Hillslope-scale spatiotemporal differentiation of fractional vegetation cover in the hilly and gully region of the Loess Plateau

Abstract

The hilly and gully region of the Loess Plateau is characterized by complex topography and pronounced spatial heterogeneity in vegetation-cover change. Conventional raster-based analyses may not fully capture the influence of complex terrain from the perspective of geomorphic process units. In this study, 20 typical sample sites were selected in this region. Based on Landsat imagery from 2001 to 2024, a 30 m fractional vegetation cover dataset was generated on the Google Earth Engine platform, and a geomorphologically meaningful hillslope-type classification framework was established by integrating terrain relief, slope position, and aspect. Using the Theil–Sen slope estimator, Mann–Kendall test, and analysis of variance, we examined the spatiotemporal differentiation and trends of vegetation cover among hillslope types at annual and monthly scales. The results showed significant differences among hillslope types in multiyear mean vegetation cover and interannual stability. Variance decomposition showed that site background explained the largest proportion of vegetation-cover variation, while hillslope type remained significant after accounting for site differences. Shaded non-gully slopes in negative and positive terrain generally exhibited relatively high vegetation cover, whereas gully-bottom slopes had relatively low cover and pronounced variability. Monthly results were broadly consistent with annual patterns, and hillslope-type differences became more pronounced during the growing season. Gully-bottom slopes remained at relatively low levels, whereas the relative advantage of shaded slopes became more evident during vigorous growth. During 2001–2024, vegetation cover generally increased, but restoration intensity, stability, and degradation risk differed substantially among sample sites and hillslope types. Overall, shaded hillslope types showed stronger recovery signals, non-gully slopes in positive terrain exhibited better stability, and gully-bottom slopes showed stronger fluctuations and higher degradation risk. This study improves our understanding of the relationship between hillslope types and vegetation-cover differentiation and informs zoned ecological restoration and soil and water conservation management in the Loess Plateau.

Graphical abstract

Keywords

hillslope type / fractional vegetation cover / Loess Plateau / vegetation dynamics / spatiotemporal differentiation / ecological restoration

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