When it comes to nutritional value, grasses are probably the world’s most important plant family. All our cereals, including maize, wheat, sorghum, rye, millet, rice, barley and sugarcane are grasses. Grasses also provide fodder for animals that supply us with meat, leather and milk. Within the South African agricultural sector, cereal accounted for about 30% of total gross agricultural production in the country in 2017, with total output of 18.6 million tonnes for wheat and maize in 2020-21 (Agriseta 2019; Lyddon 2021). Besides food, grasses are also cultivated for lawns, landscaping, erosion control, scaffolding and furniture (e.g. bamboos) and the production of biofuel (Gibbs Russell 1988; Clark & Poll 1996).
Grasses also produce a peculiar-looking microscopic plant particle in the form of mineralized silicon dioxide (Metcalfe 1960; Piperno 2006) (Figure. 1).
Known as phytoliths (Gr. phyto = plant & lithos = stone) these silica bodies are very tough and hardy and can remain intact in soils and sediments for hundreds of thousands, or even millions of years, long after their grassy host has died and disappeared. Not only do they preserve well, grass phytoliths are also very diagnostic and identifiable (Rossouw 2009; Rossouw and Scott 2011). These oddly-shaped silica bodies occur primarily in leafs and stems of grasses and are so small that a microscope is required to study them. They are formed in grass silica short cells, which are specialized silica cells or idioblasts, found in both C3 – and C4 – type grasses. It is located in the costal and intercostal zones of the leaf epidermis, overlying the vascular bundles and their associated sclerenchyma (Metcalfe 1960; Kok 1968) (Figure. 2).
Their size generally varies between 5 and 40 micron in length (1 micron = 1 thousandth of a millimetre) and their shape can tell us something about the ecological niche of the grass that produces it; for example, whether it is from a water-loving or arid adapted species, or a species that prefers to grow in winter-rainfall or summer-rainfall regions (Rossouw 2016). This is because the distribution of grasses is primarily linked to growing season temperature, which seems to account for the geographic distribution of C3 and C4 grasses (Vogel et al. 1978; Ellis et al. 1980; Cerling et. al. 1997; Ehleringer et al. 1991, Ehleringer et al. 1997; Sage 2004). C3 and C4 respectively refer to three-carbon and four-carbon molecules being the first products in photosynthesis (Ehleringer and Monson 1993). They represent two photosynthetic pathways that exist among grasses – the ancestral C3 (Calvin–Benson) photosynthetic pathway, which is utilized by grasses generally thriving under cool, dry to mesic winter-rainfall conditions, and the C4 (Hatch–Slack) photosynthetic pathway, which favour elevated temperatures during the growing season (Vogel et al. 1978; Ellis et al. 1980).
Although grass silica short cells comprise only a portion of the total siliceous residue and have restricted distributions within grasses, they provide the most taxonomically useful types of grass phytoliths (Twiss et al. 1969, Mulholland 1989; Fredlund and Tieszen 1997; Rossouw and Scott 2011; Rossouw 2016) (Figure. 3).
When fossil phytoliths are successfully extracted from ancient cave sediments and soils, carnivore dung or even fossilized teeth, they can provide valuable information about past environmental conditions by offering a wider perspective on the extent of climate variability (Foster et al. 1990; Henry et al. 2012; Scott et al. 2016; Ecker et al. 2018). For example, a mass death site made up four fossilised bone beds was discovered buried near Senekal in the eastern Free State in 2003 (Backwell et al. 2018). The site contained the densely packed and articulated skeletons of red hartebeest, black wildebeest, leopard, jackal and warthog, suggesting that these animals must have perished together under catastrophic conditions. Grass phytoliths extracted from the ancient soils associated with the bone bed yielded a proportionately high number of morphotypes that are primarily produced by arid-adapted grass species not prevalent in the area today (Figure. 4),
indicating a trend towards severe desertification and drought in the eastern FS between 3840 and 3500 years ago. Thus, whereas ecologists generally study organisms on a time-scale of less than 100 years, palaeocological analyses focussing on fossil grass phytoliths can provide an expanded temporal perspective on community and organism responses going back for thousands of years. More importantly, with modern global warming trends seen as the result of mainly human-induced activities, the bone bed study shows that grass phytoliths can be helpful when comparing long-term patterns in climate change during periods before extensive anthropogenic influences.
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