|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...