Basal bud banks are the primary determinants of survival in woody legume seedlings after clipping

Abstract

Disturbances determine the structure of savannas by influencing growth at all stages of a tree's life. Since the seedling stage is such a significant part of a tree's life cycle, greater knowledge of this stage informs woody management and encroachment prevention. In this study, we tested the effect of a suite of seedling functional traits on seedling survival after experimentally-induced disturbance. Twelve mimosoid legume tree species, an important group in African savannas, with different provenances, were grown from seed under experimental conditions. Seedlings were clipped at 1 cm above ground at age 30 days, to simulate disturbance, and their survival recorded. A number of above- and below-ground traits quantifying height, biomass, bud bank and architecture were measured prior to clipping in order to assess what traits could be responsible for post-clipping survival. We also quantified root contraction by calculating the change in height of the lowest bud between the ages of 6 and 60 days. Fire regime and browser biomass of the native range of each species were calculated to test whether these factors predicted seedling survival. Seedling survival was primarily predicted by the whether the plant possessed a bud below 1 cm above ground level, i.e. below the height of clipping. For the first time we report that, in several Vachellia species, root contraction occurred, bringing basal buds closer to the ground, and in some cases below the ground, thus increasing seedling survival after clipping. No environmental variables were significant predictors of survival. Our results suggest that, at this young age, buds close to the soil surface are the most important trait predicting seedling survival, while other traits that are important for saplings and adults do not significantly influence the survival of seedlings. At the seedling stage, herbivory and fire are possibly inflicting the same degree of selection pressures and thus little trait differentiation is evident across the environmental gradient.