The savanna-grassland 'treeline': why don't savanna trees occur in upland grasslands?

摘要

Treeless grasslands with climates that can support tree growth are common in upland regions around the world. In South Africa, the upland grasslands are adjacent to lowland savannas in many areas, with an abrupt boundary between them that could be termed a savanna-grassland 'treeline'. Both systems are dominated by C₄ grasses and burn regularly, yet fire-tolerant savanna trees do not survive in the grasslands. The upland grasslands experience lower temperatures throughout the year and frost in winter, compared with the warmer savannas. We tested whether frost in the dormant season or slow growth in the growing season in conjunction with frequent fires may explain the tree-less state of grasslands. We measured Acacia seedling growth for a year in a transplant experiment at ten sites across an altitudinal gradient (42-1704 m) from savannas to grasslands. The effect of frost on seedlings was scored during the following winter. Across all species, height (t=-6.04, d.f.=471, P<0.001), biomass (t=-4.56, d.f.=228, P<0.001) and height increase (t=-3.40, d.f.=471, P<0.001) were significantly higher at savanna sites. As the plants were irrigated and initially supplied with nutrients, the main factor affecting growth was likely to be growing season temperature. Saplings that experience slow growing conditions will take longer to reach a height above the flame zone and will therefore have a lower probability of reaching adult tree height and surviving fires. Day length may be the most important cue for the end of the growing season in savanna trees, as growth decreased with shortening day length in February-March while temperatures were still high and plants were not water limited. Synthesis. Savanna trees grew more slowly in cooler upland grassland sites compared with lower elevation warm savanna sites and, under frequent fire regimes, would be prevented from reaching maturity. This may be true globally for similar grasslands where tree growth can occur and could partly explain the lack of trees in grasslands.