Identifying and understanding alternative states of dryland landscape: A hierarchical analysis of time series of fractional vegetation-soil nexuses in China’s Hexi Corridor

Abstract

Understanding and early identification of state transitions in dryland systems’ landscape pattern are challenging but critical for conservation planning and sustainable management strategies. Currently the alternative states of drylands have been identified based on dynamics of single vegetation index (i.e., normalized difference vegetation index), with the interactions between vegetation and soil components often being neglected. Here, we proposed a framework for identifying the alternative states in relation to land degradation based on the interactive pathways of fractional vegetation and soil-related endmember nexuses. In this framework, we initially generated 16-day fractional green vegetation endmember (green vegetation [GV]) and soil-related endmembers (sand [SL], saline [SA], dark surface [DA]) time series using a standardized linear spectral mixture analysis model in the Hexi Corridor during 2001–2017. A discrete wavelet transform based trends and turning points detection (DWT-TTD) algorithm was proposed to detect trends, turning point types, and timing of these time series records. These metrics were subsequently used to identify eight state transition pathways for each endmember and synthetic degradation processes at the appropriate 5–10 year scale. We found that the unmixed fractional endmembers exhibited a consistent agreement with both field abundance values and estimated fractions from high-spatial-resolution Google Earth images. Based on the DWT-TTD algorithm derived alternative pathways, we found that four positive pathways of GV and DA fractions can be observed in the vegetated areas were integrated with corresponding negative pathways of SA and SL fractions. These pathways resulted in remarkably increasing GV (+4551.81 km2) and DA (+9749.26 km2) and significantly declining SA (−1452.70 km2) and SL (−9555.57 km2) from 2001 to 2017. These processes were linked to increased precipitation and temperature and reduced drought degree. Also, the timing of the turning points (around 2007 and 2013) was largely coincidental with the timing of ecological management, suggesting that ecological governance played a crucial role in the state shifts. However, because of drought trends in the barren regions, land desertification exhibited an increasing occurrence in extent (+2.5% of total pixels) during the study period, which may increase in the future, as indicated by the large pixel proportion of initially positive reversal pathway for SL (9.31%). Compared to state-of-the-art approaches, the DWT-TTD algorithm, as a multi-resolution analysis method, has the potential to detect hierarchical temporal changes and disentangle the cross-scale inherent shifts of vegetation-soil systems. The framework can help devise sustainable management practices that maintain land degradation neutrality, especially when integrated with land use cover change analysis.