Plant Litter and Litter Carbon: Comparison
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Please note this is a comparison between Version 2 by Rita Xu and Version 1 by Wanqin Yang.

      Investigations on the budget of plant litter and litter carbon in forest streams can provide a key scientific basis for understanding the biogeochemical linkages of terrestrial–aquatic ecosystems and managing forest catchments.

  • litter input and output
  • litter carbon budget
  • mountain forest stream

1. Introduction

In the geosphere, streams cover less than 3% of the forest catchment area, but function as the bridges linking terrestrial–aquatic biogeochemical cycles [1]. In particular, plant litter from neighboring forests and riparian zones is the major source and carrier of carbon in forest streams and plays crucial roles not only in maintaining stream ecosystem productivity, but also in maintaining the structure and function of the butted aquatic ecosystem [2,3][2][3]. Additionally, litter decomposition in the forest stream ecosystem can contribute significantly to the global carbon cycle [4,5,6][4][5][6]. Therefore, understanding the budget of litter and litter carbon in forest streams can provide a key scientific basis for managing forest catchments and predicting the global carbon cycle.
The forest stream might act as a sink of plant litter and bioelements in the forest catchment. Theoretically, litter input to the stream is hierarchically regulated by three interactive factors: climate, forest type, and stream characteristics [7]. First, the climate has been considered the primary factor influencing litter production [8]. Generally, average litter production decreases gradually from tropical zones to boreal alpine zones along the climate gradient [9,10][9][10]. Compared to cold temperate zones, evergreen broadleaved forests in tropical regions often have larger amounts of litter production due to the higher temperature and moisture [11[11][12],12], implying that more litter can enter the stream. Second, the dynamics of litter input vary greatly with forest types, as different tree species have different phenological phases [7[7][13],13], which in turn determine the quantity and dynamics of litter input to the forest stream [13,14][13][14]. For instance, on a local scale, evergreen and deciduous forests usually show higher litter production than dark coniferous forests in the subalpine forest region [15,16][15][16]. Plant species composition in the riparian zone differs greatly from that in the mountain forest, and the litter production of shrub and herb species in the riparian zone is lower than that in the mountain forest [7[7][14],14], implying that the litter input to the stream in the riparian zone might be lower than that directly in the forest. Third, litter input is also modulated by the stream length and width [17], and longer and wider streams can receive more litter along the stream [18,19][18][19]. Although litter input to the stream has been systematically investigated in northern America [20[20][21][22],21,22], litter input to streams has not been fully investigated around the world, limiting our understanding of the biogeochemical linkages of mountain forest and riparian zones with streams and rivers.
The litter and litter carbon output from streams are known as the major carbon sources of butted rivers. That is to say, the forest stream also acts as the source of litter and litter carbon [23,24][23][24]. In theory, the litter output from forest streams is usually regulated by the stream litter quality and quantity, stream biological community, stream characteristics, and climate [8,25][8][25]. To begin with, the magnitude of litter input to the stream determines the size of the litter source of a butted river [4,5][4][5]. Meanwhile, the scouring action of stream water on litter can directly accelerate litter fragmentation [26], which may lead to the output and confluence characteristics of litter varying with the seasons [27,28,29][27][28][29]. For example, streams with lower flow rates and slower velocities always accompany faster litter decomposition and litter deposition, leading to smaller amounts of output, and vice versa [30]. In addition, the length and width of the streams, together with their microtopography, might significantly influence litter output [29]. Finally, forest stream characteristics, such as discharge and velocity, are always regulated by seasonal precipitation (rainfall, snowfall, and snowmelt), theoretically modulating the output of litter and litter carbon from the streams. In particular, the plant rhythm with seasonal changes accompanies the seasonal dynamics of precipitation and temperature, which play important roles in controlling the output of litter from a forest stream [7,31][7][31]. Therefore, investigations into the litter output from forest streams could facilitate a better understanding of the biogeochemical linkages of mountain forests and riparian zones with aquatic ecosystems.
Carbon is the basic component in both terrestrial and aquatic ecosystems. The dynamic pattern of litter carbon in forest stream ecosystems can reveal terrestrial–aquatic carbon biogeochemical linkages [30,32][30][32]. Past investigations have found that the carbon fractions derived from upstream and neighboring ecosystems are the two major sources of dissolved carbon (DC) in forest stream ecosystems [33,34][33][34]. Most investigations of carbon biogeochemical linkages between mountain forests and butted aquatic ecosystems have employed the small-scale runoff field method [35]. However, this method has difficulty revealing the roles of forest streams in terrestrial–aquatic carbon biogeochemical linkages, especially in geographically fragile mountainous regions [36]. First, due to geological fragmentation and serious soil percolation [37], surface runoff is rarely observed in most rainfall and snowfall events, while percolating water becomes an important biogeochemical link between mountain forest ecosystems and aquatic ecosystems in fragile mountainous regions. Second, forest streams can be directly involved in the biological carbon cycle rather than indirectly involved through surface runoff, since litter from forest and riparian vegetation is a major source of carbon input to the butted aquatic ecosystem [38]. Third, the riparian zone is an important domain in the forest stream ecosystem. The decomposition of allochthonous organic materials (e.g., foliar litter) in riparian zones is often a critical factor affecting the continued availability of carbon resources in these ecosystems [39]. Hence, the systematic investigation of litter carbon dynamics in streams and riparian zones will provide baseline data for further understanding of the biogeochemical linkages of terrestrial–aquatic ecosystems.
As the second largest forest region in China, the subalpine forest region in the eastern Qinghai-Tibet Plateau is the most important freshwater conservation area and headwater region of the Yangtze River, and plays paramount and irreplaceable roles in holding water, conserving soil, and maintaining the safety of water resources and downstream aquatic ecosystems [40,41][40][41]. These forest stream ecosystems are typically cold ecosystems that experience considerable seasonal freezing and thawing events, and seasonal changes are associated with distinct changes in environmental conditions [42,43][42][43]. Therefore, a deep investigation of the budget of litter and litter carbon in these forest stream ecosystems is key to revealing the carbon biogeochemical linkages between subalpine forests and aquatic ecosystems.

2. Site Description

This study was conducted at the Long-Term Research Station of Alpine Forest Ecosystem in the Bipenggou Valley (31°14′~31°19′ N, 102°53′~102°57′ E, 2458~4619 m above sea level (masl)), Li County, Southwest China, which is located in the alpine gorge area with frequent geological breaks, clear seasonal snow cover (the maximum snow depth was about 35 cm), and frequent freeze/thaw cycles [7] (Figure 1). The mean annual precipitation is approximately 850 mm, and the annual mean air temperature is approximately 3 °C, with maximum and minimum temperatures of 23 °C (July) and −18 °C (January), respectively. The frozen season lasts from November to April, and thaw begins in late April. This subalpine forest is dominated by Minjiang fir (Abies faxoniana Rehder & E.H.Wilson), larch (Larix mastersiana Rehder & E.H.Wilson), and cypress (Sabina saltuaria Rehder & E.H.Wilson), and is interspersed with shrubs of azaleas (Rhododendron spp.), willow (Salix spp.), and barberry (Berberis sargentiana C.K.Schneid). The herbaceous plants consist mainly of ferns (Cystopteris montana (Lam.) Bernh. ex Desv) [7]. The concentrations of carbon (C), nitrogen (N), and phosphorus (P) in the surface soil (5 cm depth) was 126.0, 5.8, and 1.2 g kg−1, respectively.
Forests 12 01764 g001 550
Figure 1. The investigated streams in the Bipenggou Valley, located in the upper reaches of the Yangtze River. The letters A–O indicate the 15 sampling streams [7].

3. Monitoring the Input and Output of Litter and Litter Carbon

In order to collect litter, according to the stream length, a quadratic litter collector (0.8 m × 0.8 m) was randomly installed at the source, middle, and end of the stream (when the stream width < 0.8 m, one litter collector was positioned; when the stream width > 0.8 m and <1.6 m, two litter collectors were positioned; the stream widths are shown in Table 1), and each was installed 0.5 m above the water or ground surface (Figure 2). To avoid litter decay in the litter collectors caused by rainfall, the litter samples were collected every 15 days, but the litter was collected only once in the cold winter since litterfall in winter was rare. All of the collected litter samples were put into precleaned polyethylene bags and transported to the lab. The samples were dried to a constant weight and stored at 65 °C for less than one week until analysis.
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Figure 2. Litter and litter carbon input and output monitoring system in the investigated streams of the Bipenggou Valley, located in the upper reaches of the Yangtze River.
Table 1. Basic characteristics of 15 representative subalpine forest streams in the investigated subalpine forest catchment.
Stream Elevation

(m)		 Length

(m)		 Width

(m)		 Water Level (cm) Flow Velocity

(m		3/s) Main Plants
A 3668 220 0.63 ± 0.16 8.57 ± 3.20 0.11 ± 0.11 A. faxoniana, Cyperus spp., S saltuaria
B 3667 66 0.69 ± 0.14 5.15 ± 1.60 0.07 ± 0.07 A. faxoniana, Cyperus spp., S. saltuaria
C 3658 13 0.63 ± 0.22 7.16 ± 3.83 0.01 ± 0.02 A. faxoniana, Cyperus spp., S. saltuaria
D 3658 92.4 0.86 ± 0.19 4.81 ± 1.08 0.06 ± 0.07 A. faxoniana, Cyperus spp., S. saltuaria
E 3657 47 0.34 ± 0.30 3.73 ± 3.43 0.01 ± 0.01 A. faxoniana, Cyperus spp., S. saltuaria
F 3640 65 1.11 ± 0.30 6.90 ± 1.44 0.11 ± 0.12 S. saltuaria, R. lapponicum
G 3640 186 1.02 ± 0.33 8.96 ± 1.91 0.06 ± 0.06 S. saltuaria, Carex spp., R. weginzowii
H 3634 108 0.82 ± 0.28 6.73 ± 4.38 0.04 ± 0.07 S. saltuaria, Carex spp., S. rufopilosa
I 3634 256 1.02 ± 0.22 7.19 ± 1.80 0.13 ± 0.12 S. saltuaria, R. weginzowii, Carex spp.
J 3634 18 1.29 ± 1.00 3.85 ± 3.05 0.04 ± 0.07 S. saltuaria, R. weginzowii, Carex spp.
K 3611 36 1.00 ± 0.24 7.93 ± 2.32 0.10 ± 0.08 R. lapponicum, S. saltuaria
L 3611 11 0.93 ± 0.58 3.70 ± 1.89 0.03 ± 0.05 R. lapponicum, S. saltuaria
M 3610 12 0.85 ± 0.23 6.26 ± 1.42 0.03 ± 0.08 R. lapponicum, S. saltuaria
N 3607 28 0.84 ± 0.32 11.72 ± 4.88 0.15 ± 0.14 S. mastersiana, Cyperus spp., S. rufopilosa
O 3607 17 0.65 ± 0.41 4.34 ± 3.84 0.02 ± 0.04 S. mastersiana, S. rufopilosa, Cyperus spp.

4. 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−2) was observed in the LGS and then decreased gradually, reaching the minimum value (18.91 g m−2) in the SMS. The second peak (59.44 g m−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−2, and the annual value was 20.14 g m−2 for the 15 investigated streams during this one-year investigation.
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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−2), and then decreased gradually from April to June (SMS) to reach its minimum value (7.98 mg m−2) in SMS (Figure 3B). The second peak (25.12 mg m−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−2, and the annual value was 8.61 mg m−2 for all streams in this one-year investigation.

5. 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−2) appeared in the LGS, and then the value decreased gradually, reaching the minimum value in the SMS (0.46 g m−2). The second peak (1.72 g m−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−2, and the annual average value was 0.56 g m−2 during this one-year investigation.
Forests 12 01764 g004 550
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−2) and EGS (0.64 mg m−2), and the lowest point was observed in the SMS (0.31 mg m−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−2, and the annual value was 0.45 g m−2 during this one-year investigation.

6. 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).
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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.
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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.

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