Defoliation depletes the carbohydrate reserves of resprouting Acacia saplings in an African savanna

摘要

Over the past century there has been a global trend towards tree expansion and densification in rangelands and savannas. This phenomenon is commonly referred to as bush encroachment. In South Africa Acacia karroo is one of the key species responsible for bush encroachment. It has been suggested that the combination of fire and browsing might limit bush encroachment by A. karroo more effectively than either browsing or fire alone. We hypothesized that these repeated disturbances progressively deplete root carbohydrates and compromise resprouting ability. This was tested by burning and then manually defoliating A. karroo once a month for 1 year. Manual defoliation did not inhibit the rapid shoot elongation after topkill of A. karroo saplings. During this initial phase, the growth of the new shoots of A. karroo was dependent more on mobilised root reserves than on photoassimilates from the new shoots. Frequent manual defoliation of resprouting A. karroo saplings prevented the replenishment of starch reserves. We suggest a mechanism for how the interaction of browsing and fire can suppress and perhaps reverse bush encroachment in African savannas. Saplings that have reduced starch reserves at the end of dry season due to browsing will struggle to resprout if they are burnt. Even if they do not die, they will be less able to escape fire damage in the next fire than if they had been able to resprout unimpeded.