Past lives caught in the dust of trees
I’m currently working at the Annals of Botany to help out with their social media side. There’s a bit more to it than subtly dropping links to their site, like this one. At the moment I’m struggling with the Facebook integration, but there’s a fun side too. I wouldn’t have browsed AoB if I’d not been hired, and that means I would have missed out on papers like Phytoliths in woody plants from the Miombo woodlands of Mozambique by Julio Mercader and his team at Calgary. I’ll admit the article title doesn’t say much to the layman, but it’s actually something deeply cool that I didn’t find out about till my MPhil.
If megaliths are big stones and microliths are small stones like arrowheads, then phytoliths are clearly phyto-stones. Phyto– in this case meaning plant.
Phytoliths are microscopic stones formed in some plants. When a plant’s roots draw up water they also draw up the minerals dissolved within it. In the case of the silica this gets pulled out of the water and deposited either in the cells or between the cells. The exact shape of the phytoliths varies on the part of the plant the silica is deposited in, the availability of silica and, most excitingly for archaeologists, the species of the plant.
Phytoliths are useful because normally biological material doesn’t hang around long in the soil. Once something is dead it’s lunch for something else. Phytoliths are biological material but not organic, so they don’t break down in the same way. Mercader et al. are clear that’s is not an unambiguous relaltionship. Time still has an effect, but it’s easier to find phytoliths than it is to find organic remains for plants. Still as useful as they are, phytoliths alone are not enough. A handful of phytoliths under a microscope would just look like a nobbly (or smoothish) thing. If you haven’t seen what a baobab phytolith looks like, you’re not likely to guess from simply looking at the phytolith and this is where Mercader et al step in.
If you’re interested in studying the palaeoecology of Africa in the past you’ve been relatively limited to north of the equator. Mercader spotted that the biggest phytochorion (plant ecosystem) south of the Sahara is the Miombo woodlands. It’s huge. It runs from Angola and Namibia in the west to Mozambique in the east and from the Tanzanian shores of Lake Victoria in the north to Botswana and South Africa in the south. The dominant tree is Miombo, hence the name, which refers to a number of trees of the same genus, but with different species. Obviously it’s a crucial zone for understanding the ecology of sub-Saharan Africa, but no-one has described the phytoliths of the region.
The area studied was a transect through the forest between the Lake Niassa shore at Metangula and the highlands at Njawala, a distance of 50km and a rise from 465m above sea-level to 1841 above sea-level. They also used indigenous collectors to sample the flora in a 5km radius around Metangula and Njawala. They estimate they got over 90% of the species used by the native peoples. Given that a lot of usage is likely to be dominated by relatively few species, that’s a lot of plant material. There’s then a LOT of listing and description of phytoliths.
The common feature that amazes me is how small many of these phytoliths are. Some are just 20–40 μm long. A micrometre (μm) is one thousandth of a millimetre. Despite this Mercader et al, point to the phytoliths at the other end of the scale, some are around 200μm in length and over half are over 50μm. This means if you use standard techniques to sieve for phytoliths using a 50.238 to 63.246μm cut-off, you’ll miss all these larger phytoliths. That’s going to matter if what you want to find evidence of a ‘Zambezian’ forest at an archaeological site.
It’s the sort of science that is easy to overlook. The authors don’t conclude that whole textbooks need to be re-written or that our understanding of Africa’s past has to be rebuilt from scratch. It’s also the kind of science that’s easy to whine about. Here they are, picking flowers to examine tiny stones in the stems rather than just appreciating the beauty.
But it’s also the kind of science that increases the amount of beauty and mystery in the world.
Until I took my MPhil, I was completely ignorant of phytoliths. I could view the same plants an archaeobotanist, but saw a lot less. Before I read this paper I didn’t know that that the Miobos woodlands were unexamined. Knowing that these things are out there opens up new possibilities for what can be done. At Çatalhöyük they’re examining phytoliths left behind in what are almost shadows of woven baskets to flesh out details of human life in the past. In the case of this paper, it provides a benchmark for measuring future studying against. It’s detailed, meticulous and sometimes opaque to the non-specialist, but it’s also a description with lasting value. Currently publications are often judged on citations garnered over a few years. That misses the value of this paper as it will be important for decades. Indeed, if this ecosystem suddenly becomes a target for economic development it could even be important for centuries as a snapshot of the current state of the Miombos woodlands.
If you want to see the phytoliths they found, you can download the paper for free.
Mercader, J., Bennett, T., Esselmont, C., Simpson, S., & Walde, D. (2009). Phytoliths in woody plants from the Miombo woodlands of Mozambique Annals of Botany, 104 (1), 91–113 DOI: 10.1093/aob/mcp097