Past lives caught in the dust of trees

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ResearchBlogging.org 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.

Elephants in Miombo woodland. Photo by Jussi Mononen.

Elephants in Miombo wood­land. Photo by Jussi Mononen.

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.

Miombos Botanical Transect after Mercader et al.

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.

ResearchBlogging.orgMercader, 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

Photo credit: Elephants in Miombo wood­land. Photo by Jussi Mononen.