*Article* **Exploring the Applicability and Scaling Effects of Satellite-Observed Spring and Autumn Phenology in Complex Terrain Regions Using Four Different Spatial Resolution Products**

**Fangxin Chen 1, Zhengjia Liu 1,2,3,\*, Huimin Zhong 1,2 and Sisi Wang 4**



**\*** Correspondence: liuzj@igsnrr.ac.cn

**Citation:** Chen, F.; Liu, Z.; Zhong, H.; Wang, S. Exploring the Applicability and Scaling Effects of Satellite-Observed Spring and Autumn Phenology in Complex Terrain Regions Using Four Different Spatial Resolution Products. *Remote Sens.* **2021**, *13*, 4582. https://doi.org/ 10.3390/rs13224582 the <

Academic Editors: Xuanlong Ma, Xiaolin Zhu, Jiaxin Jin, Yuke Zhou and Qiaoyun Xie

Received: 24 September 2021 Accepted: 11 November 2021 Published: 15 November 2021

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**Abstract:** The information on land surface phenology (LSP) was extracted from remote sensing data inmany studies. However, few studies have evaluated the impacts of satellite products with different spatial resolutions on LSP extraction over regions with a heterogeneous topography. To bridge this knowledge gap, this study took the Loess Plateau as an example region and employed four types of satellite data with different spatial resolutions (250, 500, and 1000 m MODIS NDVI during the period 2001–2020 and ~10 km GIMMS3g during the period 1982–2015) to investigate the LSP changes that took place. We used the correlation coefficient (r) and root mean square error (RMSE) to evaluate the performances of various satellite products and further analyzed the applicability of the four satellite products. Our results showed that the MODIS-based start of the growing season (SOS) and end of growing season (EOS) were highly correlated with the ground-observed data with r values of 0.82 and 0.79, respectively (*p* < 0.01), while the GIMMS3g-based phenology signal performed badly (*r* 0.50 and *p* > 0.05). Spatially, the LSP that was derived from the MODIS products produced more reasonable spatial distributions. The inter-annual averaged MODIS SOS and EOS presented overall advanced and delayed trends during the period 2001–2020, respectively. More than two-thirds of the SOS advances and EOS delays occurred in grasslands, which determined the overall phenological changes across the entire Loess Plateau. However, both inter-annual trends of SOS and EOS derived from the GIMMS3g data were opposite to those seen in the MODIS results. There were no significant differences among the three MODIS datasets (250, 500, and 1000 m) with regard to a bias lower than 2 days, RMSE lower than 1 day, and correlation coefficient greater than 0.95 (*p* < 0.01). Furthermore, it was found that the phenology that was derived from the data with a 1000 m spatial resolution in the heterogeneous topography regions was feasible. Yet, in forest ecosystems and areas with an accumulated temperature ≥10 ◦C, the differences in phenological phase between the MODIS products could be amplified.

**Keywords:** land surface phenology; data suitability; satellite data; spatial scaling effects; the Loess Plateau
