Seedling growth and spinescence of three Vachellia species' populations in valleys formed by tectonic uplift

Abstract

The Southeastern Coastal Hinterland geomorphic province of South Africa is dissected by deep valleys created by tectonic uplift in the early Miocene and late Miocene/early Pliocene. We hypothesised that seedlings of Vachellia nilotica, V. tortilis and V. karroo grown from seeds from different valleys would differ in growth and spinescence because of geographical separation. Ninety seedlings (30 per species) were grown for 3 months in uniform conditions in a greenhouse. Mean height of V. nilotica and V. tortilis seedlings was greatest for the Phongolo populations and lowest for the Thukela populations. Maximum stem diameter of both species was attained by individuals from the Phongolo population. Mean spine dry mass of V. nilotica seedlings was greatest for the Phongolo population, suggesting a growth‐defence trade‐off, but did not differ for V. tortilis. Results suggested that Phongolo populations of V. nilotica and V. tortilis may be characterised by morphotypes clearly distinct from those in the other valleys. Unclear and inconsistent results for V. karroo in this study suggested subtly different morphotypes occur in each valley. We propose that distinct Phongolo morphotypes of V. nilotica and V. tortilis are explained as evolutionary adaptations to moderate fire and high browsing pressure compared to populations South of Phongolo.