First rains arriving over Manyara Ranch, Nov 2011 |
This is one of my favourite times to be
in the bush, as the rains arrive and the savannah turns green. I love
the excitement of the birds as they greet the rain, and the miracle
of new grass appearing in just a few days and I probably get as
excited by the first thunderstorms as my children! But as we all
know, the timing of these events can change year to year. In fact,
never more so that recently – one of the first impacts of global
climate change that we see here in East Africa. Despite the changing
season being such a profound event in the savannah, there's a
surprising amount that we don't know about the patterns of seasonal
change that we see.
For example, it's obvious that grass
growth responds directly to rainfall – or at least to soilmoisture. If the rains are late, the grass stays dry, if the rains
are early, it turns green early. But how does it know? To all intents
and purposes the grass (or the seed) seems completely dead until
something tells it the soil is moist and it's time to start growing
again. In this case, actually, it's fairly straightforward – the
moisture in the soil is in direct contact with the grass roots (or
seed) and as that moisture is absorbed the cell cycles are started up
again.
Pre-rains green flush in Brachystigia woodland, Kafue NP, Oct 2011 (pic. H. Frederick) |
Other patterns are harder to
understand, and the one that fascinates me most is the green flush
that we see in miombo woodlands (and on Commiphora and several
Combretum species too) before the rains. Not just immediately before
the rains either, but some weeks before. How, and why, do they do
that?
Let's remember first that savannah
woodlands are deciduous (the trees loose their leaves) because during
the dry season their leaves would loose too much water to allow the
tree to survive. Add water, and there's no problem, so you'll see
evergreens in the savannah only in riverine and kopjie habitats. So
why, just when water is in shortest supply, do some species 'choose'
to use some of their remaining stores of water and put out leaves
before the rains come – and not just before, but a long time
before? One of the things that we do know that might help us
understand this is that once the leaves are out, the plants once
again 'switch off' until the rains arrive. They've got leaves out,
but they're not photosynthesising and respiration (plants respiretoo, of course) is pretty much dormant too. But then, once the rains
do come, they're active within 24hrs. And another clue might come
from the fact that we know there's a flush of nutrients (particularlynitrogen) associated with the first rains, that rapidly declines
after the first few days of rain. So there's obviously a strong
advantage if you can be ready and waiting for the rain – other
trees that aren't ready will spend those first few nutrient rich days
busy growing leaves and 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 come (by essentially shutting down as much as
possible), it seems plausible that the benefits could outweigh the
costs (and clearly, for some species they do, or they wouldn't
survive!). One thing that suggests this idea might be right is the
fact that legumes – like Vachellia and Senegalia (I will get you toforget about Acacias!) - don't do it, they respond to soil moisture
and, as we know, being legumes hey have no shortage of nitrogen,
unlike other savannah species.
More pre-rain greening, Kafue NP (H. Frederick) |
But why, then, be so early – why not
just wait until the week before the rain before growing leaves and
further minimise your costs that way? And here is where we really run
out of hard facts and enter the realms of interesting scientific
speculation – my guess is that because the date when the rains
start is variable, you can't predict it that accurately. If you want
to take advantage of that first nutrient flush, you've got to be
ready for the earliest possible date the rains might fall – which
(like this year) might be several weeks before the rains begin in
normal years. I'm far from certain this is right – among other
things, it requires that the benefits of being ready for that first
flush are extremely strong, such that plants that catch it every year
have a meaningful evolutionary advantage over plants that only catch
it most years, which is testable but not guaranteed. But it's a good
theory to work on for now.
The next part of the story that I'm
interested in, of course, is how they do it? How do these plants
'know' when it's October and the rain is coming in a few weeks time?
Unlike the grasses that simply detect water, these plants must keep
track of the changing date directly. In the north where these
processes have been studied in extraordinary detail, plants (andanimals) use changes in day length to keep track of the seasons –
in spring and autumn in Aberdeen where I used to live from one day to
the next day length could change by as much as five or ten minutes.
But I find it hard to conceive that the same process is possible here
where day length changes only by two minutes across the entire year –
the difference from one day to the next can only be measured in
seconds or fractions of seconds, and I find it hard to believe this
can actually be the cue. But, amazingly, no-one's studied it so we
just don't know.
There are other biological events that
depend on precise seasonal timing, of course – like the millions of
birds that spend months here until April, then head north to breed,
but even here we don't always know the signals that the birds are
using and why, for example, so many species seem to have been rather
late arriving this year. But this has already been a long enough blog
for one day, so that will have to wait for another time...
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