Primary Productivity and Woody growth: a 35 years Landsat TM NDVI time series investigation across desert-fringe in the south-eastern Mediterranean

Abstract

This research investigates the spatiotemporal landscape characteristics within the Mediterranean Basin in the context of climate change in the South-Eastern Mediterranean. Desert-fringe ecosystems in general and the arid margins of Mediterranean regions had undergone numerous cycles of climate and human disturbance which built their resilience. Following current climate change indications, fundamental questions concerning changes in climate conditions and their corresponding surface changes are raised on one hand, there is limited data regarding the spatial distribution of climate parameters and their change in time, and on the other hand, resilient ecosystems may “absorb” climate changes to a certain extent. Landsat TM offers 35 years, from 1986 to 2021, of monitoring surface conditions. When used in conjunction with corresponding climatic data there is an opportunity to investigate impacts of climate change at desert margins and to better understand relationships between climatic conditions and surface conditions at these important zones. During the time frame of these years, there had been an extreme drought period. Thus, an important aspect of our study concerns the response of Mediterranean ecosystems to such an anomaly. The research area consists of the climatic gradient between the Judean mountains and the Negev Desert (Beit Shemesh to Lehavim). This area is characterized by notable variations in both precipitation and temperature over a relatively small geographic area and represents diverse desert-fringe ecosystems. Surface properties are represented by a time series of NDVI corresponding to relative shrubs and dwarf shrubs cover and their photosynthetic activity. Even though numerous remote sensing studies analyzed relationships between spectral vegetation indices and climatic parameters, the uniqueness of this study concerns the differentiation between Primary Productivity (PP) representing the total new herbaceous growth and new shrub leaves at the end of the winter and Woody Growth (WG) representing the greenness of the shrubs at the end of the summer. The PP is obtained as the difference between the end of the winter and the end of the summer NDVI, and WG is represented by the NDVI at the end of the summer. These vegetation forms are affected not only by the immediate yearly rainfall but also by the long-term balance of the water accumulated in the subsoil. The main goals of the present work are to study the impact of climate shifts on PP and WG patterns across a climatic gradient, spanning three and a half decades and to assess the changes in the geographical extent of aridity based on the PP and WG time series, with a particular focus on locations that had experienced vegetation loss and transitioned to bare soil. Examining both PP and WG through a temporal lens sheds light on dominant long-term trends in the ecosystem. We suggest that despite fluctuations in the vegetation conditions and their NDVI following droughts and other climatic changes, no definitive northward migration of aridity could be confirmed.