Phases or regimes? Revisiting NDVI trends as proxies for land degradation

EASDALE, MARCOS; BRUZZONE, OCTAVIO; MAPFUMO, P,; TITTONELL, PABLO

Land Degradation and Development

Wiley

Año: 2018

1099-145X

One of the main challenges in land degradation assessment is that a rigorous and systematicapproach to addressing its complex dynamics is still missing. The development and applicationof operative tools at regional and global scales remain a challenge. Land degradation is usuallydefined as a long‐term decline in ecosystem function and productivity. Due to its temporal andspatial resolution as well as data availability, the use of time series of spectral vegetation indexesobtained from satellite sensors has become frequent in recent studies in this field. Slope of lineartrends of the normalized difference vegetation index is usually considered an accurate indicatorand is widely used as a proxy for land degradation. Yet this method is built on a number ofsimplifying conceptual and methodological assumptions that prevent capturing more complexdynamics, such as cyclic or periodic behaviors. Our aim was to examine the limitations associatedwith using linear normalized difference vegetation index trends as proxies for land degradation bycomparing outcomes with an alternative methodological procedure based on waveletautoregressive methods. We explored these issues in 5 case studies from Africa and SouthAmerica. We observed that trend explained a marginal portion of total temporal variability,whereas monotonic functions, such as linear trends, were unable to capture dynamics that werenon‐unidirectional, resulting in misinterpretation of actual trends. Wavelet autoregressivemethod results were encouraging as a step towards the application of more accurate methodsto provide sound scientific information of land degradation and restoration.