The plants in the savannah that feel the rain…

When the rains come again in the savannah everything is amazing and that is a good time to be there and observe what happens next.

Birds joyfully greet the rain and new fresh grass suddenly appears in just a few days.

But as everybody knows, the timing of these events can change every year and the impacts of global climate change is well visible here in East Africa.

Despite this, it really seems that nature has its own weapons to be able to survive.







The grass growth responds directly to rainfall or, at least, to soil moisture.

It is a matter of course, but if the rains are late, the grass remains dry, if the rains are early, it turns green early.

But how does it know?

The grass appears to be completely dead until something tells it that the soil is moist and it is time to start growing again.

The moisture in the soil is in direct contact with the grass roots or seeds and when the moisture is absorbed the growth is started up again.

But there are other processes to understand.

One is the green flush that is seen in Miombo, Commiphora and several Combretum woodlands  before the rains.

And not just immediately before the rains, but several weeks earlier.

How, and why, do these plants do that?

Savannah woodlands are deciduous (the trees loose their leaves) because during the dry season their leaves would loose too much water because of evaporation, and the tree would surely die.

In fact there are still evergreens in the savannah along the rivers, for example.

Miombo woodland (Brachystegia)

So why, just when water is in shortest supply, do some trees “choose” to use their remaining water reservoir and put out leaves before the rains?

The leaves are out, yes, but the plants switch off again, until the rains arrive.

This means that they are not photosynthesising and breathe.  They are actually dormant.

But then, when the rains do arrive, these plants can be active within 24hrs.

And what is most important is the  flush of nutrients, especially nitrogen, associated with the first rains, that rapidly declines after the first few days.

Other trees which are not ready will spend those first few nutrient rich days busy growing leaves and they will not be able to take advantage of the nutrient flush.

So, as long as you can minimise the costs of having leaves before the rains, it seems reasonable that the benefits could outweigh them (and definitely it is true or they wouldn’t have survived!).

This is also confirmed by the fact that some legumes, like Acacias (even if acacias is not their scientific name anymore, being now Vachellia or Senegalia), don’t behave like this.

In fact they respond to soil moisture and they don’t have shortage of nitrogen, unlike other savannah species.

And the next question is: why so early?

Why not just wait until the week before the rain before growing leaves , for example?

Here we can only guess the answer because there are no scientific evidence.

Probably it is because the date when the rains start is variable.

You can’t predict it so accurately.

If you want to take advantage of that first nutrient flush, you’ve got to be ready for the earliest possible date, and in fact the rain could fall several weeks later.

Another interesting question is: how do they do it?

How do these trees know that it’s October and the rain is coming in a few weeks time?

Differently than the grass which simply detects water, these trees must keep track of the changing date directly.

In the north, plants and animals use changes in day length to keep track of the seasons.

But where I live, in the Kenya coast, from one day to the next day, length could change by as much as five or ten minutes, half an hour maximum. On the equator line it is even not noticeable.

So it is hard to conceive that the same process is possible here. But, amazingly, no one has studied it so we just don’t know.

There are other biological events that depend on precise seasonal timing, like for example the millions of migrating birds that spend months here until March, then head north to breed, but even here we are not sure about the signals that the birds are using to do that (signs suggest they are triggered by weather and its effect on food supply).

But we can talk more about this the next time..

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