Long-term Satellite NDVI Data Sets: Evaluating Their Ability to Detect Ecosystem Functional Changes in South America

BALDI, G; NOSETTO, M D; ARAGÓN, R; AVERSA, F; PARUELO, J M; JOBBÁGY, E G

SENSORS

MOLECULAR DIVERSITY PRESERVATION INTERNATIONAL-MDPI

Lugar: Basel; Año: 2008 vol. 8 p. 5397 - 5425

1424-8220

In the last decades, South American ecosystems underwent important functional modifications due to climate alterations and direct human intervention on land use and land cover.  Among  remotely  sensed  data  sets,  NOAA-AVHRR  ?Normalized  Difference Vegetation  Index?  (NDVI)  represents  one  of  the most  powerful  tools  to  evaluate  these changes  thanks  to  their  extended  temporal  coverage.  In  this  paper  we  explored  the possibilities and  limitations of  three commonly used NOAA-AVHRR NDVI series  (PAL, GIMMS  and  FASIR)  to  detect  ecosystem  functional  changes  in  the  South  American continent. We performed pixel-based  linear  regressions  for  four NDVI variables  (average annual, maximum  annual, minimum  annual  and  intra-annual  coefficient  of  variation)  for the  1982-1999  period  and  (1)  analyzed  the  convergences  and  divergences  of  significant multi-annual  trends  identified across all  series,  (2)  explored  the degree of  aggregation of the  trends using  the O-ring statistic, and  (3) evaluated observed  trends using  independent information  on  ecosystem  functional  changes  in  five  focal  regions.  Several  differences arose  in  terms of  the patterns of change  (the sign,  localization and  total number of pixels with  changes).  FASIR  presented  the  highest  proportion  of  changing  pixels  (32.7%)  and GIMMS the lowest (16.2%). PAL and FASIR data sets showed the highest agreement, with a convergence of detected trends on 71.2% of the pixels. Even though positive and negative changes  showed  substantial  spatial  aggregation,  important  differences  in  the  scale  of aggregation  emerged  among  the  series,  with  GIMMS  showing  the  smaller  scale  (≤11 pixels). The independent evaluations suggest higher accuracy in the detection of ecosystem changes  among  PAL  and  FASIR  series  than with GIMMS,  as  they  detected  trends  that match expected shifts. In fact, this last series eliminated most of the long term patterns over the continent. For example, in the ?Eastern Paraguay? and ?Uruguay River margins? focal regions,  the  extensive  changes  due  to  land  use  and  land  cover  change  expansion  were detected by PAL and FASIR, but completely  ignored by GIMMS. Although  the  technical explanation of the differences remains unclear and needs further exploration, we found that the evaluation of this type of remote sensing tools should not only be focused at the level of assumptions  (i.e.  physical  or mathematical  aspects  of  image  processing),  but  also  at  the level of results (i.e. contrasting observed patterns with independent proofs of change). We finally present the online collaborative initiative ?Land ecosystem change utility for South America?,  which  facilitates  this  type  of  evaluations  and  helps  to  identify  the  most important functional changes of the continent