Water and CO2 fluxes over semiarid alpine steppe ...

Water and CO2 fluxes over semiarid alpine steppe and humid alpine meadow ecosystems on the Tibetan Plateau Lei Wang, Huizhi Liu, Yaping Shao, Yang Liu & Jihua Sun

Theoretical and Applied Climatology ISSN 0177-798X Theor Appl Climatol DOI 10.1007/s00704-016-1997-1

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Author's personal copy Theor Appl Climatol DOI 10.1007/s00704-016-1997-1

ORIGINAL PAPER

Water and CO2 fluxes over semiarid alpine steppe and humid alpine meadow ecosystems on the Tibetan Plateau Lei Wang 1 & Huizhi Liu 1 & Yaping Shao 2 & Yang Liu 1 & Jihua Sun 3

Received: 7 August 2016 / Accepted: 31 October 2016 # Springer-Verlag Wien 2016

Abstract Based on eddy covariance flux data from July 15, 2014, to December 31, 2015, the water and CO2 fluxes were compared over a semiarid alpine steppe (Bange, Tibetan Plateau) and a humid alpine meadow (Lijiang, Yunnan) on the Tibetan Plateau and its surrounding region. During the wet season, the evaporative fraction (EF) was strongly and linearly correlated with the soil water content (SWC) at Bange because of its sparse green grass cover. In contrast, the correlation between the EF at Lijiang and the SWC and the normalized difference vegetation index (NDVI) was very low because the atmosphere was close to saturation and the EF was relatively constant. In the dry season, the EF at both sites decreased with the SWC. The net ecosystem exchange (NEE) at Bange was largely depressed at noon, while this phenomenon did not occur at Lijiang. The saturated NEE at Bange was 24% of that at Lijiang. The temperature sensitivity coefficient of ecosystem respiration at Bange (1.7) was also much lower than that at Lijiang (3.4). The annual total NEE in 2015 was 21.8 and −230.0 g C m−2 yr−1 at Bange and Lijiang, respectively, and the NEE was tightly controlled by the NDVI at the two sites. The distinct differences in the water and CO2 fluxes at Bange and Lijiang are attributed to the large SWC difference and its effect on vegetation growth.

* Huizhi Liu [email protected]

1

State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China

2

Institute for Geophysics and Meteorology, University of Cologne, 50937 Cologne, Germany

3

Yunnan Institute of Meteorological Sciences, Kunming 650034, China

1 Introduction Alpine grasslands (alpine steppe and alpine meadow) are the main grassland types in China, covering 26.8% of the total grassland area (Liu et al. 2008). They are primarily located on the Tibetan Plateau and its surrounding regions, which have been categorized as areas that are sensitive to climate change (Kang et al. 2010). Surface observations have revealed a more pronounced warming at high elevations in these areas (Liu and Chen 2000; Fan et al. 2011), and vegetation has responded to this change, as indicated by remote sensing data (Ding et al. 2013). The responses of the water and CO2 fluxes to environmental changes are crucial for the water cycle and global carbon budget, respectively. In past decades, field experiments have examined the land-atmosphere interaction on the Tibetan Plateau (Ma et al. 2003, 2008; Gu et al. 2008; Li et al. 2015; Ma et al. 2015). On the seasonal scale, the most important environmental factors in surface energy partitioning and CO2 flux for an alpine meadow in Haibei, Qinghai, China, were the soil water content (SWC) and air temperature (Ta), respectively (Gu et al. 2005; Fu et al. 2009). On the annual scale, the Haibei alpine meadow is characterized by a low ratio of annual evapotranspiration (ET) to annual precipitation (PPT) (approximately 60%) (Gu et al. 2008), while the annual ET is similar to the annual PPT for the alpine steppe in Shuanghu, Tibet, China (Ma et al. 2015). The area of the plateau is approximately 2.5 million km2, spanning 25° of longitude and several thousand meters of altitude. The water and CO2 exchanges over grassland ecosystems vary in different climate zones. The evaporative fraction (EF) is a key variable that links land surface forcing to the atmosphere, which is coupled with

Author's personal copy L. Wang et al.

boundary layer development, cloud characteristics, and PPT in atmospheric models (Findell and Eltahir 2003; Gentine et al. 2013). Observations indicate that the EF and SWC are weakly coupled over dense grassland (leaf area index (LAI) >4 m2 m−2) (Phillips and Klein 2014), and vegetation has a greater correlation with the EF relative to soil moisture metrics (Williams and Torn 2015). However, when the LAI is very low over sparse grassland, the coupling strength of the EF and SWC (or LAI) has not been determined. The assumption is that available soil water over sparse grasslands is readily evaporated given sufficient surface radiative heating and that it is neither restricted nor aided by vegetation (Williams and Torn 2015). Under the support of the Third Tibetan Plateau Scientific Experiment, the Bange flux measurement site was set up in the middle of July 2014. The flux observations at the Lijiang site have been continuously recorded since June 2011 (please see Wang et al. (2016a) for more details). Based on the simultaneous collection of data at the Bange and Lijiang sites from July 15, 2014, to December 31, 2015, the aims of this study were (1) to characterize the diurnal and seasonal variations in water and CO2 fluxes and (2) to determine differences in the factors that control the EF and water and CO2 fluxes over the semiarid alpine steppe and humid alpine meadow ecosystems. Fig. 1 a Location of Bange and Lijiang stations (map from Google Earth), b the picture of the Bange site, and c the picture of the Lijiang site

2 Materials and methods 2.1 Site description The observational sites include two alpine grassland ecosystems (Fig. 1). Bange (31° 25′ N, 90° 2′ E; 4700 m above sea level (asl)) is a semiarid alpine steppe ecosystem on the Tibetan Plateau, which is located in Bange County, approximately 230 km north of Lhasa. Lijiang (27° 10′ N, 100° 14′ E; 3560 m asl) is an alpine meadow ecosystem in the eastern Hengduan Mountains, which are southeast of the Tibetan Plateau. It is located at Maoniuping, which lies east of Yulong Snow Mountain, Lijiang, Yunnan Province (please see Wang et al. (2016a) for more details). The Bange site is in a subfrigid area, and its 30-year mean annual air temperature is −2.8 °C. This area is controlled by the semiarid plateau monsoon climate. The 30-year mean annual air temperature is 12.6 °C for Lijiang City (2400 m a.s.l.). The Lijiang site is in a subtropical plateau monsoon climate area, and it is influenced by both the southwest and southeast monsoons. The 30-year mean annual total PPT (1980–2010) for the Bange and Lijiang sites are 308.3 and 980.3 mm, respectively. At both sites, more than 85% of the PPT occur from June to October (the wet season). The wet season is from June to September for Bange and from June to October for Lijiang.

Author's personal copy Water and CO2 fluxes over semiarid alpine steppe and humid alpine

The alpine steppe at Bange is dominated by Stipa purpurea, with a height less than 3 cm, throughout the entire wet season, while the dominant species of the alpine meadow at Lijiang is Kobresia willd. Grass, with a maximum height greater than 20 cm. The mean aerodynamic roughness lengths at Bange and Lijiang during the wet season are 0.24 and 1.72 cm, respectively. The soil at Bange is sandy loam with a light color, while the soil at Lijiang is dark brown loamy soil. The terrain at the Bange site, which is over 300-min circumference, is flat with sufficient fetch for the application of the eddy covariance (EC) technique. The terrain at the Lijiang site has a slight slope of 10°. 2.2 Measurements EC systems were used to measure the sensible heat, latent heat, and CO2 fluxes between the land surface and atmosphere at the two sites (Baldocchi 2003). The EC system consisted of a three-dimensional sonic anemometer (CSAT3, Campbell, USA) and an open-path CO2/H2O infrared gas analyzer (LI7500A, LI-COR, USA). The EC measurement height was 2 m for Bange and 2.5 m for Lijiang. The measurement frequency was 10 Hz for both sites. A 10-m tower was set up at Bange to install the instruments for gradient meteorological observations. At Bange, the air temperature, relative humidity (HMP45C, Campbell), and wind speed (010C, Campbell) were measured at three levels (2-, 4-, and 10-m heights), and wind direction (020C, Campbell) was measured at 10 m. At Lijiang, the measurement height for air temperature, relative humidity (HMP45C, Campbell), wind speed, and wind direction (034B, Campbell) was 2.5 m. At both sites, net radiation (CNR4, Kipp & Zonen, the Netherlands) and photosynthetic active radiation (LI190SB, LI-COR) were measured at 1.5 m. The soil heat flux was measured at 5 and 10 cm below ground (HFP01, Hukseflux, the Netherlands). The soil temperature (109-L, Campbell) and the SWC (CS616, Campbell) were measured at 5, 10, 20, 50, and 100 cm below ground. The SWC at 50 and 100 cm for Bange was underestimated because of the large proportion of gravel, and the values were not included in the analysis. A tipping bucket rain gauge (52202, Young, USA) was used to measure rainfall at both sites. In addition, a weighing bucket PPT gauge (T-200B, Geonor, Norway) was used to measure solid PPT during the winter at Lijiang.

et al. 2001) was used at Lijiang due to the slope of the terrain. The fluxes were corrected in several steps, including a Schotanus correction for H (Schotanus et al. 1983); spectral loss corrections for H, LE, and NEE (Moore 1986); and WPL corrections for LE and NEE (Webb et al. 1980). A quality check (QC) of the half-hourly fluxes was conducted as described by Foken and Wichura (1996). The QC procedures included stationary and integral turbulence characteristics tests. The critical u* was 0.1 m s−1, and the nighttime CO2 flux was removed in further analysis when u* was less than 0.1 m s−1. The quality of turbulent flux measurements is usually checked by the energy balance closure. The energy balance ratios (EBRs) over the measurement period were 0.71 and 0.97 for Bange and Lijiang, respectively. Both EBRs were within the normal range (0.7 to 1.0) of other observational sites (Wilson et al. 2002). 2.4 Data analysis The EF was calculated as follows: EF ¼

LE H þ LE

ð1Þ

The van’t Hoff equation was used to describe the relationship between nighttime NEE (ecosystem respiration; μmol CO2 m−2 s−1) and soil temperature at the 5-cm depth (Ts; °C) (Aires et al. 2008). NEEnighttime ¼ aexpðbT s Þ

ð2Þ

where a and b are the regression parameters. The temperature sensitivity coefficient (Q10) of RE was calculated using the following equation: Q10 ¼ expð10bÞ

ð3Þ

The Michaelis-Menten model was used to evaluate the relationship between daytime NEE and photosynthetic active radiation (PAR) (Falge et al. 2001). NEEdaytime ¼

αNEEsat PAR þ RE αPAR þ NEEsat

ð4Þ

where α is the apparent quantum yield (μmol CO2 μmol−1 photons), NEEsat is the NEE at a saturating light level, and RE is a bulk estimate of ecosystem respiration.

2.3 Flux calculation and data processing Based on the 10-Hz raw data, the half-hourly sensible heat flux (H), the latent heat flux (LE), and the net ecosystem exchange of carbon dioxide (NEE) were calculated using EddyPro software (version 5.1, LI-COR). A double rotation (Kaimal and Finnigan 1994) was performed to transform the coordinate system at Bange, while the sector-wise planar fit method (Wilczak

3 Results 3.1 Meteorological conditions During the 2015 wet season, the global solar radiation at Bange (Rg) (mean value 23.85 MJ d−1 m−2) was substantially

Author's personal copy L. Wang et al. Table 1 The average of the solar radiation (Sin; MJ m−2 d−1), net radiation (Rn; MJ m−2 d−1), sensible heat flux (H; MJ m−2 d−1), latent heat flux (LE; MJ m−2 d−1), Bowen ratio, soil water content (SWC; m3 m−3), air temperature (Ta; °C), soil temperature at the 5-cm soil Site Bange

Lijiang

depth (Ts_5 cm; °C), wind speed (WS; m s−1) and vapor pressure deficit (VPD; kPa), and the sum of evapotranspiration (ET; mm) and net ecosystem exchange (NEE; g C m−2) at Bange and Lijiang for the wet season, the dry season, and the entire years in 2015

Period

Sin

Rn

H

LE

Bowen

SWC

Ta

Ts_5 cm

WS

VPD

ET

NEE

Wet season

23.85

9.30

3.71

2.63

1.41

0.08

9.23

13.8

3.79

0.71

142.6

13.4

Dry season Entire year

19.45 20.92

5.15 6.54

2.56 2.94

0.57 1.26

4.49 2.33

0.06 0.06

−4.60 0.02

−2.1 3.2

4.34 4.16

0.37 0.49

61.7 204.3

8.4 21.8

Wet season

10.69

6.11

1.63

4.91

0.33

0.28

10.49

13.3

0.94

0.17

322.5

−234.7

Dry season

16.66

6.07

2.64

2.56

1.03

0.21

3.99

6.4

1.84

0.37

220.2

4.7

Entire year

14.66

6.09

2.30

3.35

0.69

0.23

6.16

8.7

1.54

0.30

542.7

−230.0

higher than at Lijiang (mean value 10.69 MJ d−1 m−2) (Table 1 and Fig. 2a), because it was much cloudier and experienced more rainfall. During the dry season, Rg at Lijiang was slightly higher on clear days because of the lower latitude and higher solar angle. Bange had similar mean air temperature (Ta) to that of Lijiang during the wet season but significantly lower vapor pressure deficit (VPD) (Table 1). The daily mean Ta at Bange Fig. 2 a Integrated global solar radiation (Rg) and net radiation (Rn) over the course of a day, the daily average b air temperature (Ta), c vapor pressure deficit (VPD), and d wind speed (WS) at the Bange and Lijiang sites

decreased to less than 0 °C in late October and increased to greater than 0 °C in early May. The minimum daily mean Ta were −23.6 and −3.5 °C at Bange and Lijiang, respectively. Lijiang had a much warmer dry season and, consequently, a higher VPD than Bange. The daily mean wind speed was higher at Bange than at Lijiang throughout the entire year (Table 1 and Fig. 2d). The prevailing wind direction during the wet season was similar


(from southeast to southwest) for the two sites. In the dry season, the prevailing wind direction at Bange was from the west, while at Lijiang, it was from the northwest (data not shown), which was consistent with the monsoon. The SWC was obviously higher at Lijiang than at Bange. At both Bange and Lijiang, the SWC generally decreased with soil depth in the 0–20-cm layer (Fig. 3a, b), which indicates that the SWCs of the surface soil (0–20 cm) are mainly affected by PPT. At Lijiang, the SWCs at lower depths (50 and 100 cm) were generally greater than those of the upper layers. There was always a response delay of the SWC at the lower layers to PPT, and the SWC at the 5-cm depth for Lijiang was highest after PPT events (Fig. 3b). In contrast, the SWCs at different depths at Bange responded to PPT almost simultaneously. The soil temperature (Ts) at lower depths was higher from October 2014 to April 2015 for Bange and from midSeptember 2014 to February 2015 for Lijiang (Fig. 3c). At other times, Ts decreased with soil depth at Bange, while the difference was less than 1 °C during the wet season of 2014 at Lijiang. During the winter, Ts at Lijiang was always greater than 0 °C and it was much higher than at Bange. Ts at the 5-cm depth at Bange reached −17.6 °C in January 2015. 3.2 NDVI and annual PPT The NDVI increased gradually at Bange starting in May, while the NDVI at Lijiang increased starting in the middle of April. The NDVI reached its maximum in late August (or early September) at Bange and in July at Lijiang. The Fig. 3 Daily average soil water content (SWC) and daily sum precipitation (PPT) at a Bange and b Lijiang and c daily average soil temperature (Ts) at Bange and Lijiang at 5-, 10-, 20-, 50-, and 100-cm depths

NDVI declined sharply in September for Bange and in October for Lijiang, with natural plant senescence (Fig. 4). The annual PPT at Bange were 351.5 and 149.8 mm in 2014 and 2015, respectively. Consequently, the maximum NDVI at Bange was 0.30 in the normal PPT year of 2014, but it was much lower (0.22) in the dry year of 2015. In contrast, the annual maximum NDVI at Lijiang was similar (0.72) in both years because the PPT at Lijiang were 1204.8 and 1257.4 mm in 2014 and 2015, respectively. The large difference in NDVI between sites was mainly caused by the different PPT and SWC in the wet seasons (Table 1). 3.3 Seasonal variation in albedo Albedo was averaged from 9:00 to 15:00 (local time) to avoid the impact of the sunrise and sunset angles. During most of the year, the albedo at Bange was larger than that at Lijiang (Fig. 5). At the beginning of the measurements, the albedo difference between the sites was greater than 0.05. Then, the albedo at Bange decreased with vegetation growth, while the albedo increased with vegetation growth at Lijiang. In midAugust, the albedo at the two sites was similar. This difference in the seasonal patterns was due to the soil characteristics at the two sites. The soil at Bange reflects more solar radiation than grass, while the soil at Lijiang had a lower albedo. During the dry season, the albedo was greater than 0.7 for both sites when the surface was covered by snow.

Author's personal copy L. Wang et al. Fig. 4 Sixteen-day average normalized difference vegetation index (NDVI) at Bange and Lijiang from July 2014 to December 2015

3.4 Diurnal and seasonal variations in energy fluxes During the 2015 wet season, Rn at Lijiang was much lower than that at Bange (Fig. 6a, c). At both sites, the soil heat flux (G) was lower than H and LE, and G at Bange exhibited a much larger diurnal variation than at Lijiang. During the day, H was similar to the LE at Bange, which was consistent with the flux results observed at the Anduo and Naqu stations on the Tibetan Plateau (Ma et al. 2003). In contrast, the LE was significantly larger than H at Lijiang. During the dry season, the LE at Bange was close to zero throughout the day, while the maximum LE at Lijiang was approximately 50 W m−2 (Fig. 6b, d). G at Bange was slightly lower than H and was much larger than G at Lijiang. The Rn at Bange reached its maximum value (179.8 W m−2) in summer (July) (Fig. 7a), while the maximum value of R n at Lijiang occurred in spring (May; 201.4 W m−2) (Fig. 7b). G on the daily scale was a small component of the surface energy exchange at Bange and Lijiang, with annual means of 2.6 and −0.9 W m−2, respectively. The maximum H values were 94.2 and 116.2 W m−2 at Bange and Lijiang, occurring in June and March, respectively. The H values at Bange and Lijiang were much larger during spring (45.1 and 44.1 W m−2, respectively) than during the wet season (19.8 and 16.7 W m−2, respectively). Conversely, the LE values at Bange and Lijiang were larger during the wet Fig. 5 Daily average albedo at Bange and Lijiang from July 15, 2014, to July 14, 2015

season (63.9 and 53.6 W m−2, respectively) than spring (13.0 and 41.8 W m−2, respectively). In spring, the large difference in LE between sites was attributed to their different SWCs. In winter, both H and LE were low and constrained by the low Rn. The EF values at Bange and Lijiang were similar during the wet season of 2014, and they ranged between 0.6 and 0.9 (Fig. 7c). At most other times, the EF at Lijiang was larger due to the much higher SWC during winter and the early vegetation germination during the spring of 2015. When the surface was covered by snow, the EF was close to 1. 3.5 Diurnal and seasonal variations in the NEE The diurnal variation of the NEE in different growth stages is shown in Fig. 8. During the peak growth stage (August 2014), the maximum CO2 uptake and release for the monthly average diurnal cycles at Bange were −3.4 and 1.3 μmol m−2 s−1 (a negative NEE represents CO2 uptake), respectively. The amplitude of the NEE of the diurnal cycles at Lijiang was much larger, with maximum values of −11.1 μmol m−2 s−1 for CO2 uptake and 3.7 μmol m−2 s−1 for CO2 release. The maximum CO2 uptake occurred at 9:00 for Bange and 11:00 for Lijiang. The CO2 uptake was obviously smaller in the afternoon than in the morning at Bange, which has been referred to as a depression of CO2 exchange in previous studies of semiarid

Author's personal copy Water and CO2 fluxes over semiarid alpine steppe and humid alpine Fig. 6 Average diurnal variations in net radiation (Rn), soil heat flux (G), sensible heat flux (H), and latent heat flux (LE) for a the wet season at Bange, b the dry season at Bange, c the wet season at Lijiang, and d the dry season at Lijiang

typical steppes (Fu et al. 2006; Wang et al. 2016b). However, this phenomenon was not clear at Lijiang, where the mean NEE values in the morning and afternoon were similar. In the transition period from the wet season to the dry season (October), photosynthesis in the daytime and respiration in the nighttime were weakened compared with the peak growth stage. During the winter, the NEE was close to zero throughout the day at the two sites. During the early growing season (May), the maximum CO2 uptake occurred in the morning,

and it was related to the low levels of photosynthesis and light saturation. The NEE at Bange and Lijiang showed clear seasonal variation (Fig. 8c). During the wet season, CO2 uptake at Lijiang was much greater than that at Bange. The maximum daily CO2 uptake at Lijiang was −3.7 g C m−2 d−1 in July, while the maximum value for Bange was −1.2 g C m−2 d−1 at the end of August. The NEE at both sites increased clearly in September and became positive in late October 2014. The two ecosystems released CO2 in the dry season, and the maximum CO2 release (1.43 g C m−2 d−1) occurred at Lijiang on April 8. The Lijiang alpine meadow started to absorb CO2 in April 2015, which was earlier than the ecosystem at Bange. It is reasonable that the mean CO2 uptake values for Bange and Lijiang were −0.13 and −0.80 g C m−2 d−1, respectively, in May 2015. Subsequently, the drought in June decreased CO2 uptake. For example, the mean CO2 uptake at Lijiang decreased from 1.73 g C m−2 d−1 on May 23 to 0.03 g C m−2 d−1 on June 15.

4 Discussion 4.1 Controls of EF and ET

Fig. 7 Seasonal variations in daily average of net radiation (Rn), soil heat flux (G), sensible heat flux (H), and latent heat flux (LE) at a Bange and b Lijiang and c EF at both sites from July 15, 2014, to July 14, 2015

The daily mean EF at Bange was highly correlated with the SWC in the wet season (R2 = 0.89; Fig. 9a). This was consistent with the relationship between evaporation and SWC, which has been found to be strong and linear (Moran et al. 2009; Wang et al. 2016c). A possible reason is that evaporation was dominant in ET at Bange because of the low and

Author's personal copy L. Wang et al. Fig. 8 Monthly average diurnal variations in NEE at a Bange and b Lijiang for August 2014, October 2014, February 2015, and May 2015 and c seasonal variation in NEE at Bange and Lijiang from July 15, 2014, to December 31, 2015

sparse grass (the grass height was 2–3 cm, and the green cover ratio was less than 20%). In addition to the SWC, the 16-day average NDVI also significantly affected the EF at Bange (Fig. 9c) because the SWC and NDVI were tightly coupled. This dominant vegetation control of the EF also occurred for moist grassland in the Southern Great Plains of the USA (R2 = 0.65) because of the large proportion of transpiration to ET (Williams and Torn 2015). However, the coupling strength of the EF and SWC was very weak over this grassland area in the USA (R2 = 0.21; Phillips and Klein 2014). Specifically, both the SWC and vegetation growth controls on the EF at Lijiang were weak (Fig. 9b, c) because the atmosphere at Lijiang was close to saturation and the range of the EF was much narrower (0.75 ± 0.07; mean ± SD). On a daily scale, the control of LE depends on the soil water conditions. At Bange, the main controlling variable of the LE varied by year. In the 2014 wet season, the soil at Bange was relatively wet, and the mean SWC was 0.15 m3 m−3. The LE at Bange was mainly correlated with Rn (R2 = 0.58), and the R2 between the LE and SWC was 0.06. In the 2015 wet season, when the soil was dry (the mean SWC was 0.08 m3 m−3), the LE at Bange was highly correlated with the SWC (R2 = 0.68), but the correlation between LE and Rn Fig. 9 Relationship between the EF and SWC at a Bange and b Lijiang and c relationship between the 16-day average EF and NDVI

was very low (R2 = 0.08). At Lijiang, the LE was primarily controlled by Rn (R2 = 0.60 in 2014 and 0.64 in 2015) because the mean SWC was high (approximately 0.30 m3 m−3) in both wet seasons. On an annual basis, the ratio of ET to PPT (ET/PPT) in the dry year of 2015 at Bange was 1.36 (204.3/149.8 mm), and a portion of the evaporated water came from belowground. The ratio at Lijiang was 0.43 (542.7/1257.4 mm), and most of the PPT was lost in the form of streamflow. Both ratios are within the normal range for grassland ecosystems (40–150%; Li et al. 2007). For example, the ratio for Lijiang was close to the ratio reported for an annual grassland in California (54–59%; Baldocchi et al. 2004) because the evaporative water was near the threshold of supporting grass. At Bange, the ET exceeded the PPT, which also occurred for rangeland in Oklahoma (Meyers 2001), especially during drought periods. 4.2 Control of CO2 exchange At the half-hourly scale, the NEE deceased with increasing PAR until a critical value of PAR was reached (Fig. 10a). This critical value was approximately 1200 μmol m−2 s−1 for Bange and Lijiang. The Michaelis-Menten model was used


Fig. 10 Relationship between a the daytime NEE and PAR for August and September 2014, b the nighttime NEE and Ts at the 5-cm depth, and c the 16-day average NEE and NDVI at Bange and Lijiang. The dependent

and independent variables were averaged with a PAR bins of 100 μmol m−2 s−1 and b Ts bins of 1 °C

to derive the regression between the daytime NEE and PAR. Under similar radiation conditions, CO2 uptake was much larger at Lijiang than at Bange. During the peak growth stage (August), the NEEsat at Bange was −5.7 μmol m−2 s−1, 24% of that at Lijiang (−23.6 μmol m−2 s−1). The NEEsat at Lijiang was larger than that for an alpine meadow on the QinghaiTibetan Plateau (20.4 μmol m−2 s−1; Kato et al. 2004) but smaller than that for a Mediterranean C3/C4 grassland (−31.0 μmol m−2 s−1; Aires et al. 2008). The NEEsat at Bange was smaller than that for a semiarid typical steppe (−8.9 μmol m−2 s−1; Wang et al. 2016b) and similar to that for a desert grassland (−4.9 μmol m−2 s−1; Yang et al. 2011) a n d a S t i p a k r y ro i i s t e p p e i n I n n e r M o n g o l i a (5.8 μmol m−2 s−1; Zhang et al. 2007). At both Bange and Lijiang, NEE s a t decreased in September (−3.9 and 19.1 μmol m−2 s−1, respectively). Q10 is always used to determine ecosystem respiration in models. The difference of Q10 between Bange and Lijiang was large. The Q10 values at Bange and Lijiang were 1.7 and 3.4, respectively (Fig. 10b), both of which were within the normal range for grassland ecosystems (1.2 to 3.7) (Falge et al. 2001). The Q10 at Bange was similar to that of a desert grassland (1.6; Yang et al. 2011) and smaller than that of a semiarid typical steppe (Zhang et al. 2007; Wang et al. 2016b). The Q10 at Lijiang was larger than those for an alpine meadow and a Mediterranean grassland (2.3 and 2.4; Kato et al. 2004; Aires et al. 2008). This is possible because of the large soil heat capacity and the narrow range of soil temperature at Lijiang. On a 16-day time scale, the NEE was related to the NDVI at both Bange and Lijiang (Fig. 10c). Because the annual maximum NDVIs in 2015 were 0.22 and 0.72 at Bange and Lijiang, respectively, the annual CO2 uptake at Lijiang (230.0 g C m−2 yr−1) was much larger than that at Bange (−21.8 g C m−2 yr−1).

investigated based on 1.5-year EC measurements. The mean SWCs of the 2015 wet seasons were 0.08 and 0.28 m3 m−3, and the maximum NDVIs were consequently 0.22 and 0.72 for Bange and Lijiang, respectively. These caused distinct differences in the EF, LE, and NEE and their controls at Bange and Lijiang. In the wet season, the daily mean EF at Bange was highly correlated with the SWC, while the EF at Lijiang was relatively constant, and its relationship with the SWC and NDVI was weak. The main factors controlling the daily mean LE were Rn in the normal year of 2014 and SWC in the dry year of 2015. Under good soil water conditions, Rn primarily controlled the daily mean LE at Lijiang throughout the measurement period. On an annual scale, the ratios of ET to PPT were 1.36 and 0.43 at Bange and Lijiang, respectively. The NEEsat at Bange was 24% of that at Lijiang due to the large difference in the NDVI. The Q10 at Bange and Lijiang were 1.7 and 3.4, respectively, within the normal range. Over longer time scales, the NEE was mainly controlled by the NDVI. The annual NEEs at Bange and Lijiang were 21.8 and −230.0 g C m −2 yr −1 , respectively. In the future, the partitioning of ET (evaporation and transpiration) and NEE (ecosystem production and respiration) at Bange and Lijiang is still needed to further analyze the controls of the water and CO2 fluxes.

5 Conclusions The factors that control the EF, water, and CO2 exchange over a semiarid alpine steppe and a humid alpine meadow were

Acknowledgements This study was supported by the National Natural Science Foundation of China (grant nos. 91537212 and 41275023), the Third Tibetan Plateau Scientific Experiment: Observations for Boundary Layer and Troposphere (GYHY201406001), and the NSFC project (41305012). The staffs from Bange and Lijiang Meteorological Administration are also greatly appreciated for their helps in the maintenance of the measurements.

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