lowest litter standing crop carbon in low MAT(−2.85 ◦C–−0.83 ◦C), and in medium MAT (−0.83 ◦C–1.18 ◦C), the litter standing crop carbon was moderate (Figure 4e). Similarly to desert shrublands, and the range of MAT was between −1.14 ◦C and 5.47 ◦C. The litter standing crop carbon showed a positive trend with MAT in desert shrublands. Specifically, the highest litter standing crop carbon was in high MAT (3.65 ◦C–5.47 ◦C), while the lowest litter standing crop carbon was in low MAT (−1.14 ◦C–0.42 ◦C), and in medium MAP (0.42 ◦C–3.65 ◦C), the litter standing crop carbon was moderate (Figure 4f). Generally, multiple linear regressions showed that in total shrublands, litter standing crop carbon = −0.001 MAP + 0.01 MAT + 0.25 (*r* = 0.40, *p* < 0.01).

**Figure 4.** Relationships among mean annual precipitation (MAP) and mean annual temperature (MAT) and litter standing crop carbon, (**<sup>a</sup>**,**d**) total shrublands, (**b**,**<sup>e</sup>**) alpine shrublands, (**<sup>c</sup>**,**f**) desert shrublands.

### *3.4. E*ff*ects of Aboveground Biomass and Climatic Factors on Litter Standing Crop Carbon*

Increasing aboveground biomass can stimulate litter standing crop carbon (Figure 5a) (*p* < 0.01), specifically, the positive trend also existed in both alpine shrublands and desert shrublands (Figure 5b). Generally, multiple linear regressions showed that litter standing crop carbon = −0.001 MAP + 0.01 MAT + 0.0001Aboveground biomass + 0.11 (*r* = 0.42, *p* < 0.01). Furthermore, the relationships among litter standing crop carbon, aboveground biomass, MAT, and MAP were shown in Figure S1.

**Figure 5.** Relationships between aboveground biomass and litter standing crop carbon, (**a**) total shrublands, (**b**) red circle means alpine shrublands, green triangle means desert shrublands.
