Is the lack of leguminous savanna trees in grasslands of South Africa related to nutritional constraints?

Abstract

As with many grasslands globally, the Highveld grasslands of South Africa are tree-less, despite having a climate that can support tree growth. Models predict that fire maintains these grasslands. The question arises as to why fire-tolerant savanna trees do not survive in these ecosystems? Savanna tree survival in mesic areas is restricted by demographic bottlenecks, specifically limitations to sapling-escape from fire. It was hypothesised that ancient highly leached soils from grassland areas would prevent saplings from growing fast enough to escape the fire-trap. Growth rates of savanna tree seedlings (Acacia karroo Hayne and Acacia sieberiana Burtt Davy) were measured in a common garden experiment using soils from ten sites collected along a savanna-grassland continuum. Soils from grassland sites were relatively nutrient-poor compared to those from savannas with lower pH, and associated cations. A. sieberiana growth rates responded to pH and these nutrients, whereas A. karroo growth was less strongly linked to specific nutrients. Even so, both species accumulated more biomass when grown in soils from savanna sites compared to grassland sites. An exception was a low elevation low nutrient savanna site that resulted in poor growth, yet sustains high tree biomass in situ. Differences between growth in grassland and savanna soils were small. They may contribute to, but are unlikely to explain, the treeless nature of these grasslands.

Shade, nutrients, and grass competition are important for tree sapling establishment in a humid savanna

“…However, moist-dystrophic savannas may host increased woody vegetation of low nutritional quality (du Toit 1995). Additionally, Acacia sapling growth rates were higher in nutrient-rich soils when compared to growth in nutrient-poor grassland soils (Wakeling et al 2010). In contrast, certain studies have found that abundant soil nutrients improve herbaceous productivity which in turn suppresses tree sapling establishment (Cohn et al 1989, Kraaij and Ward 2006, van der Waal et al 2011).…”

“…3. Nutrients usually enhance the establishment success of tree seedlings (Wakeling et al 2010). Hence, nutrients should enhance the survival and growth of tree saplings.…”

Future of African terrestrial biodiversity and ecosystems under anthropogenic climate change

“…Chief among these is the role of soil nutrient availability in limiting species ranges 72 , and in supporting a sustained CO 2 fertilization response 73,74 . Plant nutrient status has commonly been found to become limiting in long-duration CO 2 fertilization experiments, though few of these experiments have been conducted under dry and hot tropical to subtropical conditions 7 where CO 2 fertilization leads to replacement of one plant functional type by another.…”

Herbivory effects on saplings are influenced by nutrients and grass competition in a humid South African savanna

“…2. Nutrient addition will increase the survival and growth of tree saplings (Wakeling et al, 2010). Plants invest in root biomass more than stem biomass when soil nutrients are limited (Poorter et al, 2012).…”