Textural Analysis of Historical Aerial Photography to Characterize Woody Plant Encroachment in South African Savanna

Hudak, Andrew T
Wessman, Carol A
Remote Sensing of Environment
Publication Year: 
Transitions from grassland to shrubland through woody plant encroachment result in potentially significant shifts in savanna ecosystem function. Given high resolution imagery, a textural index could prove useful for mapping woody plant densities and monitoring woody plant encroachment across savanna landscapes. Spatial heterogeneity introduced through mixtures of herbaceous and woody plants challenges quantitative assessments of changing woody plant density using remotely sensed imagery. Moreover, woody plant encroachment occurs across decadal time scales, restricting remote sensing analyses to historical aerial photograph records. Heterogeneity in vegetation structure has a significant influence on local pixel variance in high resolution images. We scanned black and white aerial photographs for 18 sites of varying woody plant density, producing images of 2-m grain size. Omnidirectional variograms derived from these images had ranges of approximately 10 m and sills highly sensitive to woody plant density, prompting us to use a textural index to indicate landscape variation in woody plant density. For validation purposes, we measured several woody overstory structural parameters in the field; a factor analysis revealed woody stem count as the best correlate with image texture. Significance of the regression of image texture on woody stem count declined as grain size of the 2-m images was coarsened to simulate that of SPOT and Landsat satellite sensors. At 10-m resolution, our textural index proved a significant indicator of woody plant density. We mosaicked sequential aerial photographs scanned at 10-m resolution and then applied our textural filter, producing maps of historical woody plant distribution that reflected patterns in soil and vegetation type. More accurate maps of canopy structure and structural change are needed to explore potential effects of woody plant encroachment on biophysical and biogeochemical processes at large scales.
Name of Journal: 
Remote Sensing of Environment
Resource Type: 
Document Type: 
Journal Issue/Article
Grassland Society of Southern Africa

The Grassland Society of Southern Africa (GSSA) is involved and concerned with the science and practice of range and pasture management. This broad field involves primarily the use and conservation of natural resources. It encompasses applied fields such as livestock production, wildlife management, nature conservation, water catchment management and range and mine-dump rehabilitation. The disciplines include, amongst others, ecology, botany zoology, range and pasture science, animal science, soil science and genetics. This collection includes journal articles from the African Journal of Range and Forage Science as well as related articles and reports from throughout the Southern African region.