Detecting Vegetation Variations and Main Drivers over the Agropastoral Ecotone of Northern China through the Ensemble Empirical Mode Decomposition Method

Abstract

Vegetation is the major component of the terrestrial ecosystem. Understanding both climate change and anthropogenically induced vegetation variation is essential for ecosystem management. In this study, we used an ensemble empirical mode decomposition (EEMD) method and a linear regression model to investigate spatiotemporal variations in the normalized difference vegetation index (NDVI) over the agropastoral ecotone of northern China (APENC) during the 1982–2015 period. A quantitative approach was proposed based on the residual trend (RESTREND) method to distinguish the effects of climatic (i.e., temperature (TEM), precipitation (PRE), total downward solar radiation (RAD), and near surface wind speed (SWS)) and anthropogenic effects on vegetation variations. The results showed that the NDVI exhibited a significant greening trend of 0.002 year−1 over the entire study period of 1982–2015 and that areas with monotonous greening dominated the entire APENC, occupying 40.97% of the region. A browning trend was also found in the central and northern parts of the APENC. PRE presented the highest spatial correlation with the NDVI and climate factors, suggesting that PRE was the most important factor affecting NDVI changes in the study area. In addition, the RESTREND results indicated that anthropogenic contributions dominated the vegetation variations in the APENC. Therefore, reusing farmland for grass and tree planting made a positive contribution to vegetation restoration, while deforestation, overgrazing, and the reclamation of grasslands were the opposite. In addition, with the continuous implementation of national ecological engineering programs such as the Grain to Green Program, positive human activity contributions to vegetation greening significantly increased. These results will support decision- and policy-making in the assessment and rehabilitation of ecosystems in the study region.

Vegetation restoration projects intensify intraregional water recycling processes in the agro-pastoral ecotone of Northern China

“…Specifically, we examined the changes in precipitation, ET, and moisture recycling in the APENC. We found that, over the period of 1995-2015, there were varying but increasing trends in the annual P in the APENC, a finding similar to previous studies (Xue et al 2019). Annual mean regional ET exhibited significant increasing trends from 1995 to 2015, indicating that the revegetation has enhanced regional ET, confirming the results of previous study (Wang et al 2020).…”

Effects of land use changes on the nonlinear trends of net primary productivity in arid and semiarid areas, China

“…The trend reversal from decrease to increase in the region joining Shaanxi, Gansu, and Ningxia on the Loess Plateau was predominantly attributed to land development and ecological protection efforts (Liu et al, 2015). The extensive cultivation and the rapid development of husbandry industries caused vegetation degradation during the early years of the study period, whereas the implementation of the Grain to Green Program promoted vegetation growth and led to the trend reversal from decrease to increase (Xue et al, 2019). For the high mountains in Xinjiang, the weakly increasing precipitation and warming‐related snowmelt supplied plenty of water for boosting vegetation growth (He et al, 2015a).…”

“…The ensemble empirical mode decomposition (EEMD) method is capable of decomposing time‐series data into several frequency‐decreasing intrinsic mode functions ( IMF i , i = 1, 2…, n ) and a secular trend R n (Huang et al, 1998; Liu et al, 2016a). EEMD adds white noise to the raw series and then taking the ensemble mean to cancel out white noise (Pan et al, 2018; Wu & Huang, 2009; Xue, Zhang, He, & Shao, 2019). The decomposition steps can be described as follows:…”

Assessing the Effects of Spatial Scales on Regional Evapotranspiration Estimation by the SEBAL Model and Multiple Satellite Datasets: A Case Study in the Agro-Pastoral Ecotone, Northwestern China

“…The Yuyang District is located at 37.81 • -38.92 • N, 108.94 • -110.41 • E (Figure 1), Shaanxi Province, China, and covers a total area of~7000 km 2 . This area is part of the ecologically fragile transition zone between the desert-grassland area and the Loess Plateau region [13,[49][50][51][52]. The region has a semi-arid continental monsoon climate, with an average annual temperature of 8.4 • C and an average annual precipitation of 402 mm that gradually decreases from the southeast to northwest.…”