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The Tibetan Plateau (TP) is one of the most important areas for the study of the carbon budgets of terrestrial ecosystems. However, the estimation of the net ecosystem productivity (NEP) remains uncertain in this region due to its complex topographic properties and climatic conditions. Using CO2-eddy-covariance-flux data from 1982 to 2018 at 18 sites distributed around the TP grassland, we analyzed the spatial–temporal patterns of the grassland NEP and its driving factors from 1982 to 2018 using a random forest (RF) model. Our results showed that the RF model captured the size of the carbon sink (R2 = 0.65, p < 0.05) between the observed and simulated values for the validation samples. During the observation period, the grassland acted as a carbon sink of 26.2 Tg C yr−1 and increased significantly, by 0.4 g C m−2 yr−1. On a regional scale, the annual NEP gradually increased from the northwest to the southeast, and a similar pattern was also observed in the long-term trends. Furthermore, the moisture conditions, such as the specific humidity and precipitation, were proven to be the main driving factors of the carbon flux in the southeastern areas, while the temperature predominantly controlled the carbon flux in the northwest. Our results emphasize the net carbon sink of the TP and provide a reliable way to upscale NEP from sites. [ABSTRACT FROM AUTHOR]

• Elevated CO 2 significantly advanced spring phenology in kobresia pygmaea. • Alpine plants on the tibetan plateau are more sensitive to elevated CO 2. • Elevated CO 2 reduced overlapping flowering between species. • Advancement in K. pygmaea's phenology was linked to higher water-use efficiency. Phenological changes in global vegetation are often attributed to climate warming. However, climate warming and elevated atmospheric CO 2 concentration (e CO 2) are two co-occurring global change factors, and how e CO 2 would affect vegetation phenology has received less attention. The partial pressure of atmospheric CO 2 on the Tibetan Plateau (TP) is lower than that in regions of lower altitudes. Consequently, the growth and phenology of alpine plants in this region could be more sensitive to e CO 2 , but this hypothesis is not yet supported by empirical evidence. Here we explored the effect of e CO 2 on plant phenology (including phenophases of green-up, budding, and flowering) through a 5-year field manipulation experiment in a high-altitude (4600 m above sea level) alpine grassland on the TP. Our results showed that e CO 2 significantly advanced the spring phenology of an early-flowering species (Kobresia pygmaea), while it had no impact on the phenology of two mid-flowering species (Potentilla saundersiana and Potentilla cuneata). Compared to other low-altitude regions, plant phenology on the TP underwent greater alterations under e CO 2 , which supports our hypothesis that the growth of high-altitude plants is more sensitive to e CO 2. Furthermore, we found that e CO 2 significantly reduced the overlapping of flowering between contrasting plant species, mainly due to the phenological advancement of the K. pygmaea induced by e CO 2. The observed advancement of the spring phenology in K. pygmaea under e CO 2 was associated with increasing ecosystem water-use efficiency (WUE), thereby advancing its subsequent phenological development, such as budding and flowering. Our findings provide experimental evidence that atmospheric CO 2 enrichment can accelerate plant growth processes in high-altitude regions, and suggest that large-scale model simulations should consider the effects of elevated atmospheric CO 2 concentration on plant growth and phenology. [Display omitted] [ABSTRACT FROM AUTHOR]

Aim: Ecosystem carbon use efficiency (CUEe) is a core parameter of ecosystem process models, but its relationships with climate are still uncertain, especially for ecosystems with harsh environments. Large inconsistencies in climate impacts on the CUEe have been reported among various spatial scales. The goal of this study was to examine whether warming promotes or restricts the CUEe and whether the CUEe responds to a warming gradient in a linear or nonlinear manner. Location: Tibetan Plateau. Time period: 2000–2018. Major taxa studied: Alpine grassland ecosystem. Methods: We integrated multiple‐source data of carbon fluxes and CUEe, including warming experiments at a site scale, eddy covariance observations at a landscape scale and synthesized warming experiments and ecosystem process models at a regional scale. Next, we deployed a statistical model to examine the warming impacts on the CUEe across scales; the effects of biotic and abiotic factors on the CUEe and its components were summarized based on the results of standardized major axis tests and routines, structural equation modelling and nonlinear models. Results: This study reported a suppressive warming impact on the CUEe, which followed a nonlinear curve with severe inhibition in the high‐level warming treatment. With a warming threshold of 1.5–2.0°C, CUEe response patterns transitioned from no change to a significant decrease. The restriction effects can be ascribed to the joint adverse and asymmetric effects of warming on CUEe components under multiple‐level warming. Warming‐modified relationships among CUEe components and the nonlinear effects of biotic and abiotic factors led to the nonlinear responses of CUEe to warming. Main conclusions: This study revealed suppressive and nonlinear effects of warming on the CUEe, including especially dramatic CUEe decreases with high‐level warming. These findings are critical for optimizing model parameters and improving predictions of the carbon sequestration capacity of alpine grasslands. [ABSTRACT FROM AUTHOR]

As the recent global warming hiatus and the warming on high elevations are attracting worldwide attention, this study examined the robustness of the warming slowdown over the Tibetan plateau (TP) and its related driving forces. By integrating multiple-source data from 1982 to 2015 and using trend analysis, we found that the mean temperature (Tmean), maximum temperature (Tmax) and minimum temperature (Tmin) showed a slowdown of the warming trend around 1998, during the period of the global warming hiatus. This was found over both the growing season (GS) and non-growing season (NGS) and suggested a robust warming hiatus over the TP. Due to the differences in trends of Tmax and Tmin, the trend of diurnal temperature range (DTR) also shifted after 1998, especially during the GS temperature. The warming rate was spatially heterogeneous. The northern TP (NTP) experienced more warming than the southern TP (STP) in all seasons from 1982 to 1998, while the pattern was reversed in the period from 1998 to 2015. Water vapour was found to be the main driving force for the trend in Tmean and Tmin by influencing downward long wave radiation. Sunshine duration was the main driving force behind the trend in Tmax and DTR through a change in downward shortwave radiation that altered the energy source of daytime temperature. Water vapour was the major driving force for temperature change over the NTP, while over the STP, sunshine duration dominated the temperature trend. [ABSTRACT FROM AUTHOR]

Climate change is receiving mounting attentions from various fields and phenology is a commonly used indicator signaling vegetation responses to climate change. Previous phenology studies have mostly focused on vegetation greening-up and its climatic driving factors, while autumn phenology has been barely touched upon. In this study, vegetation phenological metrics were extracted from MODIS NDVI data and their temporal and spatial patterns were explored on the Tibetan Plateau (TP). The results showed that the start of season (SOS) has significantly earlier trend in the first decade, while the end of season (EOS) has slightly (not significant) earlier trend. In the spatial dimension, similar patterns were also identified. The SOS plays a more significant role in regulating vegetation growing season length than EOS does. The EOS and driving effects from each factor exhibited spatially heterogeneous patterns. Biological factor is the dominant factor regulating the spatial pattern of EOS, while climate factors control its inter-annual variation. [ABSTRACT FROM AUTHOR]

• This is the CO 2 enrichment field experiment conducted on the highest elevation. • Elevated CO 2 (eCO 2) stimulates both GEP and ER, and causes a neutral effect on NEP. • Simultaneous N addition and eCO 2 stimulate carbon sink. • Excessive precipitation suppresses the eCO 2 effects on carbon fluxes. It's generally believed that elevated CO 2 (eCO 2) could stimulate plant growth and the ecosystem carbon (C) sink. However, great uncertainties exist in terms of the CO 2 fertilization effect (CFE) magnitude, and how it is regulated by other global change factors. The lack of experimental evidence from the Alpine Region also limits our cognition on the CFE. By conducting a five-year manipulative field experiment in a semi-arid grassland of the Tibetan Plateau, we are aimed to explore the behavior of ecosystem C exchange in response to eCO 2 and N availability under contrasting natural precipitation regimes. The experiment showed that eCO 2 stimulated both gross ecosystem productivity (GEP) and ecosystem respiration (ER), and resulted in a neutral effect on net ecosystem productivity (NEP). The reduction of leaf N concentration under eCO 2 constrained the eCO 2 effects on C fluxes, especially on GEP and NEP. As N addition replenishes N availability in soil and leaf, GEP benefited more from the N addition than the ER. The eCO 2 strengthened the C sink when exogenous N was added simultaneously. Furthermore, precipitation variability played an importance role in mediating the eCO 2 effect among growing seasons. The eCO 2 effects on C fluxes tended to decline with increased water availability. The CFE was suppressed with excessive precipitation when the water-use efficiency (WUE) response was weak and eCO 2 -induced water-saving disappeared. The negative impact of precipitation on the CFE may also be attributed to the short precipitation intervals and insufficient radiation caused by high-frequency precipitation. Our study demonstrates that eCO 2 only stimulates net C uptake under conditions of N addition or during drier periods. Given the widespread N limitation, the efficacy of terrestrial ecosystems in mitigating climate change under rising CO 2 may be weaker than projected and is closely related to the precipitation variability. [ABSTRACT FROM AUTHOR]

The Tibetan Plateau is the highest and largest plateau in the world, hosting unique alpine grassland and having a much higher snow cover than any other region at the same latitude, thus representing a "climate change hot-spot". Land surface phenology characterizes the timing of vegetation seasonality at the per-pixel level using remote sensing systems. The impact of seasonal snow cover variations on land surface phenology has drawn much attention; however, there is still no consensus on how the remote sensing estimated start of season (SOS) is biased by the presence of preseason snow cover. Here, we analyzed SOS assessments from time series of satellite derived vegetation indices and solar-induced chlorophyll fluorescence (SIF) during 2003–2016 for the Tibetan Plateau. We evaluated satellite-based SOS with field observations and gross primary production (GPP) from eddy covariance for both snow-free and snow covered sites. SOS derived from SIF was highly correlated with field data (R2 = 0.83) and also the normalized difference phenology index (NDPI) performed well for both snow free (R2 = 0.77) and snow covered sites (R2 = 0.73). On the contrary, normalized difference vegetation index (NDVI) correlates only weakly with field data (R2 = 0.35 for snow free and R2 = 0.15 for snow covered sites). We further found that an earlier end of the snow season caused an earlier estimate of SOS for the Tibetan Plateau from NDVI as compared to NDPI. Our research therefore adds new evidence to the ongoing debate supporting the view that the claimed advance in land surface SOS over the Tibetan Plateau is an artifact from snow cover changes. These findings improve our understanding of the impact of snow on land surface phenology in alpine ecosystems, which can further improve remote sensing based land surface phenology assessments in snow-influenced ecosystems. Unlabelled Image • Four satellite indicators are tested to estimate spring phenology trends over the Tibetan Plateau. • The confounding effect of snow cover is evaluated. • The Normalized Difference Phenology Index provides best results for snow conditions. • Normalized Difference Vegetation Index SOS trends are biased by snow trends. [ABSTRACT FROM AUTHOR]

Global warming exerts profound impacts on terrestrial carbon cycles and feedback to climates. Ecosystem respiration (ER) is one of the main components of biosphere CO2 fluxes. However, knowledge regarding how ER responds to warming is still lacking. In this study, a manipulative experiment with five simulated temperature increases (+0℃ [Control], +2.1℃ [warming 1, W1], +2.7℃ [warming 2, W2], +3.2℃ [warming 3, W3], +3.9℃ [warming 4, W4]) was conducted to investigate ER responses to warming in an alpine meadow on the Tibetan Plateau. The results showed that ER was suppressed by warming both in dry and wet years. The responses of ER to warming all followed a nonlinear pattern. The nonlinear processes can be divided into three stages, the quick‐response stage (W1), stable stage (W1–W3), and transition stage (W4). Compared with the nonlinear model, the linear model maximally overestimated the response ratios of ER to warming 2.2% and 3.2% in 2015 and 2016, respectively, and maximally underestimated the ratio 7.0% and 2.7%. The annual differences in ER responding to warming were mainly attributed to the distinct seasonal distribution of precipitation. Specially, we found that the abrupt shift response of ER to warming under W4 treatment in 2015, which might be regulated by the excitatory effect of precipitation after long‐term drought in the mid‐growing season. This study highlights the importance of the nonlinearity of warming effects on ER, which should be taken into the global‐C‐cycling models for better predicting future carbon–climate feedbacks. The responses of ER to warming all followed a nonlinear pattern. The nonlinear processes can be divided into three stages, the quick‐response stage (W1), stable stage (W1–W3), and transition stage (W4). The annual differences in ER responding to warming were mainly attributed to the distinct seasonal distribution of precipitation. Specially, we found that the distinct abrupt shift response of ER to warming under W4 treatment in 2015, which might be regulated by the excitatory effect of precipitation after long‐term drought in the mid‐growing season. [ABSTRACT FROM AUTHOR]

The Tibetan Plateau has experienced higher-than-global-average climate warming in recent decades, resulting in many significant changes in ecosystem structure and function. Among them is albedo, which bridges the causes and consequences of land surface processes and climate. The plateau is covered by snow/ice and vegetation in the non-growing season (nGS) and growing season (GS), respectively. Based on the MODIS products, we investigated snow/ice cover and vegetation greenness in relation to the spatiotemporal changes of albedo on the Tibetan Plateau from 2000 through 2013. A synchronous relationship was found between the change in GSNDVI and GSalbedo over time and across the Tibetan landscapes. We found that the annual average albedo had a decreasing trend, but that the albedo had slightly increased during the nGS and decreased during the GS. Across the landscapes, the nGSalbedo fluctuated in a synchronous pattern with snow/ice cover. Temporally, monthly snow/ice coverage also had a high correspondence with albedo, except in April and October. We detected clear dependencies of albedo on elevation. With the rise in altitude, the nGSalbedo decreased below 4000 m, but increased for elevations of 4500–5500 m. Above 5500 m, the nGSalbedo decreased, which was in accordance with the decreased amount of snow/ice coverage and the increased soil moisture on the plateau. More importantly, the decreasing albedo in the most recent decade appeared to be caused primarily by lowered growing season albedo. [ABSTRACT FROM AUTHOR]

• Atmospheric and soil droughts together greatly depressed GPP. • Atmospheric water deficits had stronger instant inhibited effects on GPP. • Drought-induced H enhancements provided positive feedback and aggravated GPP loss. Under global climate change, warmer temperatures and changing precipitation patterns will increase the relative importance of soil and atmospheric droughts in limiting productivities across different ecosystems, especially in the fragile and sensitive ecosystem on the Tibetan Plateau. Therefore, the present study examined the effects of atmospheric drought, soil drought and compound drought on alpine meadow productivity based on seven years of continuous observational data. The results demonstrated that water conditions dominated the variations in alpine meadow productivity in northern Tibet. Gross prime productivity (GPP) in the growing season was significantly inhibited during soil drought (defined as soil water content (SWC) < 0.11 m3 m−3) or atmospheric drought (defined as vapor pressure deficit (VPD) > 0.61 kPa) conditions. Pooling of the moisture data according to these thresholds revealed that soil drought occurred more frequently and accounted for 20.4% of all observed days. Atmospheric drought occurred in 2.4% of all days, and the frequency of compound drought (when SWC < 0.11 m3 m−3 and VPD > 0.61 kPa) was 8.7%. Compound drought reduced GPP by the largest extent to as much as 1.47 g C m−2 d−1. Atmospheric drought reduced GPP by 1.10 g C m−2 d−1. However, the more frequent soil drought reduced GPP by the smallest extent, with a value of 0.69 g C m−2 d−1. In spite of this, soil drought played an important role in restricting the accumulated GPP of this alpine meadow via its higher occurrence frequency and longer duration. All three types of drought significantly increased sensible heat flux (H), which provided positive feedbacks to droughts and led to further decreases in GPP. The results of the present study suggest that drought greatly threatens the alpine meadow ecosystem in northern Tibet. Therefore, more attention should be paid to water conditions under the global climate change context. [ABSTRACT FROM AUTHOR]

As a key biotic factor, phenology exerts fundamental influences on ecosystem carbon sequestration. However, whether spring phenology affects the subsequent seasonal ecosystem productivity and the underlying resource limitation mechanism remains unclear for the alpine grasslands of the Tibetan Plateau (TP). In this study, we investigated the direct and lagged seasonal responses of net primary productivity (NPP) to the beginning of growing season (BGS) along a precipitation gradient by integrating field observations, remote sensing monitoring and ecosystem model simulations. The results revealed distinct response patterns of seasonal NPP to BGS. Specifically, the BGS showed a significant and negative correlation with spring NPP (R = −0.73, p < 0.01), as evidenced by the direct boosting effects of earlier BGS on spring NPP. Moreover, spring NPP was more responsive to BGS in areas with more annual precipitation. The boosting effects of earlier BGS on NPP tended to weaken in summer compared with that in spring. Sequentially, BGS exhibited stronger positive correlation with autumn NPP in areas with less annual precipitation, which suggested the enhanced lagged suppressing effects of earlier spring phenology on ecosystem carbon assimilation during the later growing season under aggravated water stress. Overall, the strengthened NPP in spring was offset by its decrement in autumn, resulting in no obvious relationship between BGS and annual NPP (R = −0.34, p > 0.05) for the entire grasslands on the TP. The findings of this study imply that the lagged effects of phenology on the ecosystem productivity during the subsequent seasons should not be neglected in the future studies. [ABSTRACT FROM AUTHOR]

• The soil water content and temperature determined the maximum photosynthesis. • Sufficient soil water elevated the optimal temperature and maximum photosynthesis. • The optimal environmental configuration for the alpine meadow was detected. • If soil water is high, higher temperature and VPD are beneficial for carbon uptake. The changes in the carbon sink function of sensitive and fragile alpine ecosystems under climate change are widely concerned. Radiation, temperature, and water resources play important roles in regulating carbon assimilation. Therefore, it is important to detect the optimal configuration of environmental factors for the photosynthesis of alpine meadow ecosystems. In this study, based on 10-year flux and corresponding environmental observations in a typical alpine meadow ecosystem, the optimal environmental configuration that could realistically occur in the natural world for photosynthesis was detected. The results indicated that the temperature and water conditions were the primary limiting factors, as radiation resources are abundant on the Tibetan Plateau. The optimal temperature for photosynthesis (TA opt) was determined to be 11.92 ℃ based on the entire observation dataset, with a maximum photosynthetic capacity (GPP max) of 0.20 mg CO 2 m−2 s−1. However, the TA opt and the corresponding GPP max could be elevated when the soil water content (SWC) was abundant. When SWC exceeded 0.26 m3 m−3, TA opt increased to 13.41 ℃, and the corresponding GPP max reached as high as 0.30 mg CO 2 m−2 s−1, which is the maximum the ecosystem could reach. Thus, the optimal environmental configuration for photosynthesis that could realistically occur in the natural world in this alpine meadow was air temperature (TA)=13.41 ℃, SWC=0.28 m3 m−3, vapor pressure deficit (VPD)=0.61 kPa, net radiation (RN)=411.53 J m−2 s−1. According to this study, it could be inferred that within a reasonable range, higher temperatures and VPD would benefit photosynthesis in this alpine meadow as long as the soil water supply is sufficient. The climate on the Tibetan Plateau is arid with high VPD and is experiencing warming due to climate change. Consequently, alpine ecosystems situated in relatively humid regions with sufficient soil water resources may exhibit greater potential for carbon sequestration than previously estimated. [ABSTRACT FROM AUTHOR]

Greening of the Earth is among the world's most important ecological changes in recent decades, but its effects and triggers are multifaceted and varied across the globe. Climate change is the primary driver for greening of the Tibetan Plateau (TP), where vegetation is of great significance for resident survival, carbon sequestration and climate feedbacks. However, the spatial variability of greening and its drivers are not well-understood on the TP. Here, we compiled multi-source vegetation indices, climate and atmospheric data to investigate the greenness spatiotemporal variability and its linkage with large-scale climate oscillations. We found that greening dominated the TP during 1980s–2010s and positive trends in satellite-based vegetation indices account for >70% of the TP. Specifically, the arid and semi-arid TP experienced a significant and higher greening rate due to the remarkable increase in precipitation. We further clarified the domains of the South Asian monsoon (SAMI1 and SAMI2), the westerly wind (WI) and the plateau monsoon (TPMI) on the TP, and realized that the wetness increase in the arid and semi-arid was correlated with a weakening WI. Detecting the linkage between WI with large-scale climate oscillations, we suggested that the decadal weakening WI was induced by a warmer North Atlantic SST (positive AMO phase), which triggered wave train anomalies and regulated zonal and meridional vapor transport across Eurasia. Generally, during the weak/negative WI phase, anticyclone anomalies centered over Mongolia may strongly reduce vapor output across the east boundaries, while cyclone anomalies centered over the northwest outside the TP likely enhance meridional vapor transport into the western TP from the south ocean. Eventually, precipitation over the arid and semi-arid TP increased significantly in the past four decades, that stimulated plant growth and greening pattern on the TP. Our results established the linkage among climate oscillations, regional climate alteration and vegetation greening, providing a more thorough understanding in terrestrial greening. • Approximately 70% areas of Tibetan Plateau (TP) were greening at the past four decades. • The greening of TP was dominated by the wetting in arid and semi-arid areas. • The decadal weakening of westerly wind dominated wetting over the TP, thus actuated the greening of TP. [ABSTRACT FROM AUTHOR]

• Water conditions dominated plant phenology and photosynthetic capacity. • Plant phenology and photosynthetic capacity co-determined to annual GPP. • Plant photosynthetic capacity dominated annual GPP. Under global climate change, climate warming and changing precipitation patterns would greatly affect plant phenology and photosynthetic capacity and hence affect the gross primary productivity (GPP) of the sensitive alpine meadow. To deeply understand the variations in GPP, it is important to clarify the joint effects of plant phenology and photosynthetic capacity. In this study, 10-year continuous flux and microclimatic data and in situ phenology period observation data were used to explore the roles of growing season length (GSL) and the seasonal maximal capacity of carbon uptake (GPP max) on annual cumulative gross primary productivity (GPP ann). The results indicated that the temperature and water conditions, especially water conditions, dominated GSL and GPP max and hence determined GPP ann. Both GSL and GPP max were dominated by annual precipitation, and the soil water content in August also showed a strong influence over them. GSL and GPP max together could explain 87% of the variation in GPP ann. Compared with GSL, GPP max had stronger effects on the interannual variability (IAV) in GPP ann. This study provides a new perspective on clarifying the proximate causes of IAV in GPP ann. The IAV in GPP ann of alpine ecosystems could be determined by these two key underlying processes, the length of the carbon uptake period and amplitude. These results are of critical importance for improving our ability to predict future changes in alpine meadow ecosystems. [ABSTRACT FROM AUTHOR]

Summary Cosmic-ray neutron sensing (CRNS) is a new method for continuously monitoring mean soil water content (SWC) on a hectometer scale. To evaluate the application and accuracy of the method for SWC observation in an alpine meadow ecosystem (AME), we installed the CRNS in a flat meadow near the Naqu prefecture on the northern Tibetan Plateau. We collecting soil samples and applying the system by the oven-drying method. A weather station was also installed near the CRNS for monitoring basic meteorological variables and the soil temperature and water content at various depths. Three Em-50 instruments for monitoring SWC and soil temperature were buried in three sub-quadrats northwest, northeast and southeast of the CRNS at distances of 460, 370 and 373 m, respectively, to observe the variation of SWC at the various depths. The footprint of the CRNS for SWC observation in the meadow was about 580 m, and the mean measuring depth was about 31 cm according to the general calculation equations. The reference neutron flux for dry soil ( N 0 ) had a mean and coefficient of variation of 8686 and 3%, respectively, and remained substantially invariant throughout the measuring period. The five SWCs from the independent field samples almost passed through the SWC trend of the CRNS, the root mean square error (RMSE) was 0.011 m 3 m −3 for the CRNS and oven-drying method. The time series of SWC measured by the CRNS agreed well with the mean SWC series to a depth of 20 cm measured by the weather station. The trend of SWC measured by the Em-50s generally agreed with the trend of SWC measured by the CRNS, but some values and variations of SWC differed between the Em-50s and CRNS data. Because of the good agreement between the CRNS and independent field samples, we suspect that this disagreement is due to an insufficient representativeness of point observations and the distances of the points from the CRNS. The diurnal variation of hourly SWC from the CRNS was sinusoidal during a dry period, peaking at 11:00 and was minimum at 18:00 (Beijing time), with a range of 1%. Overall, the CRNS measured SWC in the AME with an acceptable accuracy, providing a scientific basis for the promotion and application of the CRNS in high, cold ecosystems. [ABSTRACT FROM AUTHOR]

The Tibetan Plateau (TP), known as "The Roof of World", has expansive alpine grasslands and is a hotspot for climate change studies. However, cropland expansion and increasing anthropogenic activities have been poorly documented, let alone the effects of agricultural activities on food security and environmental change in the TP. The existing cropland mapping products do not depict the spatiotemporal characteristics of the TP due to low accuracies and inconsistent cropland distribution, which is affected by complicated topography and impedes our understanding of cropland expansion and its associated environmental impacts. One of the biggest challenges of cropland mapping in the TP is the diverse crop phenology across a wide range of elevations. To decrease the classification errors due to elevational differences in crop phenology, we developed two pixel- and phenology-based algorithms to map croplands using Landsat imagery and the Google Earth Engine platform along the Brahmaputra River and its two tributaries (BRTT) in the Tibet Autonomous Region, also known as the granary of TP, in 2015–2019. Our first phenology-based cropland mapping algorithm (PCM1) used different thresholds of land surface water index (LSWI) by considering varied crop phenology along different elevations. The second algorithm (PCM2) further offsets the phenological discrepancy along elevational gradients by considering the length and peak of the growing season. We found that PCM2 had a higher accuracy with fewer images compared with PCM1. The number of images for PCM2 was 279 less than PCM1, and the Matthews correlation coefficient for PCM2 was 0.036 higher than PCM1. We also found that the cropland area in BRTT was estimated to be 1979 ± 52 km2 in the late 2010s. Croplands were mainly distributed in the BRTT basins with elevations of 3800–4000 m asl. Our phenology-based methods were effective for mapping croplands in mountainous areas. The spatially explicit information on cropland area and distribution in the TP aid future research into the effects of cropland expansion on food security and environmental change in the TP. [ABSTRACT FROM AUTHOR]