Intraspecific Responses of Seedlings of Three Vachellia Species to Simulated Browsing Reflect Adaptive Traits of Older Life Stages

Abstract

Intraspecific variation in adaptation to herbivory has been studied in juvenile (sapling) and adult (reproductive) stages of woody species in African savannas, but has not been studied at the early seedling stage. We hypothesized that, among Vachellia species commonly occurring in African savannas, spinescence increases and growth rate decreases after herbivory, but these responses would be expressed most strongly in populations with slower growing seedlings. Seedlings of V. nilotica , V. tortilis, and V. karroo were grown from seeds of different populations within the Southeastern Coastal Hinterland geomorphic province of South Africa. Seedlings were grown in a greenhouse and clipped at three intensities when they were 3 months old. Responses were determined for seedlings harvested 3 months later. Statistically significant (p < 0.05) interacting effects of clipping and population were rare. Clipping increased the spine mass fraction of V. tortilis seedlings from one population. Clipping reduced the relative height growth of V. nilotica and V. tortilis seedlings, while populations of V. nilotica and V. karroo differed in relative growth rate. We interpret weak vertical regrowth of V. nilotica and V. tortilis seedlings as reflecting adaptation to herbivory reported for saplings and adults in other studies. Conversely, we interpret strong height regrowth of V. karroo seedlings as reflecting adaptation to fire in association with herbivory or shading, as reported for older plants elsewhere. The study highlights the importance of studying plant traits relevant to herbivory in different populations and at different life stages to better understand adaptations to herbivory.