Fire effects on vegetation in a grassy dwarf shrubland at a site in the eastern Karoo, South Africa

Fire is rare in semi-arid eastern Karoo dwarf shrublands, South Africa, and responses to fire are largely unknown. Recent increased grassiness, and hence fuel loads, at Grootfontein in the Eastern Cape allowed an accidental fire (24.3 ha) to carry, and afforded the opportunity to examine compositional and structural effects of fire on a grassy dwarf shrubland. Sampling seven months after the fire, 108 species (102 perennial) were encountered, of which 74 were resprouters, six were fire sensitive (non-sprouters), and the remainder (rare, non-perennial or herbaceous) had an unknown response. The dominant pre-fire shrub, Eriocephalus ericoides, was extirpated by the fire, as was the unpalatable and sometimes invasive Ruschia intricata. All grass species resprouted, and grass became the dominant life-form after the fire, indicating a possible conferred competitive advantage. Resprouting shrubs grew to only a small fraction of their pre-burn size. The unpalatable, aromatic shrub Stachys rugosa was the dominant post-burn shrub. Extirpated species will need to regenerate from seeds, but no seedlings were found in the gaps where shrubs had stood. The widespread ability to resprout confers resilience on this vegetation, but fire does induce changes in structure, in the dominance of life forms, and it decreases canopy cover.Keywords: dwarf shrubs, grass, Nama-Karoo, non-sprouter, resproute

Interactions of grazing and rainfall on vegetation at Grootfontein in the eastern Karoo

Rainfall and grazing are primary drivers of vegetation composition in the Nama-Karoo. Increased rainfall increases grassiness, to where Nama-Karoo transitions to grassland. Severe grazing treatments (e.g. continuous or summeronly) increase abundance of grazing-tolerant dwarf shrubs and annual grasses, and decrease perennial grasses. Grootfontein, which is ecotonal between the Nama-Karoo and Grassland Biomes, houses long-term grazing trials. The area has experienced higher-than-average rainfall in recent decades. Plant basal cover data from the 1960s and 2010s allow several hypotheses to be addressed: (1) historical severe grazing (until 1985) will limit subsequent grassiness (grazing legacy effect); (2) severe grazing will preclude increases in grassiness, independent of rainfall (herbivore trap effect); and (3) historically leniently-grazed sites will transition to grassland with increased rainfall (biome shift effect). Rainfall was lower from 1957–1966 (350 mm) than from 2003–2012 (490 mm). The grazing legacy effect was supported based on the abundance of Aristida diffusa, despite all sites becoming much grassier. The herbivore trap effect was not supported. The biome shift effect was supported in that shifts to grassland sometimes occurred. Results suggest that increasing rainfall has prompted a shift to much increased grassiness and decreased abundance of dwarf shrubs, and that grazing had a smaller secondary effect.Keywords: biome shift, grassy dwarf shrubland, grazing legacy, herbivore trap, Nama-Karo

Long-term livestock grazing increases the recruitment success of epigeal termites: insights from a >75-year grazing experiment in the Karoo, South Africa

<p>Epigeal termites alter ecosystem-level processes through foraging and mound building. These effects are proportional to density. Whilst much is known about the factors influencing mound density in natural settings, how termites respond to livestock grazing has been overlooked. We compared the characteristics and density of <i>Trinervitermes trinervoides</i> (Isoptera: Termitidae) mounds in the Karoo (South Africa) within an exclosure and continuously-grazed plot from a >75-year grazing experiment. Specifically, we asked whether long-term livestock grazing impacted (1) termite mound density, volume, basal area and size, (2) mound occupancy and age, (3) vegetation characteristics and (4) soil properties. Long-term grazing increased mound density two-fold. This was coupled with a decrease in mean mound size, an increase in the size of the largest mounds and range of mound sizes, and an increase in the proportion and numbers of young, active mounds. However, these changes did not alter landscape-scale mound basal area or volume. Vegetation was unaffected by long-term grazing but it did cause soil phosphorus, magnesium and calcium depletion. We suggest that livestock grazing provides additional forage resources for termites through litter breakup and dung production, leading to greater mound recruitment and thus densities, whilst allowing mounds to achieve greater maximum size.</p

Different clades and traits yield similar grassland functional responses

Plant functional traits are viewed as key to predicting important ecosystem and community properties across resource gradients within and among biogeographic regions. Vegetation dynamics and ecosystem processes, such as aboveground net primary productivity (ANPP), are increasingly being modeled as a function of the quantitative traits of species, which are used as proxies for photosynthetic rates and nutrient and water-use efficiency. These approaches rely on an assumption that a certain trait value consistently confers a specific function or response under given environmental conditions. Here, we provide a critical test of this idea and evaluate whether the functional traits that drive the well-known relationship between precipitation and ANPP differ between systems with distinct biogeographic histories and species assemblages. Specifically, we compared grasslands spanning a broad precipitation gradient (∼200-1,000 mm/y) in North America and South Africa that differ in the relative representation and abundance of grass phylogenetic lineages. We found no significant difference between the regions in the positive relationship between annual precipitation and ANPP, yet the trait values underlying this relationship differed dramatically. Our results challenge the trait-based approach to predicting ecosystem function by demonstrating that different combinations of functional traits can act to maximize ANPP in a given environmental setting. Further, we show the importance of incorporating biogeographic and phylogenetic history in predicting community and ecosystem properties using traits