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Journal of Arid Land  2026, Vol. 18 Issue (1): 56-83    DOI: 10.1016/j.jaridl.2026.01.006     CSTR: 32276.14.JAL.20250380
Research article     
Multi-source remote sensing and machine learning reveal spatiotemporal variations and drivers of NPP in the Tianshan Mountains, China
LI Jiani1,2,3,4, XU Denghui1,2,3,4, XU Zhonglin1,2,3,4, WANG Yao1,5,*(), YANG Jianjun1,2,3,4
1College of Ecology and Environment, Xinjiang University, Urumqi 830017, China
2Key Laboratory of Oasis Ecology, Xinjiang University, Urumqi 830017, China
3Xinjiang Jinghe Observation and Research Station of Temperate Desert Ecosystem, Ministry of Education, Urumqi 830017, China
4Technology Innovation Center for Ecological Monitoring and Restoration of Desert-Oasis, Urumqi 830001, China
5Institute of Desert Meteorology, China Meteorological Administration, Urumqi 830002, China
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Abstract  

Arid mountain ecosystems are highly sensitive to hydrothermal stress and land use intensification, yet where net primary productivity (NPP) degradation is likely to persist and what drives it remain unclear in the Tianshan Mountains of Northwest China. We integrated multi-source remote sensing with the Carnegie-Ames-Stanford Approach (CASA) model to estimate NPP during 2000-2020, assessed trend persistence using the Hurst exponent, and identified key drivers and nonlinear thresholds with Extreme Gradient Boosting (XGBoost) and SHapley Additive exPlanations (SHAP). Total NPP averaged 55.74 Tg C/a and ranged from 48.07 to 65.91 Tg C/a from 2000 to 2020, while regional mean NPP rose from 138.97 to 160.69 g C/(m2•a). Land use transfer analysis showed that grassland expanded mainly at the expense of unutilized land and that cropland increased overall. Although NPP increased across 64.11% of the region during 2000-2020, persistence analysis suggested that 53.93% of the Tianshan Mountains was prone to continued NPP decline, including 36.41% with significant projected decline and 17.52% with weak projected decline; these areas formed degradation hotspots concentrated in the central and northern Tianshan Mountains. In contrast, potential improvement was limited (strong persistent improvement: 4.97%; strong anti-persistent improvement: 0.36%). Driver attribution indicated that land use dominated NPP variability (mean absolute SHAP value=29.54%), followed by precipitation (16.03%) and temperature (11.05%). SHAP dependence analyses showed that precipitation effects stabilized at 300.00-400.00 mm, and temperature exhibited an inverted U-shaped response with a peak near 0.00°C. These findings indicated that persistent degradation risk arose from hydrothermal constraints interacting with land use conversion, highlighting the need for threshold-informed, spatially targeted management to sustain carbon sequestration in arid mountain ecosystems.



Key wordsnet primary productivity (NPP)      Carnegie-Ames-Stanford Approach (CASA)      Hurst exponent      land use change      Extreme Gradient Boosting (XGBoost)      SHapley Additive exPlanations (SHAP)      hydrothermal thresholds     
Received: 25 September 2025      Published: 31 January 2026
Corresponding Authors: *WANG Yao (E-mail: wangyao@idm.cn)
Cite this article:

LI Jiani, XU Denghui, XU Zhonglin, WANG Yao, YANG Jianjun. Multi-source remote sensing and machine learning reveal spatiotemporal variations and drivers of NPP in the Tianshan Mountains, China. Journal of Arid Land, 2026, 18(1): 56-83.

URL:

http://jal.xjegi.com/10.1016/j.jaridl.2026.01.006     OR     http://jal.xjegi.com/Y2026/V18/I1/56

Fig. 1 Elevation (a) and land use type spatial distribution (b) in the Tianshan Mountains in Xinjiang Uygur Autonomous Region, China. DEM, digital elevation model.
Indicator Year Resolution Data source
Land use type 2000-2020 Annual, 500 m MCD12Q1 dataset (https://www.earthdata.nasa.gov/data/catalog/lpcloud-mcd12q1-061)
NDVI 2000-2020 Monthly, 1 km MOD13A3 dataset (https://www.earthdata.nasa.gov/data/catalog/lpcloud-mod13a3-061)
NPP (g C/(m2•a)) 2020 Annual,
1 km
MOD17A3 dataset (https://www.earthdata.nasa.gov/data/catalog/lpcloud-mod17a3hgf-061)
Solar radiation (W/m2) 2000-2020 Monthly, 4 km TerraClimate (https://climate.northwestknowledge.net/TERRACLIMATE/)
Temperature (℃) 2000-2020 Monthly, 4 km TerraClimate (https://climate.northwestknowledge.net/TERRACLIMATE/)
Precipitation (mm) 2000-2020 Monthly, 4 km TerraClimate (https://climate.northwestknowledge.net/TERRACLIMATE/)
DEM (m) 2020 Annual, 30 m Geospatial Data Cloud (https://www.gscloud.cn/)
Slope (°) 2020 Annual, 30 m Calculated by DEM
Aspect 2020 Annual, 30 m Calculated by DEM
Population density (persons/km2) 2020 Annual,
1 km
Resource and Environmental Science Data Platform (https://www.resdc.cn)
GDP density
(×104 CNY/km2)
2020 Annual,
1 km
Resource and Environmental Science Data Platform (https://www.resdc.cn)
Nighttime light index 2020 Annual,
1 km
Resource and Environmental Science Data Platform (https://www.resdc.cn)
Human footprint 2020 Annual,
1 km
Scientific data (https://doi.org/10.6084/m9.figshare.16571064)
Grazing intensity (SU/hm2) 2020 Annual,
1 km
Global Resource Data Cloud (https://www.gis5g.com/)
Distance to the road (km) 2020 OpenStreetMap (https://www.openstreetmap.org/)
Distance to highway (km) 2020 OpenStreetMap (https://www.openstreetmap.org/)
Distance to water body (km) 2020 OpenStreetMap (https://www.openstreetmap.org/)
Distance to government center (km) 2020 National Bureau of Statistics (https://www.stats.gov.cn/)
Table 1 Data sources
Fig. 2 Flowchart of the analytical workflow. MODIS, Moderate Resolution Imaging Spectroradiometer; NDVI, normalized difference vegetation index; NPP, net primary productivity; CASA, Carnegie-Ames-Stanford Approach; CV, coefficient of variation; XGBoost, Extreme Gradient Boosting; SHAP, SHapley Additive exPlanations.
Product pair Variable Spearman Pearson RMSE MdAE MBE Threshold Number of hotspots Share of grids (%)
IGBP vs. CLCD Built% 0.80* 0.78* 2.34 0.00 0.11 5 314 3.03
Crop% 0.77* 0.86* 10.98 0.00 0.01 5 1660 16.01
IGBP vs. LUCC Built% 0.83* 0.78* 3.72 0.00 0.76 5 505 4.87
Crop% 0.82* 0.83* 12.31 0.00 0.55 5 1909 18.41
CLCD vs. LUCC Built% 0.80* 0.74* 3.96 0.00 -0.64 5 611 5.89
Crop% 0.88* 0.96* 5.92 0.00 -0.54 5 1080 10.42
Table S1 Consistency of Built% and Crop% among land-cover products at the 6 km grid scale
Fig. S1 Inter-product scatter plots of Built% and Crop% at the 6 km grid. The 1:1 line and ±5.00% bands are shown. (a), Built% compared between IGBP and CLCD; (b), Built% compared between IGBP and LUCC; (c), Built% compared between CLCD and LUCC; (d), Crop% compared between IGBP and CLCD; (e), Crop% compared between IGBP and LUCC; (f), Crop% compared between CLCD and LUCC. IGBP refers to the International Geosphere-Biosphere Programme (IGBP) land-cover classification scheme used by the MODIS land-cover product (MCD12Q1), LUCC refers to the Land Use and Cover Change (LUCC) dataset, and CLCD refers to the China Land Cover Dataset (CLCD).
Fig. S2 Differences between the National Tibetan Plateau Data Center (NTPDC) temperature/precipitation (1 km) and TerraClimate temperature/precipitation (4 km resampled to 1 km) in 2020. (a), pixel-wise temperature difference; (b), pixel-wise precipitation difference; (c), grid-level temperature difference aggregated to the analysis grid; (d), grid-level precipitation difference aggregated to the analysis grid.
Significance level Slope P (two-sided)
Extremely significant reduction Slope<0.0000 P<0.01
Significant reduction Slope<0.0000 0.01≤P≤0.05
No significant change P>0.05
Significant increase Slope>0.0000 0.01≤P≤0.05
Extremely significant increase Slope>0.0000 P<0.01
Table 2 Significance level and corresponding parameters for NPP trend
Category Parameter Note
Strong persistent degradation (SPD) Slope< -0.0005 and 0.65<H<1.00 Strong historical decline with high persistence, likely to continue as severe degradation.
Weak persistent degradation (WPD) Slope< -0.0005 and 0.50<H<0.65 Moderate historical decline with moderate persistence, likely to continue as mild degradation.
Weak anti-persistent degradation (WAPD) Slope< -0.0005 and 0.35<H<0.50 Weak trend reversal with mild degradation, likely to reverse to weak degradation.
Strong anti-persistent degradation (SAPD) Slope< -0.0005 and 0.00<H<0.35 Strong trend reversal with significant degradation, likely to reverse to strong degradation.
Strong persistent improvement (SPI) Slope≥0.0005 and 0.00<H<0.35 Strong trend reversal with continuous improvement, likely to continue with significant improvement.
Weak anti-persistent improvement (WAPI) Slope≥0.0005 and 0.35<H<0.50 Weak trend reversal with mild improvement, likely to continue with weak improvement.
Moderate anti-persistent improvement (MAPI) Slope≥0.0005 and 0.50<H<0.65 Moderate trend reversal with improvement, likely to reverse to moderate improvement.
Strong anti-persistent improvement (SAPI) Slope≥0.0005 and 0.65<H<1.00 Strong trend reversal with significant improvement, likely to reverse to strong improvement.
No significant change (NSC) -0.0005<Slope<0.0005 Stable state with no significant trend, slight fluctuation.
Table 3 Classification of future trends and corresponding parameters
Fig. 3 Area (a-c), amplitude (d), and percentage (e) variations of different land use types in the Tianshan Mountains from 2000 to 2020
Fig. 4 Comparison of CASA NPP with MODIS NPP (a) and NPP of field sampling data (b)
Variable Year Pearson Spearman RMSE MdAE MBE
Temperature 2000 0.99 0.99 0.78 0.22 -0.10
2005 0.99 0.99 0.80 0.26 0.11
2010 0.99 0.99 0.80 0.25 -0.00
2015 0.99 0.99 0.83 0.23 0.15
2020 0.99 0.99 0.90 0.42 0.17
Precipitation 2000 0.98 0.99 22.74 8.07 -3.30
2005 0.98 0.98 24.69 9.00 -0.01
2010 0.98 0.98 30.01 10.68 -12.51
2015 0.98 0.98 30.05 12.60 -11.25
2020 0.96 0.96 33.57 20.89 9.49
Table S2 Year-specific consistency between TerraClimate and National Tibetan Plateau Data Center (NTPDC) meteorological datasets at the 6 km grid
Fig. 5 Temporal variation of NPP in the Tianshan Mountains during 2000-2020. (a), total NPP; (b), annual mean NPP of different land use types; (c), average NPP in representative years; (d), area proportion of NPP classes. "Others" in Figure 5b include water body and unutilized land.
2000 2010
Construction land Cropland Forest Grassland Unutilized land Water body Wetland Transfer out
Construction land 35.20 35.20
Cropland -0.02 245.97 0.04 -186.35 -2.33 -5.17 0.21 52.33
Forest -0.13 23.65 -152.96 -16.14 0.20 -145.39
Grassland -0.01 783.67 11.54 23.81 -1382.36 -4.16 5.97 -561.54
Unutilized land 0.07 16.34 1.24 2849.02 655.97 0.51 3.00 3526.16
Water body 1.26 1.04 23.34 4.62 30.26
Wetland -0.21 1.73 -298.52 -11.53 -0.97 2.17 -307.34
Transfer in 35.23 1045.63 38.19 2236.27 -755.37 13.55 16.18 2629.68
2010 2020
Construction land Cropland Forest Grassland Unutilized land Water body Wetland Transfer out
Construction land 15.79 15.79
Cropland -0.54 -44.25 -402.79 -0.96 0.02 -448.52
Forest 26.60 -14.83 -3.87 0.96 8.86
Grassland -0.11 710.67 68.26 2632.71 -644.64 -1.35 77.50 2843.04
Unutilized land 52.21 1.40 2099.15 792.07 2.67 5.88 2953.38
Water body 2.20 -0.19 13.85 1.69 17.55
Wetland 1.39 -9.89 -1.15 -1.58 25.43 14.20
Transfer in 15.14 718.63 97.66 4306.54 141.27 13.59 111.48 5404.30
Table 4 Transmission matrix of NPP variations between 2000 and 2020 (Unit: ×109 g C)
Fig. 6 Spatial patterns of NPP and its variability and trends in the Tianshan Mountains during 2000-2020. (a), multi-year mean NPP; (b), CV of annual NPP; (c), Theil-Sen trend slope of NPP; (d), significance level of NPP trends based on the Mann-Kendall test. ESR, extremely significant reduction; SR, significant reduction; NSC, no significant change; SI, significant increase; ESI, extremely significant increase. Inserted pie charts in Figure 6c and d show the area proportions of the corresponding categories.
Fig. 7 Spatial distribution of the Hurst exponent (a) and projected future NPP trend types (b) in the Tianshan Mountains. SPD, strong persistent degradation; WPD, weak persistent degradation; WAPD, weak anti-persistent degradation; SAPD, strong anti-persistent degradation; SPI, strong persistent improvement; WAPI, weak anti-persistent improvement; MAPI, moderate anti-persistent improvement; SAPI, strong anti-persistent improvement; NSC, no significant change.
Fig. 8 SHAP-based attribution of factors influencing NPP in the Tianshan Mountains. (a), SHAP summary (beeswarm) plot for the 15 predictors; (b), SHAP feature importance ranked by mean absolute SHAP values. X1, land use; X2, temperature; X3, precipitation; X4, DEM; X5, slope; X6, aspect; X7, population density; X8, gross domestic productivity (GDP) density; X9, nighttime light index; X10, human footprint; X11, grazing intensity; X12, distance to road; X13, distance to highway; X14, distance to water body; X15, distance to government center.
Fig. 9 SHAP dependence plots showing nonlinear responses of NPP to the top six drivers in the Tianshan Mountains during 2000-2020. (a), land use (X1); (b), precipitation (X3); (c), temperature (X2); (d), population density (X7); (e), DEM (X4); (f), grazing intensity (X11). Points show SHAP values for each sample, and the red curve indicates the locally estimated scatterplot smoothing (LOESS)-smoothed trend.
Fig. 10 Interaction plots of factors influencing NPP. The color gradient represents the feature values of the driving factors, of which red indicates high values, while blue represents low values.
Human disturbance type Rule-based classification Producer's Accuracy (%)
5297 0 30 67 98.20
70 79 40 26 36.70
429 25 684 81 56.10
835 0 77 2628 74.30
User's accuracy (%) 79.80 76.00 82.30 93.80
Overall accuracy=83.79% Expected agreement=0.43 Kappa coefficient=0.71
Table S3 Confusion matrix between human disturbance four-type hydrothermal-regime clustering (HD4) and the rule-based classification (6 km grids; n=10,368)
Fig. S3 Spatial distribution of human disturbance four-type hydrothermal-regime clustering (HD4; a) and rule-based classification in 2020. I corresponds to grids dominated by high-altitude or desert landscapes with minimal human impact; II represents urban-industrial settings characterized by high imperviousness and elevated population density and GDP; III denotes agricultural-irrigated grids dominated by cropland and irrigation; and IV corresponds to pastoral-grazing settings where grassland use and grazing pressure are prevalent. The HD4 derives the four types via clustering based on hydrothermal conditions and human-pressure proxies, whereas the rule-based scheme assigns the same types using predefined thresholds of Built%, Crop%, grazing intensity, population density, and GDP.
Fig. S4 Near-term ecosystem productivity risk in the Tianshan Mountains in 2024 at the 2 km grid. (a), synergy hotspot; (b), high-risk grid distribution; (c), NPP low tail. Synergy hotspots are grids with NPP in the lowest 30.00% of all 2 km grids, together with hydroclimatic stress (precipitation in the lowest 40.00% or temperature in the highest 40.00%) and elevated human pressure (population density in the highest 30.00% or a built-up proportion of at least 10.00%). High-risk grids are synergy hotspots where hydroclimatic conditions further deviate from the identified safe ranges. Specifically, precipitation falls below 300.00 mm or exceeds 400.00 mm, or temperature falls below -5.00°C or exceeds 5.00°C. NPP low tail includes grids with NPP in the lowest 30.00% only, regardless of hydroclimatic or human-pressure conditions. The 30.00%, 40.00%, and 30.00% thresholds are percentile-based cutoffs computed across all 2 km grids, whereas the 10.00% built-up cutoff is an absolute threshold.
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