Frost tolerance in Vachellia sieberiana var. woodii in the high-altitude grasslands of southern Africa

Abstract

High altitude grasslands are frequently within a climatic zone that should support trees, yet trees are absent. Low temperature is often cited as a possible mechanism that excludes trees. We investigated the effect of white frost and black frost on Vachellia sieberiana saplings, possible preadaptation to frost; a possible strategy employed to tolerate frost; and whether seeds of V. sieberiana are viable after exposure to sub-zero temperatures. Seeds of V. sieberiana collected from trees at two different altitudes were germinated and the saplings potted out. Saplings were subjected to simulated white frost and to simulated black frost. Tissue samples from chilled saplings were analysed for soluble sugars in the cell sap. Seeds were also exposed to temperatures between 0 and -9°C and then germinated. V. sieberiana saplings were able to coppice after extensive damage to above-ground tissue as a result of exposure to white frost. However, saplings were unable to compensate adequately in one growing season for the loss of above-ground growth after more than 25% topkill. This may keep saplings within the frost trap leaving them vulnerable to further frost, fire and browsing. Saplings exposed to black frost at temperatures down to -6°C were also able to coppice after damage to above-ground tissue, but prolonged exposure to black frost at less than -6°C resulted in sapling mortality. Saplings grown from seed from a low-altitude site where frost events were frequent exhibited preadaptation to low temperatures, while those grown from high altitude seed experiencing less frost did not. V. sieberiana did not increase soluble sugars within the cell sap in response to chilling stress. The seeds of V. sieberiana are resistant to damage at sub-zero temperatures, maintaining limited viability after exposure to -9 °C. Frost and very low temperatures, therefore, limit the establishment of V. sieberiana saplings, but not recruitment (seed germination). Once a sapling has escaped the frost trap, it will be able to establish itself successfully.