Relationship between herbaceous biomass and 1‐km² Advanced Very High Resolution Radiometer (AVHRR) NDVI in Kruger National Park, South Africa

Abstract

The relationship between multi‐year (1989–2003), herbaceous biomass and 1‐km² Advanced Very High Resolution Radiometer (AVHRR) Normalized Difference Vegetation Index (NDVI) data in Kruger National Park (KNP), South Africa is considered. The objectives were: (1) to analyse the underlying relationship between NDVI summed for the growth season (ΣNDVI) and herbaceous biomass in field sites (n = 533) through time and (2) to investigate the possibility of producing reliable herbaceous biomass maps for each growth season from the satellite ΣNDVI observations. Landsat Enhanced Thematic Mapper Plus (ETM+) and Thematic Mapper (TM) data were used to identify highly heterogeneous field sites and exclude them from the analyses. The average R ² for the ΣNDVI–biomass relationship at individual sites was 0.42. The growth season mean biomass and ΣNDVI of most landscape groups were strongly correlated with rainfall and each other. Although measured tree cover and MODIS estimates of tree cover did not have a detectable effect on the ΣNDVI–biomass relationship, other observations suggest that tree cover should not be ignored. The ΣNDVI was successful at estimating inter‐annual variations in the biomass at single sites, but on an annual basis the relationship derived from all the sites was not strong enough (average R ² = 0.36) to produce reliable growth season biomass maps. This was mainly attributed to the fact that the biomass data were sampled from very small field sites that were not fully representative of 1‐km² AVHRR pixels. Supplementary field surveys that sample a larger area for each field site (e.g. 1 km² or larger) should account for the variability in biomass and may improve the strength of ΣNDVI–biomass relationships observed in a single growth season.

Acknowledgements

This work was partially funded by NASA Earth Systems Sciences Fellowship O2‐0000–0130, SA Department of Agriculture (Directorate: Land Use and Soil Management) and SA Department of Science and Technology (LEAD project funding). We are grateful to SANParks for allowing this research and to all the KNP personnel for collecting the field data over the past 14 years. We thank Dr Ralph Dubayah (UMD) for comments on the manuscript.