Tai et al. (2025) Declining sensitivity of transpiration fraction to vegetation greening in China’s arid grasslands: Role of soil moisture and vapor pressure deficit

Identification

• Journal: Journal of Hydrology Regional Studies
• Year: 2025
• Date: 2025-11-21
• Authors: Yang Tai, Cong Wang, Xingwu Duan, Zongshan Li, Xiaoming Feng, Xing Wu, Yihe Lü, Bojie Fu
• DOI: 10.1016/j.ejrh.2025.102963

Research Groups

• State Key Laboratory of Regional and Urban Ecology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China
• Yunnan Key Laboratory of Soil Erosion Prevention and Green Development, Institute of International Rivers and Eco-Security, Yunnan University, Kunming 650500, China
• Shaanxi Yan’an Forest Ecosystem National Observation and Research Station, Beijing 100085, China
• National Observation and Research Station of Earth Critical Zone on the Loess Plateau in Shaanxi, Xi’an 710061, China

Short Summary

This study investigated how vegetation greening affects the land-atmosphere water cycle in China's arid grasslands under coupled soil moisture (SM) and vapor pressure deficit (VPD). It found a widespread decline in the sensitivity of the transpiration-to-evapotranspiration ratio (T:ET) to Leaf Area Index (LAI) changes, primarily driven by boundary damping effects in relatively humid regions and increasing VPD in drier regions.

Objective

• Examine the historical evolution of vegetation-driven water cycle processes in response to changing environmental conditions.
• Analyze the environmental factors influencing the relationship between vegetation dynamics and the transpiration fraction (T:ET) under varying soil–atmosphere conditions, specifically investigating the spatial heterogeneity and variation patterns in the sensitivity of T:ET to LAI.

Study Configuration

• Spatial Scale: Arid and semi-arid regions of China (73°26′−127°04′E, 30°07′−50°44′N), focusing on grasslands (grid cells with over 80% grassland coverage at 0.25° resolution).
• Temporal Scale: Vegetation growing season (June–August) from 2001 to 2021, with sensitivity analyzed using a decadal moving window.

Methodology and Data

• Models used:
  • Theil-Sen median slope estimation method (for trend analysis)
  • Mann-Kendall test (for single-pixel significance testing)
  • Fisher’s combined test (for regional significance testing)
  • Segmented linear regression (to identify LAI thresholds)
  • Bayesian Information Criterion (BIC) (to determine optimal number of segments)
  • K-Means clustering algorithm (for temperature zoning)
• Data sources:
  • Land Cover: MCD12Q1 product (LC_Type1 band, 500 m spatial resolution, from Google Earth Engine)
  • LAI: Optimized MODIS LAI datasets (0.25° spatial resolution)
  • Atmospheric Pressure: ERA5-Land (0.25° spatial resolution, from Google Earth Engine)
  • Evapotranspiration (ET), Transpiration (T), Root-zone Soil Moisture (SM): GLEAM v3.8a dataset (0.25° spatial resolution)
  • Temperature, Precipitation, Relative Humidity: CN05.1 dataset (0.25° spatial resolution)
  • Xylem hydraulic conductance: Field measurements from 52 forb species (root samples, 3–10 cm depth)

Main Results

• A widespread decline in the sensitivity of T:ET to LAI (μ) was observed across China’s arid grasslands from 2001 to 2021.
• The sensitivity (μ) dropped from 30 % (m² m⁻²)⁻¹ to near 0 % (m² m⁻²)⁻¹ in High-Temperature zones and from 20 % (m² m⁻²)⁻¹ to near 0 % (m² m⁻²)⁻¹ in Low-Temperature zones.
• In regions with increasing soil moisture and decreasing vapor pressure deficit (SM↑VPD↓), LAI exceeded boundary damping thresholds (1.189 m² m⁻² in High-Temperature zones, 1.024 m² m⁻² in Low-Temperature zones), indicating that reduced sensitivity was primarily due to this effect.
• In regions with increasing soil moisture and increasing vapor pressure deficit (SM↑VPD↑), LAI remained below thresholds (1.054 m² m⁻² in High-Temperature zones, 1.628 m² m⁻² in Low-Temperature zones), suggesting the decline in sensitivity was mainly driven by increased VPD.
• Higher VPD enhanced transpiration in Low-Temperature zones (with low baseline VPD) but suppressed it in High-Temperature zones (with high baseline VPD).
• Grasslands exhibited a significant greening trend (P < 0.05), with an average rate of 0.014 m² m⁻² per year.
• Both ET and T showed significant increases (P < 0.05), with ET increasing by 1.22 mm per year and T by 1.19 mm per year.
• SM showed a significant increasing trend (P < 0.05), with an annual increment of up to 0.002 m³ m⁻³.
• Precipitation exhibited an increasing trend of approximately 1.6 mm per year.
• In SM↑VPD↑ regions, the annual T:ET growth rate was approximately 0.1 % per year higher than in SM↑VPD↓ regions, but the average T:ET level in SM↑VPD↓ regions was significantly higher (>70% compared to approximately 55%).

Contributions

• Developed a novel classification framework combining SM and VPD trends with baseline temperature to assess T:ET sensitivity to greening across diverse hydroclimatic conditions.
• Quantified the sensitivity of T:ET to LAI (μ = TrendT:ET / TrendLAI) and analyzed its temporal shifts using a decadal moving window, revealing a widespread decline in sensitivity.
• Provided distinct biophysical explanations for the observed decline in T:ET sensitivity, attributing it to boundary damping effects in relatively humid regions and VPD-driven suppression in drier regions.
• Highlighted the critical role of vegetation in regulating the land–atmosphere water cycle in temperate arid grasslands and underscored the pronounced influence of atmospheric aridity in sparsely vegetated areas.
• Emphasized the necessity of explicitly incorporating dynamic, threshold-dependent VPD responses into Earth system models to improve biophysical realism, especially under global warming scenarios.

Funding

• State Key Project of Research and Development Plan (Grant No. 2022YFF1300405, 2023YFF1305105)
• National Natural Science Foundation of China (grant number 41991233)
• Open Bidding for Selecting the Best Candidates’ Major Demonstration Engineering Project for Scientific and Technological Innovation in Desertification Prevention and Control in Inner Mongolia Autonomous Region (Grant No. 2024JBGS0005)
• Ordos City Major Science and Technology Project (Grant No. ZD20232324)

Citation

@article{Tai2025Declining,
  author = {Tai, Yang and Wang, Cong and Duan, Xingwu and Li, Zongshan and Feng, Xiaoming and Wu, Xing and Lü, Yihe and Fu, Bojie},
  title = {Declining sensitivity of transpiration fraction to vegetation greening in China’s arid grasslands: Role of soil moisture and vapor pressure deficit},
  journal = {Journal of Hydrology Regional Studies},
  year = {2025},
  doi = {10.1016/j.ejrh.2025.102963},
  url = {https://doi.org/10.1016/j.ejrh.2025.102963}
}