Evaluating the viability of Acacia populations in the Negev Desert: a remote sensing approach

Abstract

Recent surveys have pointed to massive mortality of Acacia trees in the Negev Desert, Israel. These observations were interpreted as evidence for a possible deterioration in the conservation status of these species. In this study we employed a demographic approach based on image processing of historical (1956) and recent (1996) aerial photographs to evaluate the viability of Acacia populations in the Negev. Populations were studied in two different sites and in habitats representing contrasting runoff regimes (large wadis vs. small runnels) within each site. In contrast to expectations, all populations showed an increase, rather than a decrease in density between 1956 and 1996. Although a strong recruitment of young trees may lead to a decrease in mean tree size, we observed a significant increase in average tree size for all populations. Distributions of tree size obtained for both 1956 and 1996 were characterized by high frequency of small trees, a result consistent with the observed positive growth rates of the populations. Patterns of demographic variation were scale-dependent. The southern site exhibited significantly greater changes in density and tree size than the northern site and, for each site, wadi populations exhibited higher growth rates than populations growing in nearby small runnels. Changes in tree density were strongly correlated with changes in mean tree size across the different site/habitat combinations, but were weakly correlated at the scale of 100×100 m grid cells. In spite of the general trend of increase in tree density, many grid cells were characterized by a decline in density, indicating that demographic data obtained from relatively small research plots may lead to misleading conclusions concerning the viability of plant populations.