Alpine Meadow Sites Research Articles

Biophysical factors control the interannual variability of evapotranspiration in an alpine meadow on the eastern Tibetan Plateau

Evapotranspiration (ET) is a key parameter regulating land–atmosphere interaction processes and the water cycle. The seasonal and interannual variability of ET and its environmental controls over an alpine meadow in a subfrigid humid zone of the Tibetan Plateau (TP) are reported on. Direct measurements were made using the eddy covariance method over a 10-year period with a significant increase in growing season length (GSL). The results showed that annual ET for the alpine meadow site was 492 ± 66 mm in comparison with 635 ± 88 mm of precipitation (P), with a ratio of ET/P ranging from 0.63 (2017) to 1.04 (2010). The path analysis and Priestley–Taylor coefficient (ET/ETeq) revealed that the daily ET experienced energy-limited and water-limited seasons within the year. During the water-limited non-growing season, the daily surface conductance (Gs) and ET/ETeq increased positively with soil water content (SWC). During the energy-limited growing season, the 16-day average ET, ET/ETeq, and Gs scaled positively with the normalized difference vegetation index (NDVI). ET/ETeq increased nonlinearly with an increase in Gs, but was insensitive to increases in Gs greater than the threshold Gs*. The mean Gs and Gs* were strongly regulated by the maximum NDVI. The maximum NDVI, Gs*, and annual mean Gs explained 46%, 80%, and 68%, respectively, of interannual variation in annual ET. Thus, we concluded that biophysical factors, rather than P and GSL, mainly controlled the interannual variability in annual ET. These findings are critical for understanding the response mechanism of ET to the changing biotic and abiotic conditions in the TP.

Inversion and Validation of FY-4A Official Land Surface Temperature Product

The thermal infrared data of Fengyun 4A (FY-4A) geostationary meteorological satellite can be used to retrieve hourly land surface temperature (LST). In this paper, seven candidate algorithms are compared and evaluated. The Ulivieri (1985) algorithm is determined to be optimal for the algorithm of FY-4A LST official products. The refined algorithm coefficients for distinguishing dry and moist atmosphere were established for daytime and nighttime, respectively. Then, FY-4A LST official products under clear-sky conditions are produced. The validation results show that: (1) Compared with in-situ measured LST data at the HeBi crop measurement network, the root mean square errors (RMSE) were 2.139 and 2.447 K. Compared with in-situ measured LST data at Naqu alpine meadow site of Tibet plateau, the RMSE was 2.86 K. (2) When compared with the MODIS LST product, the RMSE was 1.64, 2.17, 2.6, and 1.73 K in March, July, October, and December, respectively. By the bias long-time change at a single site, RMSE of the XLHT (city) and GZH (desert) sites were 2.735 and 2.97 K, respectively. Overall, the preferred algorithm exhibits good accuracy and meets the required accuracy of the FY-4A mission.

Response of forage nutrient storages to grazing in alpine grasslands

Forage nutrient storages can determine livestock size and husbandry development. There is insufficient research on the response of forage nutrient storages to grazing and related driving mechanisms in alpine grasslands, especially on the Tibetan Plateau. This study conducted a grazing experiment in three alpine grassland sites along an elevation gradient (two warm-season pastures and one cold-season pasture; two alpine steppe meadow sites and one alpine meadow) of Northern Tibet. Different types of alpine grassland ecosystems, at least for forage nutrient storages, may have different responses to grazing. Warm-season grazing significantly reduced crude protein (CP) storage, acid detergent fiber (ADF) storage, and neutral detergent fiber (NDF) storage of high-quality forage by 53.29, 63.82, and 63.26%, respectively, but cold-season grazing did not significantly alter the CP, ADF and NDF storages of high-quality forage. Warm-season grazing significantly reduced CP, ADF, NDF, crude ash (Ash), ether extract (EE) and water-soluble carbohydrate (WSC) storages of the plant community by 46.61, 62.47, 55.96, 64.94, 60.34, and 52.68%, and forbs by 62.33, 77.50, 73.69, 65.05, 57.75, and 62.44% in the alpine meadow site but not the alpine steppe meadow site, respectively. Plant species and phylogenetic diversity had different relationships with forage nutrient storages. The elevation distribution of forage nutrient storages under fencing conditions were different from those under grazing conditions. Therefore, cold-season grazing can have lower negative effects on forage nutrient storages than warm-season grazing. Combined plant species with phylogenetic diversity and composition can be better in predicting forage nutrient storages. Grazing can restructure the elevation distribution of forage nutrient storages in alpine grasslands.

Embedding trait‐based ecology within indigenous knowledge to advance sustainable management of Tibetan rangeland

AbstractQuestionsTraditional pastoralists attribute high numbers of plateau pikas (Ochotona curzoniae) to impoverished soils that favor more and better forage for pika, suggesting a bottom‐up control of pika density. Conversely, government policies focus on excessive numbers of this small mammalian herbivore as the primary top‐down cause of degradation in Tibetan rangeland. Despite concerted campaigns to reduce pika abundance in recent decades, the sustainability of Tibetan rangelands remains uncertain.LocationAlpine meadows on the Tibetan Plateau.MethodsWe proposed a conceptual model based on indigenous knowledge that predicted pika numbers from soil condition and plant traits. At three alpine meadow sites lightly grazed by livestock, we tested whether spatial variation in pika burrow density could be explained by changes in the functional composition of the plant community attributable to species turnover and intraspecific trait variation associated with changes in soil fertility.ResultsDue primarily to intraspecific trait variation, changes in the functional composition of the meadow community accounted for 56%–68% of the spatial variation in pika density, changes in the proportion of plant functional groups for 62%–74%, and changes in edaphic conditions for 71%–82%. Greater pika density was associated with a decline in soil phosphorus availability and a lower‐growing vegetation profile enriched in both the quantity and quality of forage preferred by pikas.ConclusionsThese results, which are in accord with indigenous knowledge, suggest that compensating soil phosphorus losses and maintaining a well‐calibrated grazing rotation can better manage pika populations and improve the sustainability of Tibetan rangelands. By combining trait‐based ecology and traditional knowledge, our study provides new insight into both understanding the dynamic complexity of grazing regimes and managing rangeland sustainability.

Optimization of canopy resistance models for alpine meadow in the northeastern Tibetan Plateau

• Twelve combination models of the Jarvis-type model were compared for canopy resistance estimation of alpine meadows. • Six-year eddy covariance data were used to evaluate the model performance. • The model 10 (M10) performed best for the alpine meadow. • Selection of appropriate stress functions is essential for canopy resistance modeling. Canopy resistance ( r c ) is a critical parameter for estimating vegetation transpiration. The site-specific r c can be calculated using the inversed Penman-Monteith (PM) equation with the effective leaf area index (LAI), which requires meteorological and turbulent flux data. The spatial distribution of r c is difficult to characterize due to the harsh environment of the Tibetan Plateau. The Jarvis-type model for modeling r c , described as a multiplicative function of environmental variables, has been widely used. However, the differences and optimization of different Jarvis-type models for alpine meadows have not been fully addressed. Consequently, our overall objective was to determine the appropriate functions for r c estimation and improve its accuracy for the alpine meadow ecosystem. Twelve Jarvis-type models composed of different stress functions were examined and compared with the observed r c calculated using PM equation at the Arou site in the northeastern Tibetan Plateau. The results suggest that the proper air temperature function and vapor pressure deficit function could improve model performance obviously. There was no obvious difference between the two different stress functions of downward shortwave radiation. The best model (M10), which was composed of an asymptotical function of downward shortwave radiation, a linear function of air temperature, an exponential function of vapor pressure deficit and a piecewise function of soil water content, had best performance with coefficient of determination of 0.93, root mean square error of 60.2 s m −1 and Nash-Sutcliffe efficiency coefficient of 0.92. The selection of proper stress functions is important for r c modeling. Models that considered the air temperature for r c calculations produced better results than those without temperature. The sensitivity analysis of r c to environmental variables indicated that r c was most sensitive to vapor pressure deficit, followed by LAI and downward shortwave radiation, whereas r c was less sensitive to soil water content. For all optimized parameters, r c was the most sensitive to k T (a fitting parameter for temperature), followed by k D (a fitting parameter for vapor pressure deficit) and r cmin (minimum r c under the optimal physiological condition). This study addresses the selection of proper stress functions in modeling r c for the alpine meadow site, which can also provide a reference for other ecosystems.

Nitrogen Addition Affects Ecosystem Carbon Exchange by Regulating Plant Community Assembly and Altering Soil Properties in an Alpine Meadow on the Qinghai-Tibetan Plateau.

Nitrogen (N) deposition can affect the global ecosystem carbon balance. However, how plant community assembly regulates the ecosystem carbon exchange in response to the N deposition remains largely unclear, especially in alpine meadows. In this study, we conducted a manipulative experiment to examine the impacts of N (ammonium nitrate) addition on ecosystem carbon dioxide (CO2) exchange by changing the plant community assembly and soil properties at an alpine meadow site on the Qinghai–Tibetan Plateau from 2014 to 2018. The N-addition treatments were N0, N7, N20, and N40 (0, 7, 20, and 40 kg N ha–1year–1) during the plant growing season. The net ecosystem CO2 exchange (NEE), gross ecosystem productivity (GEP), and ecosystem respiration (ER) were measured by a static chamber method. Our results showed that the growing-season NEE, ER and GEP increased gradually over time with increasing N-addition rates. On average, the NEE increased significantly by 55.6 and 65.2% in N20 and N40, respectively (p < 0.05). Nitrogen addition also increased forage grass biomass (GB, including sedge and Gramineae) by 74.3 and 122.9% and forb biomass (FB) by 73.4 and 51.4% in N20 and N40, respectively (p < 0.05). There were positive correlations between CO2 fluxes (NEE and GEP) and GB (p < 0.01), and the ER was positively correlated with functional group biomass (GB and FB) and soil available N content (NO3––N and NH4+–N) (p < 0.01). The N-induced shift in the plant community assembly was primarily responsible for the increase in NEE. The increase in GB mainly contributed to the N stimulation of NEE, and FB and the soil available N content had positive effects on ER in response to N addition. Our results highlight that the plant community assembly is critical in regulating the ecosystem carbon exchange response to the N deposition in alpine ecosystems.

Small Semi-Fossorial Herbivores Affect the Allocation of Above- and Below-Ground Plant Biomass in Alpine Meadows.

Small semi-fossorial herbivores can affect plant aboveground biomass (AGB) in grasslands and possibly alter the allocation of AGB and belowground biomass (BGB). In this study, plateau pika (Ochotona curzoniae) was used to investigate such effects at three alpine meadow sites on the Eastern Tibetan Plateau, where pairs of disturbed vs. undisturbed plots were randomly selected and sampled. We also explored the relationships between soil properties and BGB/AGB across the plots in the presence and absence of plateau pikas, respectively. We found that BGB and BGB/AGB were 11.40 and 8.20% lower in the presence of plateau pikas than in their absence, respectively. We also found that the BGB/AGB was positively related to soil moisture and soil total nitrogen (STN) in the absence of plateau pikas. In contrast, BGB/AGB was positively related to STN, soil organic carbon (SOC), soil carbon/nitrogen (C/N), and soil total phosphorus in the presence of plateau pikas. These factors indicated plateau pika disturbance increased AGB allocation. The relationship between AGB and BGB of alpine meadow plants to soil variables was also different between sites with and without plateau pika disturbance. In conclusion, small semi-fossorial herbivore disturbance is likely to alter grassland carbon stock and should be well controlled for sustainable conservation and management of alpine meadows on the Tibetan Plateau.

Marginal habitats provide unexpected survival benefits to the alpine marmot

AbstractAge‐specific survival trajectories can vary significantly among wild populations. Identifying the environmental conditions associated with such variability is of primary importance to understand the dynamics of free‐ranging populations. In this study, we investigated survival variations among alpine marmot (Marmota marmota) families living in areas with opposite environmental characteristics: the typical habitat of the species (alpine meadow) and a marginal area bordering the forest. We used data collected during an 11‐year study in the Gran Paradiso National Park (Italy) and performed a Bayesian survival trajectory analysis on marked individuals. Furthermore, we investigated, at a territorial level, the relationships among demographic parameters and habitat variables by using a path analysis approach. Contrary to our expectations, for most of the marmot's lifespan, survival rate was higher in the marginal site closer to the forest and with lower visibility than in the alpine meadow site. Path analysis indicated that the number of families living close to each other negatively affected the stability of the dominant couple, which in turn affected both juvenile survival and reproduction. Given the lower number of neighboring families which inhabited the marginal site and the potentially different predation pressure by the most effective predator in the area (Aquila chrysaetos), our results suggest that species adapted to live in open habitats may benefit from living in a marginal habitat. This study highlights the importance of habitats bordering the forest in the conservation of alpine marmots.

Bias Correction for ERA5-Land Soil Moisture Product Using Variational Mode Decomposition in the Permafrost Region of Qinghai–Tibet Plateau

Soil moisture (SM) is one of the key measures to understand the land-atmosphere interaction and permafrost dynamics in the Qinghai-Tibet Plateau (QTP). ERA5-Land is a new reanalysis product with high spatial resolution (9 km), which can provide long-term SM data with a large spatial coverage as well as at multi-layer soil depths. However, preliminary comparisons with <i>in-situ</i> data show that the ERA5-Land SM product generally underestimates the seasonal variability and demonstrate a positive bias on the QTP. In this paper, we proposed to utilize the mode decomposition method to correct such bias. Specifically, through using the variational mode decomposition (VMD), we decomposed the long time-series of ERA5-Land SM data into a series of Intrinsic Mode Functions (IMFs), and found that the SM seasonal variation can be well represented by the low-frequency modes, which were then selected to feed a regression model for the bias correction. The single-site bias correction results show that our method significantly improves the accuracy of ERA5-Land SM product with bias reduced by 0.22 m³&#x002F;m³, 0.31 m³&#x002F;m³, 0.15 m³&#x002F;m³ for alpine meadow, alpine steppe, and alpine desert sites respectively. Together with the slightly reduced accuracy but still acceptable results for the cross-site bias correction, we successfully demonstrate the potential of the mode decomposition method for the bias correction of the ERA5-Land SM product at regional scale. Our method is of great use to study climate impact on regional ecohydrological processes and the permafrost changes in the QTP region.

Synergetic variations of active layer soil water and salt in a permafrost-affected meadow in the headwater area of the Yellow River, northeastern Qinghai–Tibet plateau

The coupling effects and mechanisms of water, heat, and salt in frozen soils are considered to be one of the core scientific issues in frozen soil studies. This study was based on in situ observation data of active layer soil volumetric water content (VWC), temperature, and bulk electrical conductivity (EC) obtained at an alpine meadow site from October 2016 to November 2019. The site is located in the headwater area of the Yellow River (HAYR). We analyzed the synergetic variations of active layer soil VWC, temperature, and bulk EC during the freeze and thaw processes and discussed the underlying mechanisms. When the thaw process occurred from 10 to 80 cm depths, the VWC and bulk EC at a 10 cm depth showed synchronous high-frequency fluctuations and both increased linearly. The linear decreasing rate of the VWC (bulk EC) at an 80 cm depth in the freeze depths between 0 and 40 cm was 2 (1.6–2.3) times that of the VWC (bulk EC) at an 80 cm depth in the freeze depths occurring 0–10 cm. As soil temperature decreased in the frozen layer, unfrozen water content (bulk EC) decreased nonlinearly along with the absolute value of soil temperature (|T|), following a power (logarithmic) function. This study provided data that partly elucidate the interactions among permafrost, meadow, and ecohydrological processes in the HAYR. Also, our results can be used as a scientific basis for decision making on the protection and restoration of alpine grasslands, as well as for soil salinization studies.

The response of culturally important plants to experimental warming and clipping in Pakistan Himalayas.

The relative effects of climate warming with grazing on medicinally important plants are not fully understood in Hindukush-Himalaya (HKH) region. Therefore, we combined the indigenous knowledge about culturally important therapeutic plants and climate change with experimental warming (open-top chambers) and manual clipping (simulated grazing effect) and compared the relative difference on aboveground biomass and percent cover of plant species at five alpine meadow sites on an elevation gradient (4696 m-3346 m) from 2016-2018. Experimental warming increased biomass and percent cover throughout the experiment. However, the interactive treatment effect (warming x clipping) was significant on biomass but not on percent cover. These responses were taxa specific. Warming induced an increase of 1 ± 0.6% in Bistorta officinalis percent cover while for Poa alpina it was 18.7 ± 4.9%. Contrastingly, clipping had a marginally significant effect in reducing the biomass and cover of all plant species. Clipping treatment reduced vegetation cover & biomass by 2.3% and 6.26%, respectively, but that was not significant due to the high variability among taxa response at different sites. It was found that clipping decreased the effects of warming in interactive plots. Thus, warming may increase the availability of therapeutic plants for indigenous people while overgrazing would have deteriorating effects locally. The findings of this research illustrate that vegetation sensitivity to warming and overgrazing is likely to affect man-environment relationships, and traditional knowledge on a regional scale.

Sustained increase in soil respiration after nine years of warming in an alpine meadow on the Tibetan Plateau

Soil microbes are key determinants of soil carbon (C) dynamics. The response of the soil microbial community to climate warming modulates the feedback between ecosystem C cycling and future climate change. We conducted a long-term manipulative warming (1.6 °C increase of the soil temperature at 5 cm) experiment to examine the soil respiration, microbial biomass, and community composition at an alpine meadow site on the Qinghai-Tibetan Plateau. After nine years of warming, soil respiration (3.5 μmol m2 s−1 in control in the growing season) increased in the warmed plots. In the early growing season, the increase in heterotrophic respiration (Rh) accounted for more than 90% of the increase in soil respiration. The warming effect gradually decreased during the mid and late growing season (46%). Microbial biomass C and nitrogen declined significantly in the 0–10 cm to the 30–50 cm layer. Warming did not significantly affect microbial biomass C and N in any soil depth layer. Metabolic activity of microbes in terms of Rh per unit microbial biomass C significantly increased by 66% in warmed plots. The bacterial and fungal community composition did not significantly change in the warmed plots. The relative abundance of Actinobacteria decreased at 20–30 cm and 30–50 cm soil depths, but that of Cercozoa increased in all four soil layers. The relative Actinobacteria abundance was negatively correlated with Rh and metabolic activity in the 10–20 and 20–30 cm layers. Our results indicate a decrease in Actinobacteria abundance, increases in metabolic activity, and no substrate limitation sustained the positive warming effect on soil respiration throughout the last 9 years. This implies that climate warming could trigger a substantial loss of soil C to the atmosphere in the alpine meadow on the Qinghai-Tibet Plateau.

Response of Soil Microbial Communities to Warming and Clipping in Alpine Meadows in Northern Tibet

In order to explore responses of soil microbial communities among different alpine meadows under warming and clipping, soil microorganisms of three alpine meadow sites (low altitude: 4313 m, alpine steppe meadow, 30°30′ N, 91°04′ E; mid-altitude: 4513 m, alpine steppe meadow, 30°31′ N, 91°04′ E; and high altitude: 4693, alpine Kobresia meadow, 30°32′ N, 91°03′ E) were measured using the phospholipid fatty acid (PLFA) method. Both warming and clipping significantly reduced PLFA content and changed the community composition of soil microbial taxa, which belong to bacterial and fungal communities in the alpine Kobresia meadow. Warming significantly reduced the soil total PLFA content by 36.1% and the content of soil fungi by 37.0%; the clipping significantly reduced the soil total PLFA content by 57.4%, the content of soil fungi by 49.9%, and the content of soil bacteria by 60.5% in the alpine Kobresia meadow. Only clipping changed the total fungal community composition at a low altitude. Neither clipping nor warming changed the microbial community composition at a moderate altitude. Soil temperature, soil moisture, and pH were the main factors affecting soil microbial communities. Therefore, the effects of warming and clipping on soil microbial communities in alpine meadows were related to grassland types and soil environmental conditions.

Effect of disturbance by plateau pika on soil nitrogen stocks in alpine meadows

The presence of small burrowing herbivores often cause extensive disturbance to the grassland soil nutrient stocks. This study investigated the effect of small burrowing herbivore presence, the plateau pika (Ochotona curzoniae), and variation in disturbance intensity on soil nitrogen stocks at five alpine meadow sites. Disturbed plots occupied by plateau pikas consist of burrows and their associated bare soil patches in a matrix of continuous alpine meadow. The percentage of the bare soil area in a disturbed plot was used as a proxy for disturbance intensity of plateau pikas. This study showed that the soil total nitrogen stock was lower of bare soil than vegetated soil within disturbed plots, while ammonium nitrogen (NH4+-N), nitrate nitrogen (NO3−-N), inorganic nitrogen and ammonium nitrogen/nitrate nitrogen ratio (ANR) were higher in bare soil than vegetated soil within the disturbed plots. This study further showed that the NH4+-N, inorganic nitrogen and ANR of disturbed plots were higher than those of undisturbed plots, whereas soil total nitrogen and the NO3−-N stock was not different between disturbed plots and undisturbed plots. This study also showed that the soil total nitrogen, NH4+-N and inorganic nitrogen stocks of disturbed plots were higher at intermediate disturbance intensities of plateau pikas, whereas the NO3−-N stock and ANR were not related to disturbance intensities. These results suggest that the NH4+-N and inorganic nitrogen stocks were higher in the presence of plateau pikas, and intermediate disturbance intensity of plateau pikas was beneficial for soil quality due to higher soil total nitrogen, NH4+-N and inorganic nitrogen stocks. The findings of this study present a possible approach for estimating how the presence of a small burrowing herbivore influences grassland soil nitrogen stock.

Change in soil microbial biomass and regulating factors in an alpine meadow site on the Qinghai-Tibetan Plateau

ABSTRACT The change in soil microbial biomass (SMB) content and its direct links to soil organic matter (SOM) and environmental factors are not well understood for high-elevation regions. Therefore, this research investigated the temporal variation of SMB and its relationship with SOM and environmental factors in an alpine meadow site on the Qinghai-Tibetan Plateau. The soil organic carbon (SOC) and total nitrogen (TN) contents in alpine meadows showed monthly and seasonal variations and were higher in colder months, and the soil C/N ratio was higher in winter and in autumn than it was in the other seasons (P < 0.05). In addition, the changes in the SMB C and SMB N contents were notable at monthly and seasonal scales, whereas the SMB C and SMB N contents were higher in the winter and spring than they were in the other seasons (P < 0.05); the ratio of SMB C/SOC was higher in spring than it was in the other seasons; the ratio of the SMB N/TN content was higher in cold or cool months than it was in the other months; and the ratio of the SMB C/SMB N was highest in August (P < 0.05). Moreover, the SMB C and SMB N contents were significantly positively correlated to the SOC and TN contents (P < 0.01), and the SMB content was also significantly positively correlated to soil water content and air temperature (P < 0.05), or to soil bulk density and soil pH (P < 0.05). The results suggested that the change in the SMB in the alpine meadows was highly regulated by the SOC and TN and by monthly and seasonal changes in the soil bulk density, the soil pH, soil moisture, and air temperatures.

Representation of modern pollen assemblage to vertical variations of vegetation and climate in the Yadong area, eastern Himalaya

The presence of arboreal pollen grains in pollen assemblages collected from alpine meadow and steppe sites in the central Tibetan Plateau affects the reliability of pollen-based investigations of past vegetation and climate conditions, but their origin remains uncertain. In this study, we present a 39-sample modern pollen dataset from the Yadong area on the southern slope of the Himalayas and investigate the relationships between pollen assemblages and the vertical variations in vegetation and climate and the upward transport of arboreal pollen. Our results reveal that surface soil pollen assemblages were influenced heavily by local vegetation components; however, their variations along the elevation gradient generally reflect vertical variations in vegetation and climate. For instance, arboreal pollen abundances were high at low-elevation sites, whereas herbaceous pollen abundances were high at cold and dry sites. Arboreal pollen grains (dominantly Pinus, Betula and Picea) can be transported by wind to unforested areas, and the southern slope of the Himalayas is likely a source of the arboreal pollen grains in alpine meadows and alpine steppes in the central Tibetan Plateau. Thus the arboreal pollen grains should be treated with caution in the reconstructions of past vegetation and climate in the central Tibetan Plateau.

Temporal variations in soil CO2 efflux in an alpine meadow site on the Qinghai–Tibetan Plateau

AbstractTemporal variations in soil CO2 efflux and its direct links to soil microbial biomass and environmental factors in high‐elevation regions are poorly understood. In this study, I investigated the variations in soil CO2 efflux over time and their relationships to soil organic matter, soil microbial biomass and environmental factors in an alpine meadow site on the Qinghai–Tibetan Plateau. The results showed notable monthly to seasonal changes in soil CO2 efflux in alpine meadows. The soil CO2 efflux was lower overall from January to May and from October to December than from June to September (p &lt; 0.05); on a seasonal basis, the soil CO2 efflux was lower during the spring, autumn and winter than during the summer (p &lt; 0.05). In addition, linear regression analysis showed that the temporal variations in soil CO2 efflux over the whole year decreased with soil microbial biomass carbon content (p &lt; 0.05). Soil CO2 efflux was also positively related to air temperature and snow cover depth (p &lt; 0.05). Multiregression and principal component regression analyses indicated that temporal variations in soil CO2 efflux were mainly caused by monthly and seasonal changes in air temperature and soil microbial biomass.

Dynamic response of water retention to grazing activity on grassland over the Three River Headwaters region

Known as the “Chinese water tower,” the Three River Headwaters (TRH) region serves an important water retention function. In addition, grassland in this region is mainly used for grazing. However, grazing pressure potentially threatens the balance of this alpine grassland ecosystem. Thus, the influence of grazing activity on water retention needs to be explored. Based on remote sensing, meteorological, government statistics, hydrological records, and field observation data, we used the Integrated Valuation of Environmental Services and Tradeoffs model, grazing pressure index (GPI), and regression analysis to assess water yield (WY) variations and investigate the influence of grazing activity on water retention. The results showed that the grasslands were overloaded in the southern and eastern areas, whereas they had the capacity to carry more livestock in the western and central areas. Grazing pressure negatively impacts on WY in most of the TRH region. Source regions of the Yangtze, Yellow, and Lantsang Rivers also exhibited significant negative correlations between GPI and WY. Grazing decreased the soil water content in the alpine steppe, alpine meadow, and temperate steppe sites at soil depths of 0–30 cm, and a significant decrease was observed in the alpine meadow site at a depth of 10–20 cm. In all the areas, WY tended to decrease with GPI. Regression analyses indicate that GPI could better explain the total WY variation in the alpine meadow than in temperate steppe and alpine steppe. This implies that grazing activity has a negative effect on water retention, particularly in alpine meadows.

Potential of the remotely-derived products in monitoring ecosystem water use efficiency across grasslands in Northern China

ABSTRACTWater use efficiency (WUE) has been recognized as a crucial parameter to describe the interrelationship between carbon and water cycling. Quantitative assessment to spatiotemporal dynamics of ecosystem WUE in grasslands is of vital importance, given the large proportion of grasslands on the Earth’s land surface. Through continuous eddy covariance (EC) measurements at seven grassland sites in Northern China, this study examined the seasonal and interannual variations of gross primary production (GPP), evapotranspiration (ET) and WUE across four typical grassland ecosystems along a water availability gradient. The highest WUE occurred at the alpine meadow ecosystem with 1.45 g C kg−1 H2O, followed by the temperate meadow steppe and the typical steppe. The desert steppe had the lowest WUE with 0.53 g C kg−1 H2O. In addition, the remotely-derived WUE estimates from the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Breathing Earth System Simulator (BESS) GPP and ET products, were used to characterize the variability in tower-based WUE over these grassland ecosystems. Generally, WUEBESS had more consistent seasonal trajectories with WUEEC at most grassland sites compared to the variability of WUEMOD. The highest square of Pearson correlation coefficient (R2) values of WUEBESS were achieved in the alpine meadow sites (approximately 0.83), as well as the lowest root mean square error (RMSE), which ranged from 0.21 to 0.37 g C kg−1 H2O. However, the performances of both WUEBESS and WUEMOD lacked skills at the desert steppe sites which had low vegetation productivity. These remotely-derived WUE products, particularly the WUEMOD, tended to overestimate the annual mean WUEEC across these grassland types, with exception of the alpine meadow sites, where exhibited good performance. The underlying reasons for the biases of the MODIS- and BESS-based products in capturing the seasonal dynamics of grassland WUE were also examined. In general, GPPMOD performed better than GPPBESS over an 8-day period, whereas ETBESS had a higher accuracy compared to ETMOD across the different grassland ecosystems. Our analyses may be useful for improving the remote sensing-based GPP and ET products to accurately monitor the ecosystem WUE patterns of grasslands over large areas.

Comparison of the Vegetation Effect on ET Partitioning Based on Eddy Covariance Method at Five Different Sites of Northern China

Vegetation exerts profound influences on evapotranspiration (ET) partitioning. Many studies have demonstrated the positive impact of vegetation cover on the ratio of transpiration (T) to ET. Whether it is universally true with regard to different vegetation types and different sites is understudied. In this study, five sites in Northern China with different vegetation types were selected for comparison study.ET partitioning is conducted using an approach based on the concept of the underlying water use efficiency with eddy covariance measurements. The results show various patterns of vegetation’s effects over ET partitioning and, when compared with existing studies, also reveal a new relationship between the T/ET ratio and Normalized Difference Vegetation Index (NDVI) at some of the sites. At the alpine meadow site, the T/ET ratio gradually increase when NDVI is low and rapidly increase as NDVI go beyond a certain value, whereas at the arid shrub site, the T/ET ratio rapidly increase when NDVI is low and plateaus at a certain value when NDVI reaches a relatively high value. In deciduous forest, the T/ET ratio becomes unresponsive to NDVI beyond a threshold value. This study also reveals that irrigation schemes play a major role in determining the correlation between the T/ET ratio and NDVI because the T/ET ratio becomes well correlated with NDVI in case of flood irrigation and irrelevant to NDVI in the case of mulch drip irrigation. Furthermore, this study helps us to understand ET partitioning under different sites and different human activities such as irrigation. These findings can help policymakers to better understand the connection between vegetation and climate change or human activities and provide significant information for water management policy.