Estimating Woody Plant Density from Aerial Photog raphs in Communal and Leasehold Land Tenure System s in Northwest Botswana

Abstract

Information on woody plant density (WPD) is important for assessing impacts of land use, climate variability and climate change in semi-arid lands. Previous studies in Botswana have used visual methods with the aid of a portable microscope with 40× magnification to estimate WPD from counts of apices of shadows of woody plants on air photographs. This study investigated the reliability of quantitative woody plant data established using this procedure. The investigation focused on consistency of the visual derived data, relationship of the photo and field based WPD, effect of different photo scales and film types and reliability of the data in determining trends over time. Results showed that variations in woody plant counts (WPC) made at different periods by the same observer were not statistically significant at 95% probability level. Relatively strong relationships were established between ground and photo based WPD, R^2 = 0.74: P < 0.05 and R^2 = 0.62: P < 0.05 for predominantly tree and shrub layer sites, respectively. Different photo scales and film types resulted in statistically significant differences in WPC but these differences were minimized by standardizing the data to a single base scale and film type. The standardized multi-date WPD data from different scales and film types showed consistent trends with landscape and land use changes in the area. This study showed that calibrated low cost visual based geo-spatial data analysis methods could provide required information for detailed assessment of change invegetation structure in semi-arid lands in response to land use and the indicated climate change.

Keywords: aerial photographs, canopy shadows, photo scale, photo film type, semi-arid lands, visual counts, woody plant density