**4. Discussion**

### *4.1. Size of Litter Standing Crop*

The density of the litter standing crop in the Tibetan Plateau shrublands was 0.23 kg m<sup>−</sup>2, nearly half of the litter standing crop in the forests (0.45 kg m<sup>−</sup>2) of the Tibetan Plateau [5], and was more than three times as much as the litter standing crop of the grasslands (0.05 kg m<sup>−</sup>2) of the Tibetan Plateau [5]. The mean aboveground biomass in the shrublands was 1.10 kg m<sup>−</sup><sup>2</sup> [19], less than that of 4.22 kg m<sup>−</sup><sup>2</sup> in the forests [30], and greater than that of 0.08 kg m<sup>−</sup><sup>2</sup> in the grasslands of the plateau [31]. As aboveground biomass made the greatest contribution to litter standing crop in ecosystem [1], the litter standing crop in shrublands was in the range between forest and grasslands.

The litter standing crop in the shrublands of south China was 0.32 kg m<sup>−</sup><sup>2</sup> [16], larger than our result of 0.23 kg m<sup>−</sup><sup>2</sup> for the Tibetan Plateau. This di fference may result from the greater amount of aboveground biomass in the shrublands across south China, compared to the Tibetan Plateau shrublands. Aboveground biomass can shape the input resource of litter standing crop [1] and significantly accumulate litter standing crop. It has been demonstrated that aboveground biomass in shrublands in the south region of China was 1.44 kg m<sup>−</sup><sup>2</sup> [32], greater than the 1.10 kg m<sup>−</sup><sup>2</sup> of the Tibetan Plateau shrublands [19].

### *4.2. Controlling Factors of Litter Standing Crop Carbon*

Climatic factors can significantly shape litter standing crop [33,34]. Generally, precipitation not only stimulates vegetation growth in arid regions [35], and furthermore increases input of litter standing crop, but also a ffects the leaching of chemical substances from litter and consequently limits the litter decomposition rate [2]. So precipitation has been shown to have a positive e ffect on litter production [13] and stimulates accumulation of litter standing crop. However, precipitation has restrained e ffects on accumulation of litter standing crop carbon in the Tibetan Plateau shrublands. This result was di fferent from those for other regions and biomes. For forests, precipitation did not have a significant e ffect on litter standing crop in forests of the eastern Inner Mongolia Plateau [18] or forests in China [5]. Litter standing crop carbon was stable with increasing precipitation in southern regions of China shrublands [16]. It should be noted that precipitation can also enhance litter decomposition by maximizing decomposer activity [11,36], which can reduce litter standing crop storage. Litter production and litter decomposition determine litter standing crop [37]. The restrain e ffects on the accumulation of litter standing crop carbon from precipitation may exceed the positive e ffects; consequently, increasing MAP decreases litter standing crop carbon in the shrublands, both alpine shrublands and desert shrublands, on the Tibetan Plateau.

Temperature is considered to be the most significant factor determining litter decomposition [38,39]. Rising temperature increases the litter decomposition rate [2]. In our study, MAT contributed to

the accumulation of litter standing crop carbon in the shrublands on the Tibetan Plateau, which was di fferent from other reported findings, specifically, that temperature had negative e ffects on accumulation of litter standing crop carbon in shrubland ecosystems in southern China [15] and in forests in China [5], whereas temperature was not a significant controlling factor for litter standing crop carbon in grasslands in China [5]. One of the most significant climate characteristics on the Tibetan Plateau is cold, and in the Tibetan Plateau shrublands, increasing temperature has been shown to play a significant role in shaping the amount of aboveground biomass [40]. Temperature contributes to the accumulation of aboveground biomass [40], which shapes amount of litter input, and stimulates more inputs into the litter standing crop; consequently, the litter standing crop in the shrublands of the Tibetan Plateau shows an increasing trend with MAT.

The climatic controlling factors on the litter standing crop carbon in the Tibetan Plateau shrublands, such as MAT and MAP, were di fferent from those in other regions, which may indicate that the biophysical processes that control carbon on the Tibetan Plateau may di ffer from those in other regions [20,22]. In the global warming scenario, MAT is increasing by 0.05 ◦C every year [10,41], which, combined with the positive relationship between litter standing crop carbon and temperature, contributes to the carbon sink trend in the Tibetan Plateau shrublands.

### *4.3. Distribution and Conversion Coe*ffi*cient of Litter Standing Crop to Litter Standing Crop Carbon*

Spatially, the litter standing crop carbon decreased with longitude in shrublands, which may result from climatic changes with di fferent regions. With an increasing longitude, the MAP has an increasing trend (Figure S2a), and consequently, increasing precipitation limited accumulation of litter standing crop carbon (Figure 4a–c). At the same time, MAT has a decreasing trend with longitude (Figure S2b), and decreasing temperature also limited the accumulation of litter standing crop carbon (Figure 4d–e). The climatic changes with longitude can explain the distribution of litter standing crop carbon in shrublands on the Tibetan Plateau.

It has been estimated that the conversion coe fficient for litter standing crop to litter standing crop carbon is 0.41 in the shrublands of southern China [16], which is lower than our result of 0.44. Di fferent vegetation types result in various rates of litter production and litter decomposition due to climatic conditions [33,34], and eventually, shape the conversion coe fficient for litter standing crop to litter standing crop carbon. Cold weather is the most significant climatic characteristics of the Tibetan Plateau [42], due to its high elevation. On the one hand, lower temperature suppresses decomposer activity [11,43], which contributes to the weak decomposition of litter on the Tibetan Plateau. On the other hand, plants tend to produce organic matter with a higher carbon content [44–46]. Therefore, the conversion coe fficient for litter standing crop to litter standing crop carbon in shrublands was smaller on the Tibetan Plateau compared with in shrublands of southern China. Researchers use a coe fficient of 0.50 to convert biomass to biomass carbon in woody ecosystems such as forests [47,48] and shrublands [17], while the coe fficient is smaller (0.45) in herbaceous ecosystems such as grasslands [49,50]. Our results reveal that the conversion coe fficient for branch standing crop to branch standing crop carbon was 0.45, larger than the conversion coe fficient of 0.36 for foliage standing crop to foliage standing crop carbon, which indicates that the conversion coe fficient in woody ecosystems is larger than in herbaceous ecosystems. Studies of forest and grassland ecosystem support this trend, with a conversion coe fficient of 0.46 in forests and 0.40 in grasslands in China [5]. Therefore, it is better to use specific conversion coe fficients for di fferent ecosystems to estimate litter standing crop carbon. Furthermore, even with in the same ecosystem, such as shrublands, it is better to use a specific parameter to assess carbon pools, specifically, 0.41 for the shrublands of southern China [16] and 0.44 for the Tibetan Plateau shrublands. To conduct a more accurate estimation of the carbon pool, it is necessary to use conversion coe fficients for the di fferent components of litter standing crop (in this case, 0.45 and 0.36, respectively, for branch and foliage standing crop in the shrublands of the Tibetan Plateau). Meanwhile, in the Tibetan Plateau, conversion coe fficients of branch standing crop to branch standing crop carbon and foliage standing crop to foliage standing crop carbon were 0.46 and 0.42, 0.40, and 0.27 greater in alpine shrublands than in desert shrublands, respectively, which furthermore means that in the neighborhood regions (such as the Tibetan Plateau), the given conversion coe fficient needs to be considered for more accurate estimation.
