Saturday, 11 June 2011

Acacia woodland

Next on the list of major savanna habitats must be the Acacia woodlands. The sun setting behing a lat-topped acacia provides one of the iconic images of an African savanna, and many of the most interesting game drives involve meandering through Acacia woodland. In fact, there are a very large number of Acacia species in Africa - something over 150 - and they're al a little different (let's ignore the current taxonomic discussions about Australia taking the Acacia genus for it's species leaving the African's with none...) . Ecologically, Acacia species play a vital role in the savanna ecosystem, but before we think too much about that, let's start by thinking about why Acacia woodlands are found where they are.
They make great backdrops, even for Flamongos! Lake Magadi, Serengeti NP Jan 2011

First, of course, we need to identify where the Acacia woodlands are and it's obvious if you're looking for it: Acacia woodlands are typically found on the lower slopes of hills and on the flatter land at the bottom of valleys. On the ridges there are generally broad-leafed woodlands, such as Terminalia and Combretrum woodlands, or in areas with a single rainy season you might find Miombo woodlands on the ridges. Boardering rivers, of course, you often find true riverine forest, another habitat again (although one that often features Acacia species). But between the riverine forest and the broad-leaved woodlands is the area where grasslands and Acacia woodlands are commonest. And the reasons for this position are probably to be found on those four main drivers of savanna ecology: fire, nutrients, water availability and grazing pressure. On the ridges, nutrients are very scarce thanks to millenia of washing by heavy tropial rains. Lower down there are more nutrients, but in consequence the grazing/browsing pressure is going to be higher - to survive in these areas you need to be very heavily defended - like the big throns on many Acacia species. The soil moisture content is also important: like other members of the Fabaceae (peas and beans being obvious examples, of course), Acacia species have a symbiotic relationship with nitrogen fixing bacteria (called Rhizobia) that live in nodules in their roots, and these bacteria are somewhat fussy about where they live. (In fact, they can life in soil away from the plants, but they are unable to fix nitrogen in isolation.)

This relationship with Rhizobia is responsible for what is probably the most important role Acacia woodlands have in the savanna: they're incredibly important nutrient pumps. Thanks to their nitrogen fixing bacteria, Acacia species have a pretty much unlimited supply of organic nitrogen, vital to producing proteins and growth. In the generally rather nutrient poor soils of Africa, this has a massive impact. All the browsers love a snack on the nutrient rich Acacia leaves, despite their thorns and high tannin content. But most importantly, at the end of the wet season Acacia trees drop their leaves like most other savanna plants - but because they have such a plentiful supply of nitrogen, they don't bother withdrawing all the nutrient before they do so. This is immediately obvious if you drive the savanna at this time of year: Acacia trees remain greenish right until the leaves fall, other broad-leaf species withdraw as much nutrient as possible, resulting in yellow or orange leaves, before they fall. And the consequence of this is that Acacia leaf litter is much richer than the little of other species and fertilises the soil under the trees. So effective is it, that in some places Acacia litter is used as a major fertiliser for poor soils.

Again, the impact of the richer soil is immediately obvious if you go and look under an Acacia - there's a whole lot more diversity in the herb layer than under a neighbouring non-Acacia species.
Lots of herb diversity in the understory of an Acacia woodland thanks to the fertilisation effects. Near Mbalageti River, Serengeti, Jan 2011.
Not much under a Balanites but grass (oh, and a few animals) - Grumeti GR, Sep 2010
This diversity, combined with the fertilisation effect making everything rather more nutritious than elsewhere in the savanna explains the reason you spend so much of your time on game drives in Acacia woodlands: everything loves them! Of course, the problem with this is that it attracts lions and other predators who hunt much more efficiently from the cover of woodlands, so if you're a browsing animal you've got to choose: do you got for the nutrient rich woodlands and under-story of the Acacia belt but face the higher risk of being eaten yourself, or do you avoid the richer habitats and forage in safer places where you can keep an eye on predators much more easily? If you're sensible, of course, you'll probably balance the two options up and make decisions based on exactly how much you need those nutrients at any one time – early in the dry season there's plenty of forage in the open grasslands and you've had plenty of nutrients recently during the wet season anyway, so you might spend more time in the grasslands. Later in the dry season those open areas may have been grazed to nothing and you're more in need of nutrients as you might well be preparing for pregnancy, so you might decide to take the risk and forage in the woodlands: at different seasons, different strategies make most sense and within the savanna ecosystem such movements are very sensible. Of course, you might also decide to just live on the woodland margin – nipping into the woods when you're fairly sure there are no predators around, but in easy reach of the open areas if you're more worried. This, of course, means that those ecotones – the transition from one habitat (Acacia woodland) to another (open grasslands) – are going to be fantastic places to explore on your game drive.

And that, for now, is probably enough about Acacia woodlands. There's lots more to say about both Acacias and the woodlands, but as an introduction to the habitat it's not a bad place to start.
Acacias do make for nice sunsets! Here some herons roost on a bit A. tortilis near Lake Ndutu, Serengeti, Jan 2010

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