Mapping Mediterranean scrub with satellite imagery: biomass estimation and spectral behaviour

Abstract

Millennia of intensive use have led to severe land degradation in the Mediterranean basin, much of which is now under serious risk of desertification. Very large extents are presently covered with scrub dominated by plants of the genus Cistus. The level of soil protection provided by these shrubs is proportional to their biomass, so it is important to be able to quantify this variable. The potential for mapping Cistus scrub biomass using optical satellite imagery was investigated in this study. It was found that the peculiar adaptations of the Cistus to summer drought strongly influence its spectral behaviour, which, during this season, is similar to that of vegetation of arid and semi-arid regions. During the rainy winter this pattern changes and becomes very similar to that of active green vegetation. In summer images, individual Landsat Thematic Mapper (TM) bands, including the near-infrared band, presented negative correlations with biomass. However, in spite of these inverse relationships, the Normalized Difference Vegetation Index (NDVI) had a high positive correlation with biomass, so this variable can be adequately mapped using satellite images. Seventy-four per cent of the biomass values estimated with linear regression models, using the NDVI as predictor, were within 50% of the ground measurements.

Acknowledgments

Fernando Catarino, José Miguel Cardoso Pereira and Rosa Paiva provided valuable advice. Carlos Santos, Cláudia Franco, Evelina Moura, Serafim Pinto and Sérgio Rodrigues helped during the field work. Rosa Paiva kindly reviewed the manuscript. Centro Nacional de Informaça˜o Geográfica made available the TM winter image in the Framework of Demeter Program. The City Council of Mértola provided logistic support during the field campaigns.