Plant Records: The (Almost) Invisible Giant

Name: Tiama African Mahogany
Scientific name: Entandrophragma excelsum
Known for: Being freakishly good at hiding despite its size
Record broken: Height

There was a surprising article in Biodiversity and Conservation earlier this year, Africa’s highest mountain harbours Africa’s tallest trees by Hemp et al. What caught my eye isn’t that Entandrophragma excelsum a little taller than some other trees in Africa, it’s a lot taller.

Tree reports from Africa so far suggest that trees grow not taller than 65 m (FTEA 1952–2012). The only exception is an individual of an introduced species, Sydney blue gum (Eucalyptus saligna), which grew 81.5 m tall prior to its death in 2006.

Hemp et al. report that the tallest tree they found in their survey was the height of 18 and a half double-decker buses.* Not only is this tall, it’s over seven times the height of Shaquille O’Neal** taller than other known trees in Africa. This is a tall tree.

Given its height you might be puzzled as to why this is news. Why has this only recently been discovered? The answer is a combination of two reasons. One is that Africa is far from fully surveyed. There is a lot of work to do on the continent. To make things more difficult the potential habitat for this trees is tiny, an area just one third of a thousandth the size of Wales.*** The fact that such a major find is only recently coming to light underlines how much work there is to do.

But why are mega-trees so comparatively rare? The bigger you are, the more exposed to the elements you are, so there’s value to being part of the canopy instead of standing out a long way. However, in the case of Entandrophragma excelsum, they’re in a gorge, so there’s reason for them to need to be tall. The gorge may also give them more protection so their relatively light wood can build a tall tree. There’s also comparatively little competition, and they have quite a good supply of nutrients.

If that’s the case why do they stop at that height and not drive each other up a little higher generation by generation? A story in New Scientist put me on to another paper that might explain that. It’s down to how sugars move in a plant.

Jensen and Zwieniecki have published a paper, Physical Limits to Leaf Size in Tall Trees, in Physical Review Letters. They argue that sugars pick up speed as they move through the leaf. However, once they reach the trunk, they reach a speed limit as the trunk is, effectively, a long tube, and sugars can only travel at a set speed. The taller the tree the farther the sugars have to travel to get to the roots. So there’s a maximum leaf size that’s viable for a tree that gets smaller as the tree gets taller. But there’s also a minimum leaf size that’s viable too, and the place where these limits meet is, 104±6m.

By finding this tall tree Hemp and colleagues have shown that the area they’ve investigated has special properties that are rare elsewhere in Africa, so they might have the remains of an ancient environment that has survived to the modern day. They have also found a special plant, in that it’s capable of growing to an extraordinary degree compared to other trees in Africa.

They also shown that there might well be other, quite literally, massive discoveries to be made in 2017. If a 80m tall tree can be hiding by Kilimanjaro, are the bigger discoveries to be made in locations that are harder to get to?

* The actual measure they use is 81.5 metres, but it is traditional in record-breaking posts to relate one hard-to-imagine thing in units of another hard-to-imagine thing. For example n times the size of Wales, a cliche that the author firmly intends to use if the opportunity presents itself.

** 16 metres.

*** Nailed it. The cliche ‘size of Wales’ has been put to positive use by a charity that set out to preserve an area of rainforest over six and a half times the size of Rhode Island.