In a public talk at the West Coast Fossil Park on 22 July 2023, Prof William Bond engaged the audience on the question of why trees have thorns. Spoiler alert, the answers became ever thornier the further the investigation.
Bond outlined how spinescence in woody plants (trees and shrubs) is widespread and ecologically important. Thorn trees are often the most palatable (delicious) trees in plant communities. The abundance of thorn trees in Africa indicates the intensity of herbivory by wild bovids, such as antelopes, giraffes, wildebeest and buffalo. Indeed, in Africa, thorn trees coevolved with bovids to protect their stems more than their leaves. The defensive ability of thorns is enhanced by cage-like growth formations creating a painful barrier to browsers seeking leaves.
The ecological importance of this is that plants that lack spiny defences are likely to succumb to herbivory, with forests and thickets thinning out to open savannas or grasslands with scattered thorn trees. So entire biomes are shaped by bovids driving the spinescence of constituent plants.
How does the defence work? Browsers gingerly push their muzzles past the thorns to feed on the highly palatable leaves located beyond. The most difficult to reach are well-caged leaves, enabling only small bite sizes – nibbling rather than chomping. The browser can feed faster by going for less defended branches. However, the efficacy of thorns and cages as plant defences varies with different browser species with disparate muzzles and mouth sizes. Plants feature different lengths of spines and leaves and closed versus open cage architectures at different height levels. The longest spines, shortest leaves and most convoluted cages occur at lower levels. These configurations vary according to the tree development stage, from seedlings, and saplings, to adults. Notwithstanding, well-defended thorny plants form the staple diet of many browsers, a paradox awaiting elucidation.
Frequent fires throw a spanner into the works of having a nice, neat relationship of browsers versus defences. The tightest cages with the longest and densest thorns are most vulnerable to fire, especially within the lowest metre or two above the ground.
It is, therefore, not surprising that herbivory and the accompanying plant spinescence are most intense where fires are less frequent — fire and herbivory drive savanna plant community configurations in different directions.
Bond went as far as enacting the situations goats face when tackling thorn trees. In this way he also showed how bovids, which have no hands or trunks to manipulate thorns out of the way, encounter thorns “face-on”, in this way rendering this plant defence effective as a deterrent. His presentation reflected the ceaseless fascination and endeavours of science. Nobody could fail to be inspired to tackle thorny issues through scientific careers.
. The next opportunity for intellectual stimulation a la West Coast Fossil Park will be on 23 September 2023, given by Caitlin Rabe, who will speak on sabre-tooth cats that used to roam Langebaanweg.
