New ecological change indicators using breakpoints in vegetation trends applied to a dryland pastoral catchment in the Moroccan high atlas

Abstract

This study aims to improve ecological change quantification from remote sensing-based methods by incorporating the sign of breakpoints in vegetation productivity trends and proposing a new breakpoint typology. The typology distinguishes between positive and negative breakpoints reflecting either an improvement or deterioration in ecosystem functioning. Using 35 years of Landsat NDVI data, the analysis focused on a dryland pastoral catchment in Morocco's High Atlas, including the most severe drought during this period. The frequency of negative breakpoints increased with aridity, especially in areas of scant vegetation, while positive breakpoints were more evenly distributed throughout the catchment. Regions with smaller NDVI changes over time exhibited a higher number of breakpoints with a similar share of positive and negative, compared to areas with stronger greening or browning. During the drought, positive breakpoints (positive reversals) were most common, followed by negative breakpoints (interrupted decreases). Areas with positive reversals experienced fewer total breakpoints over the study period and had a greater share of positive breakpoints than areas with interrupted decreases. The contrasting vegetation responses to drought may have reflected the interaction between grazing pressure and aridity, with positive drought responses aligning with herd collapses and higher aridity limiting recovery despite reduced grazing. These findings highlight the importance of analysing the balance of positive and negative breakpoints alongside their total count for understanding ecological change. The study also revealed significant ecosystem resilience to severe drought across much of the catchment, but underscored the potential risk of crossing resilience thresholds as aridity intensifies.