Zhang et al. (2025) EEMD disentangles climate-NPP dynamics in Southwest China’s karst area: Soil moisture overtakes precipitation as a key driver of afforestation-induced carbon sinks

Identification

• Journal: International Journal of Applied Earth Observation and Geoinformation
• Year: 2025
• Date: 2025-11-22
• Authors: Wen Zhang, Yudi Li, Limin Jiao, Hui Zheng, Bolin Fu
• DOI: 10.1016/j.jag.2025.104963

Research Groups

• Guangxi Key Laboratory of Environmental Processes and Remediation in Ecologically Fragile Regions, Guangxi Normal University, Guilin, China
• School of Resource and Environmental Sciences, Wuhan University, Wuhan, China
• Key Laboratory of Geographic Information System, Ministry of Education, Wuhan University, Wuhan, China
• College of Environment and Resources, Guangxi Normal University, Guilin, China
• College of Geomatics and Geoinformation, Guilin University of Technology, Guilin, China

Short Summary

This study used Ensemble Empirical Mode Decomposition (EEMD) to analyze Net Primary Productivity (NPP) and climate data in Southwest China's karst area (1981-2019), revealing that afforestation after 2001 significantly increased NPP, especially in new forests, and shifted the primary climate driver of NPP from precipitation to soil moisture.

Objective

• To identify the interannual variation trends in Net Primary Productivity (NPP) and climatic factors.
• To evaluate the spatial coupling relationship between NPP and climate variables at an interannual scale.
• To compare the NPP response mechanisms to climate change in old and new forests before and after afforestation.

Study Configuration

• Spatial Scale: Southwest China's karst (SCK) area, including Yunnan, Guizhou, and Guangxi, covering approximately 800,000 km². Analysis was conducted at the pixel level.
• Temporal Scale: Monthly data from 1981 to 2019 (39 years), with analysis focused on interannual variability (periods ≥ 2 years).

Methodology and Data

• Models used: Ensemble Empirical Mode Decomposition (EEMD), Theil–Sen median (TSM) method, Mann–Kendall (MK) test, Sequential MK test, Partial correlation analysis.
• Data sources:
  • Monthly Net Primary Productivity (NPP) data (1981–2019).
  • Precipitation (PRE), average temperature (TEM), and temperature difference (DIF) data from the National Tibetan Plateau Data Center.
  • Evapotranspiration (ET) data from ERA5 reanalysis (ECMWF).
  • Surface soil moisture (SMsurf) data from the Global Land Evaporation Amsterdam Model (GLEAM).
  • Forest age distribution map (1986–2018) derived from Landsat satellite images.

Main Results

• Net Primary Productivity (NPP) in the Southwest China's karst (SCK) area showed a significant overall increase from 1981 to 2019, with an average growth rate of 0.47 gC·m⁻²·month⁻¹.
• A significant breakpoint in NPP growth was detected around 2001, coinciding with the implementation of large-scale ecological restoration projects.
• After 2001, the NPP growth rate in newly afforested areas ("new forests") was 1.47 times higher than in pre-existing forests ("old forests"), indicating greater carbon sink potential in new forests.
• NPP sensitivity to climate change increased after 2001 across the region, with new forests exhibiting higher sensitivity compared to old forests.
• The dominant climatic factor influencing NPP shifted from precipitation (PRE) before 2001 to surface soil moisture (SMsurf) after 2001 in both new and old forests. The correlation strength with SMsurf increased by 142% in new forests and 123% in old forests.
• The relationship between NPP and temperature (TEM) in new forests shifted from a weak negative correlation to a positive correlation after 2001.
• NPP responses to climate factors exhibited time-lag cumulative effects, with nearly 70% of the area showing a three-month lag for precipitation.

Contributions

• Provides a novel diagnostic framework for assessing climate sensitivity of afforestation initiatives by integrating pixel-level interannual signal decomposition, forest age stratification, and pre/post-afforestation period comparisons.
• Demonstrates that large-scale afforestation projects in karst regions reinforce water constraints and shift the dominant climatic control on NPP from precipitation to soil moisture.
• Offers crucial insights for evaluating the effectiveness of ecological restoration projects and highlights the need to incorporate water and carbon trade-offs into future afforestation planning and forest management, particularly in newly afforested areas.
• Reveals that newly established forests, while having higher carbon uptake potential, also exhibit greater sensitivity and vulnerability to future climate change.

Funding

• Project on Enhancement of Basic Research Ability for Young and Middle-aged Teachers in Guangxi Universities (2023KY0069).
• Guangxi Key Laboratory of Environmental Processes and Remediation in Ecologically Fragile Regions, Guangxi Normal University (EPRZR2024-01).

Citation

@article{Zhang2025EEMD,
  author = {Zhang, Wen and Li, Yudi and Jiao, Limin and Zheng, Hui and Fu, Bolin},
  title = {EEMD disentangles climate-NPP dynamics in Southwest China’s karst area: Soil moisture overtakes precipitation as a key driver of afforestation-induced carbon sinks},
  journal = {International Journal of Applied Earth Observation and Geoinformation},
  year = {2025},
  doi = {10.1016/j.jag.2025.104963},
  url = {https://doi.org/10.1016/j.jag.2025.104963}
}