A bean tree gone wild

When I think of beans, I think of peas, or lentils, or soy – small and herbaceous plants, with long linear fruits holding a treasure of seeds down their spine. But not all bean-family plants are like this – some are a little more wild.

I found a very different bean in the mature forest near Lopé, Gabon. This kind of forest has a very high numbers of species in it. Only a few of these species are found to be common, and most are very rare, with individuals scattered far away from each other. Walk along an elephant track and you may notice dozens of one species in a minute, but only notice a single individual of another in an entire day. Or, more likely, you might not notice the difference at all – with leaves high up in the canopy, and with few differences in visible bark or branching characteristics, many species look about the same from down on the ground.

That knowledge was what surprised me when I found this immense bean pod on the forest floor one day. It had the same morphology as a garden pea – but was much, much larger. And it had to have come from somewhere. One of these rare forest species was about to reveal itself. The density of fruits was higher in some parts of the forest than others, so I walked until the density reached a maximum – and then looked up.

And there it was – a bean plant, but a very large one. A tree of about twenty meters’ height, with sparse leaves and many immense fruits hanging from its branches. It was the only individual in this patch of forest – and even after two more weeks’ determined looking, the only individual I saw on this entire field campaign.

Luckily it was not so hard to identify – there are not so many bean-family trees in this forest, and this individual happened to have already been sampled by a taxonomist for our campaign. It was a Filaeopsis discophora (Mimosaceae; Fabaceae). And it was beautiful.

I teased apart one of the dried fruits on the ground, and found it full of large winged seeds. The fruits probably open while still attached to the trunk, with the wind blowing the seeds across the forest. This dispersal mechanism perhaps explains why it is so locally rare. It was a pleasure to find a fully-grown tree on this hot and sweaty day of work, and I wonder where the rest of the population may be found.

Seeing this large bean tree here made me reflect on how important my expectations are for determining what is interesting and rare. Growing up in a temperate North American environment, growing plants in a small garden, I assumed that all beans were small herbaceous things – but perhaps if I had grown up in a tall tropical African forest instead, I would be more surprised to find these herbs than these trees. But the bean family is actually one of the largest, comprising almost twenty thousand species. Perhaps I would have simply been a more careful observer and noticed the native locust and redbud trees – also all in the same family. But as a biologist temporarily switching continents, I was glad to be surprised, and pleased to discover something new to me in this beautiful forest.

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