Eco-hydrological functioning of multi-aged dryland afforestation systems

Abstract

Continual land degradation processes adversely affect the functioning of dryland ecosystems. In recent decades, extensive afforestation activities have been undertaken in marginal lands of the semiarid northern Negev region of southern Israel to mitigate such degradation processes. However, the long-term impacts of these actions in drylands, subjected to long-term drought episodes, remain unknown. We investigated the impact of landuse change from natural lands to afforestation runoff-harvesting systems—through intensive earthworks (landforming) to establish a contour bench terrace during long-term drought—on herbaceous vegetation productivity, and assessed its temporal dynamics across the multi-aged Ambassadors’ Forest. The MODIS MOD13Q1 and MYD13Q1 maximum value composite products were used to calculate normalized difference vegetation index (NDVI) data for the hydrological years 2000 to 2020. For this study, implemented in 2021, we selected three locations within the Ambassadors’ Forest: (1) 15-year-old afforested hillslopes, (2) 11-year-old afforested hillslopes, and (3) 4-year-old afforested hillslopes. We further delineated undisturbed hillslopes near these sites as a reference treatment. We found high spatiotemporal variability in vegetation cover. Over the short term, and specifically in the first hydrological year following the establishment of the water-harvesting systems, a substantial reduction in mean annual NDVI was observed, with values ranging from 30% to 65% lower compared to the reference sites. However, the negative impact of landuse change diminished over time, suggesting that (1) self-restoration processes occurred over a longer term after landuse changes were implemented, and (2) the establishment of water-harvesting systems improve the conservation of runoff water at the hillslope. This effect was observed for the 11- and 15-year-old afforestation sites, where vegetation productivity was 8.9% and 31.0% greater, respectively, than in the respective reference sites ( p < 0.05). Although the long-term drought occurred during the construction of the water-harvesting systems, these findings are in agreement with previous studies. Specifically, this study suggests that ecological self-restoration processes in semiarid regions occur approximately a decade after runoff-harvesting systems are established through earthworks.