Resprouting versus turning up of leaning trees in a subtropical coastal dune forest in South Africa

Abstract

To survive, leaning trees on steep slopes with loose substrate can either resprout or turn upward to regain the normal vertical orientation of the growing tip. Data from 19 tree species were collected from 20 transects in coastal dune forest. Resprouting was negatively correlated with turning up and species abundance, and positively correlated with leaning and a dead primary stem. In contrast, turning up was associated with low probabilities of leaning and higher species abundance but not with a dead primary stem. Slightly inclined trees turned upwards more than severely inclined ones, which mostly resprouted. Leaning trees without exposed roots turned upwards, while severe erosion caused leaning and resprouting. Consistent with the latter, leaning trees in stable dune slacks frequently turned upwards, whereas leaning trees on narrow dune crests and steep landward slopes seldom did so. Small trees were more likely to turn upwards than big trees. Because of potentially greater costs to the individual of sprouting, stem reorientation precedes the resprouting response across the disturbance gradient. Consequently, species prone to stem reorientation and less likely to resprout, are dominant during late-succession. This study emphasizes the advantage under low levels of disturbance of turning up as an alternative strategy to resprouting for leaning trees.