Greening trends and their relationship with agricultural land abandonment across Poland

Substantial land surface greening has been observed globally over the last few decades. This greening has been primarily attributed to climate change and CO2 concentrations, but recent research studies have emphasized the large role of human land use in this process, especially agricultural land abandonment (ALA). The aim of this study was to investigate whether long-term gradual greening of agricultural land can aid in mapping abandoned land across Poland. Trend estimation and temporal segmentation were applied to the 1986-2019 annual Landsat-derived Normalized Difference Vegetation Index (NDVI) time series to detect periods of long-term greening and to assess its relationship with actual information on ALA. The results show that long-term greening is widespread in Poland (covering up to 60% of its territory), regardless of former and current land uses, most of which has stemmed from modifications within stable land use classes (as they cover 91% of Poland). The highest greening rates and intensities were observed due to conversions from agricultural to non-agricultural land use, which strongly suggests land abandonment and the proceeding succession. However, some proportion of managed agricultural land has also experienced high-intensity greening, and a large proportion of abandoned agricultural land is not greening intensively. Thus, setting an exact greening intensity threshold allowing to clearly distinguish the status of land was not possible. This study also presents the consistency of the Landsat annual time series and its good performance for long-term greening detection, indicating that temporal segmentation may better capture ALA patterns than trend estimation