Abstract
The proliferation of woody vegetation on open grass savannas constitutes degradation in rangeland quality. Historical high spatial resolution satellite imagery in archive provides possibilities for assessing increase in woody vegetation cover on the rangelands. This paper examines the quantification of woody vegetation in the field and on historical high spatial resolution imagery, using the southern granites research supersite in the Kruger National Park (KNP) as test case. The recently established research supersites represent the key characteristics of the vegetation zones in the KNP. Field quantification of woody cover was undertaken in June and September 2013 by employing a detailed procedure that quantified the area covered by individual woody (tree, shrub) canopies at twelve one hectare sample plots. The area covered by the woody individuals at the respective plots was then totalled in order to yield the fraction (%) of cover per hectare. The woody cover data were then related to test recent historical imagery in order to assess woody cover estimation procedures on historical imagery. Digital high spatial resolution aerial photographs (dated 2010) and dry period (spring/autumn) SPOT multispectral images (September 2001, April 2012) were used; the dry period dates selected so as to eliminate herbaceous vegetation from the analysis. For the newest (2012) image, sub-pixel classification correctly assigned woody cover at the field sample plots to their 2013 field-derived cover fractions. The results indicate that sub-pixel classification, validated by detailed field quantification of woody cover, can accurately map woody encroachment on savanna rangelands using historical high spatial resolution imagery.
Highlights
Savanna rangelands in Africa provide habitat for a variety of ungulates, which makes their conservation of biodiversity significance
Autumn (April) and spring (September) images were judged ideal for detecting woody cover on images, because the woody vegetation is in leaf and herbaceous vegetation dry
We recommend the woody cover field quantification and sub-pixel woody cover image classification methods utilised in this study for similar studies of savannas
Summary
Savanna rangelands in Africa provide habitat for a variety of ungulates (du Toit & Cummings, 1999), which makes their conservation of biodiversity significance. Proponents of the use of objectoriented classification in mapping bush encroachment stress the advantage that it groups adjacent pixels into contiguous multi-dimensionally homogenous clusters that represent natural land cover patterns and minimize classification errors that result from single pixels with outlier values and areas of complex spectra due to mixed coverage (Laliberte et al., 2004; McGlynn & Okin, 2006). Such approaches appear inappropriate for mapping and quantifying the actual woody cover on savannas in that aggregating woody cover that is inherently patchy in nature amounts to introducing over-estimation error. We assess the potential of subpixel classification in monitoring bush encroachment using high spatial resolution historical aerial photographs and multispectral imagery
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