Do you know why savanna trees have flat tops?
A savanna panorama is soon identifiable thanks to the flat-topped acacias.
But why? Have you ever asked yourself during a safari?
Why do so many acacias (Vachellia) and other savanna trees have such a particular structure that they have become an icon of the African grasslands?
Some answers about this topic have been given by Sally Archibald and William Bold, who studied one species called the Sweet Thorn (Vachellia karroo) that, like some of our Vachellia species in East Africa, shows a range of various forms in different habitats.
25
SEPTEMBER 2020
Botany
Savanna
Trees
Vachellia karroo – Sweet thorn
In a forest it is a tall, thin tree, while in the semi-desert of the Karroo (South Africa), it grows as a medium-sized ball of thorns.
In the savanna it has a typical medium-tall flat-topped acacia look.
It may be genetic differences within the species, but equally the differences could be caused in some species by a variable response to the environment.
It really seems that the plant is wondering whether to grow tall or wide.
As always, when we are thinking about what makes the savanna species, we have to take into consideration the savanna big four:
water availability, nutrients, fire and herbivory.
The first two processes have impacts in all biomes, while the second two are the most distinctive about savanna.
Balanites aegyptiaca – Masai Mara
We can start from here, then.
Any tree that wish to survive in a fire-dominated biome has to develop special adaptations.
Thick bark is an evident useful feature that has evolved many times by trees in the savanna.
Another important characteristic is to grow rapidly tall: trees that are grown above the “fire trap” (2-3 metre tall) are not burnt back to the roots by a fire, but they can resprout from the top.
If you have access to limited resources and live in a place where fires are common, you would put those resources into a single (or at most a few) stems and grow straight up, without branching out sideways.
Once you’ve grown tall enough to escape the fire, you are then free to branch sideways.
So there’s one good reason why in a savanna biome, you may find lots of tall trees with few branches low down, then lots more above.
Senegalia mellifera
Now we consider the other process: herbivory.
Growing tall is not so easy if you are a young tree and there are lots of herbivores around.
You may not reach your target before they have eaten you.
In this condition, you would put a lot of effort into protecting yourself with producing many thorns, and grow in a thicket type growth form.
Once grown wide enough, you may eventually decide to protect the internal branches enough to completely free them from browsing. They could then grow tall if this is useful (according to the environment).
So in a browser dominated habitat, branching low down might be a much better strategy.
Balanites aegyptiaca – Masai Mara
In the savanna biome, fire is carried by grass.
In areas where rainfall is low and therefore grass is short, you will find a browser dominated system (no grazers).
It is exactly where William and Sally found ball-shaped thorn trees.
By the contrary, in wetter areas grass and trees grow very well, and you will find both fire (carried by the grass) and herbivory.
This is a really struggle for any tree!
In these circumstances scientists think that the certainty that all seedlings will be killed by fire if they’re not tall enough, versus the possibility – though small – that a seedling my not be found by a browser would favour a fire-adapted response.
But this doesn’t change the fact that a tree can still make serious thorns and chemical defence of saplings.
To summarize, in a dry area it is more likely to have ball-shaped thorn trees.
In a wet area it is more likely to find tall tree with flat tops (like exactly in Masai Mara and Serengeti).
Now, I want to take into consideration also another environment: in a forest, trees are cramped together and their main effort is to get sufficient light to grow.
Therefore, trees have evolved two main strategies to survive: they either tolerate low light and grow very, very slowly but live a long time and eventually make it to the top of the canopy and can shade out other less-tolerate species.
Or they grow fast and die young, rapidly filling any holes that form in the canopy when a mature tree dies, but eventually being out competed by the slower-growing species.
But they need to maximise any light they can get.
Even if that light has already passed through the top leaves, they want to catch the remaining light lower down too, making the forest floor a shadowy place.
The consequences of this intense light competition are:
– mature trees tend to be very tall, always struggling to get above their neighbours;
– they have lots of leaves at all sorts of heights in the canopy, trying to catch every stray bit of light.
Vachellia tortilis (acacia a ombrello)
Coming back to the typical fire-dominated savanna, once the tree have escaped the fire trap it is free to branch.
But it is rare that a seedling breaks out of both herbivory and fire, that is why mature trees tend to be at low density in savannas.
Canopy cover in woodlands is around 20-30%.
Moreover, in these circumstances there is no significant competition for light. In other words, there’s no need for savanna trees to grow tall, and horizontal growth is enough to gather sufficient light.
What’s more, even moderately tall trees don’t always escape giraffe browsing, but a horizontal growth-form still protects the central branches.
An example is the Vachellia tortillis that only grow large thorns on the outer tips of their branches, providing enough defence to protect inner branches without further costs for thorn growth.
At last, in a fire dominated, browse affected and unlimited light environment, the flat topped tree structure is exactly what you would expect to see!
All of this, in my opinion, is simply fantastic: everything is interconnected and animals, plants and environment shape each other.
Come and discover all of this personally together with us, choosing one of our safari!
Source : safari-ecology.blogspot.com
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