**3. Results**

#### *3.1. Litter and Litter Carbon Input to the Stream*

The litter input to the stream showed two peaks (Figure 3A). The maximum peak (101.40 g m<sup>−</sup>2) was observed in the LGS and then decreased gradually, reaching the minimum value (18.91 g m<sup>−</sup>2) in the SMS. The second peak (59.44 g m<sup>−</sup>2) was found in the GS (Figure 3A), and these values were all averages per period. Meanwhile, the litter input to the stream ranged from 2.47 to 103.13 g m<sup>−</sup>2, and the annual value was 20.14 g m<sup>−</sup><sup>2</sup> for the 15 investigated streams during this one-year investigation.

**Figure 3.** Dynamics of litter ( **A**) and litter carbon (**B**) input to the subalpine forest streams in the upper reaches of the Yangtze River. LGS, SSC, SMS, EGS and GS indicate the sampling periods, i.e., later growing season (LGS: September to October), seasonal snow cover (SSC: November to April next year), snowmelt season (SMS: April to May), early growing season (EGS: May to June), and growing season (GS: July to August). The vertical coordinate is the mean of litter input accumulation of 15 streams during this period. Different lowercase letters indicate the significant difference among different periods (*p* < 0.05), while the same letter indicates no significant difference among each other.

Like litter input, the dynamic pattern of litter carbon in the streams increased gradually from September to November (LGS), reached its maximum value (44.39 mg m<sup>−</sup>2), and then decreased gradually from April to June (SMS) to reach its minimum value (7.98 mg m<sup>−</sup>2) in SMS (Figure 3B). The second peak (25.12 mg m<sup>−</sup>2) of litter carbon input to the stream was observed in the GS (Figure 3B), and these values were all average of per period. For all streams, the litter carbon input to the streams ranged from 0.11 to 40.57 mg m<sup>−</sup>2, and the annual value was 8.61 mg m<sup>−</sup><sup>2</sup> for all streams in this one-year investigation.

#### *3.2. Litter and Litter Carbon Output from the Stream*

The dynamic pattern of litter output from the stream also showed a similar pattern to that of the input. The maximum value (1.69 g m<sup>−</sup>2) appeared in the LGS, and then the value decreased gradually, reaching the minimum value in the SMS (0.46 g m<sup>−</sup>2). The second peak (1.72 g m<sup>−</sup>2) was observed in the EGS (Figure 4A), and these values were all average of per period. The litter output from the all streams ranged from 0.02 to 22.30 g m<sup>−</sup>2, and the annual average value was 0.56 g m<sup>−</sup><sup>2</sup> during this one-year investigation.

**Figure 4.** Dynamics of litter (**A**) and litter carbon (**B**) output from the subalpine forest streams in the upper reaches of the Yangtze River from 11 July 2015, to 2 August 2016. LGS, SSC, SMS, EGS and GS indicate the sampling periods, i.e., later growing season (LGS: September to October), seasonal snow cover (SSC: November to April next year), snowmelt season (SMS: April to May), early growing season (EGS: May to June), and growing season (GS: July to August). The vertical coordinate is the mean of litter input accumulation of 15 streams during this period. Different lowercase letters indicate the significant difference among different periods (*p* < 0.05), while the same letter indicates no significant difference among each other.

Consistent with the pattern of litter input to the stream, the output of litter carbon from the streams also had two peaks (Figure 4B), which also appeared in the LGS (0.63 mg m<sup>−</sup>2) and EGS (0.64 mg m<sup>−</sup>2), and the lowest point was observed in the SMS (0.31 mg m<sup>−</sup>2) (Figure 4B), and these values were all average of per period. Additionally, the litter carbon output from all streams ranged from 0.01 to 4.53 g m<sup>−</sup>2, and the annual value was 0.45 g m<sup>−</sup><sup>2</sup> during this one-year investigation.

#### *3.3. The Ratios of the Input to Output of Litter and Litter Carbon*

During the investigated period, the average ratios of input to output of litter and litter carbon were 188.17 and 270.01 at the forest catchment level, respectively. The ratios of litter input to output ranged from 23.85 to 619.67 (Figure 5A), and the ratios of the input to output of litter carbon ranged from 16.80 to 1147.91 (Figure 5B). Meanwhile, the ratios of both litter input to output and litter carbon input to output showed similar dynamic patterns with the two peaks appearing in the LGS (272.74, 329.70) and EGS (190.46, 304.13), and the lowest point observed in the SMS (37.99, 67.11) for litter and litter carbon, respectively. Additionally, the ratios of input to output for both litter and litter carbon varied greatly with the investigated streams, and the averaged ratios of input to output ranged from 16.42 to 577.91 for litter (Figure 6A), and from 16.80 to 1147.91 for litter carbon, respectively (Figure 6B).

**Figure 5.** The ratios of input to output of litter (**A**) and litter carbon (**B**) in the subalpine forest streams in the upper reaches of the Yangtze River from 11 July 2015, to 2 August 2016. The value of each dot is the ratio of the average for the investigated streams during the sampling time. Different lowercase letters indicate the significant difference among different periods (*p* < 0.05), while the same letter indicates no significant difference among each other.

**Figure 6.** The ratios of input to output of litter (**A**) and litter carbon (**B**) in the subalpine forest streams in the upper reaches of the Yangtze River from 11 July 2015, to 2 August 2016. Each bar is the average of 13 sampling times for each forest stream.A–O are the sampled streams in the study.

#### *3.4. Relationships of Litter and Litter Carbon Input/Output with Relative Variables*

Litter input to the streams was significantly and positively correlated with the temperature and length (Table 2). However, litter input was slightly and negatively correlated with the stream width, but slightly and positively related to the precipitation, water level, sediment depth, and flow velocity of the forest streams (Table 2). Correspondingly, litter carbon input to the streams was also showed the similar relationships with relative variables (Table 2).


**Table 2.** Relationships of litter and litter carbon input with stream characteristics in the subalpine forest catchment.

\* *p* < 0.05, \*\* *p* < 0.01. d.f.: degree of freedom.

Similarly, litter output from the streams was significantly and positively correlated with the precipitation, temperature, litter input, and flow velocity, and slightly and positively with the water level, but significantly and negatively with sediment depth, and slightly and positively with stream width and length. Correspondingly, litter carbon output from the streams was also the similar relationships with the variables mentioned above (Table 3).

**Table 3.** Relationships of litter and litter carbon output with stream characteristics in the subalpine forest streams.


\* *p* < 0.05, \*\* *p* < 0.01. d.f.: degree of freedom.
