Regional scale hierarchical classification of temporal series of AVHRR vegetation index

Abstract

In this paper a hierarchical approach is taken to classify temporal sequences of images of the Normalized Difference Vegetation Index (NDVI) from the Advanced Very High Resolution Radiometer (AVHRR), using Iberia as an example. Iberia is a convenient area of study because it has a high environmental diversity and very strong environmental gradients, and yet a reduced size at the spatial resolution of current global data-sets. An Iberian subset of a global temporal series of AVHRR-NDVI images facilitates test and validation of different approaches while producing results that are likely to be valid over much larger areas. Our hierarchical clustering approach yields maps with nested legends. We compare these maps to a digitized map of potential natural vegetation, which reveals a clear bioclimatic control. The highest level of the hierarchical classification separates vegetation with a Summer peak of NDVI from vegetation with a Spring peak of NDVI. Such a discontinuity corresponds to the discontinuity between Atlantic and Submediterranean vegetation in the vegetation map. Lower levels in the hierarchical classification produce maps of increasing complexity but that keep a high degree of spatial continuity. A correspondence analysis between a 16-classes NDVI map and the digitized map of potential vegetation produces an ordination that is bioclimatically coherent. According to the known characteristics of the potential vegetation units, the two first correspondence axes can be interpreted, respectively, as water availability and temperature. These results are a consequence of the temporal NDVI series being an accurate signal of vegetative phenology, which in turn is a fundamental vegetation property. A comparison of our results with several global land cover digital maps by means of the Wilk's ratio indicates that the global maps do not produce an appropriate partition of the region in terms of the NDVI temporal course. We conclude that the analysis of temporal series of NDVI yield relevant ecological information at finer scales and with more detailed legends that had not been attempted until now, and, therefore, are suitable for regional scale applications. Our results also indicate the interest of a bioclimatic analysis and modeling of the NDVI signatures for their correct ecological understanding. Maps at a global scale can be produced based on such an understanding.