Defensive traits of savanna trees - the role of shoot exposure to browsers

To investigate patterns of plant defence, a study was conducted on 13 tree species in Botswana. We tested the hypotheses that (1) shoots on small, young trees are more defended than shoots at the same height on tall, mature individuals and (2) shoots within browsing height are more defended than shoots above the reach of browsers, on mature trees. Plant traits studied were total phenolics, tannin activity, nitrogen (N), acid-detergent fibre (ADF), and degree of spinescence (in three species). We found some support for the first hypothesis. Overall young trees had a significantly higher concentration of total phenolics than mature trees and on species level these differences were significant for three of the 13 species. However, we found no differences in tannin activity, spinescence or ADF concentration between young and mature trees, and four species had higher levels of N in young trees, suggesting higher overall palatability. Contrary to our second hypothesis, shoots within browsing height were less chemically defended (total phenolics and tannin activity) than shoots above browsing height in six of 13 species. Two species had less ADF in lower shoots, and no significant differences were found in N or spinescence for any species. This study included a broad range of tree species at two sites and the results indicate that general theories of plant defence, originating from boreal and northern temperate forests, cannot be simply applied to savanna ecosystems.