Thursday, November 14, 2013

Gleanings from the Kunming conference - Underground trees

In South Africa's coastal grasslands, you can explore a forest by literally walking along its canopy. It is home to some extraordinary tree species which are called underground trees. Only the uppermost leaves and branches of the tree are visible. The rest of the tree is submerged below the deep sandy soil, creating a clonal network of underground forests.

Prof. Braam van Wyk, plant taxonomist from the University of Pretoria explains those specialized plants (the correct scientific term is pyrogenic geoxylic suffrutices):
It is a very peculiar growth form that is associated with our grasslands, and it is very much a type of growth form in Africa. It is not found in significant numbers anywhere else in the world, except perhaps to a limited degree in South America. It is a growth form where you get plants, woody plants that can be compared to underground trees, and all that you see are these green twigs which can be compared with a canopy of the tree. And this is probably one plant sitting here, or maybe even this whole area may be one plant, and it’s the canopy that just sticks out, the tips of the branches above ground. These tips may burn down every year, but the rest of the tree stays untouched underground. Why they have adopted this strategy… it is a very interesting challenge to come up with reasons. Fire, frost, a shallow water table and grazing have all been considered. There are lots of interesting things we can say about the reasons why plants have adopted this strategy and why it mainly evolved in Africa. They are called clones, and are essentially immortal, nothing can kill them, except for habitat destruction. Grazers can not kill them, fire can not kill them and they are drought resistant. They grow extremely slowly, and if you look at the diameter of some of these clones, they must be the oldest inhabitants of our grasslands. I would say easily more than a thousand years for many of these clones since the first seed arrived for that particular species. But I would not be surprised if some of them are one day shown to be perhaps more than 10 000 years old, amongst the oldest plants in the world, much older than any tree that you are going to see. They are very peculiar plants and we have quite a number of these species in our grasslands.

It was during Michelle Van der Bank's (University of Johannesburg) talk at the conference in Kunming that I first learned about these extraordinary trees. Michele was reporting on results of the TreeBOL Africa project. They have cataloged and barcoded over 50% of the estimated 2,486 woody shrub and tree species in southern Africa. The underground tree species were one example of how this dataset was used to address questions across disciplines including ecology, conservation biology, and taxonomy.

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