|Majo Moto near Boma Ng'ombe - thick riverine forest is protected from browsers and fire|
I spent the weekend with my family and friends camping at Maji Moto near Boma Ng'ombe. As the name suggests, this is a hot (well, warm at least) spring in the middle of some pretty dry Acacia – Commiphora scrub. There's a pretty impressive flow of somewhat warm water out of a cave forming a nice river that, as you might imagine, if a complete contrast to the surrounding dry, and it inspired me to think about riverine forest again. Already this year, it's pretty dry in lots of places (though I hear there's rain in western Serengeti), and everywhere is pretty dusty and dry. Except, that is, the riverine forests. Here the trees are still green, there's plenty of shade and the contrast is amazing, but this is just a hint of how different riverine forests will be from the surrounding vegetation in another couple of months as the dry season starts to bite.
Acacia commiphora scrub in the foreground with tall, green riverine forest in the back. Maji Moto, July 2011
So, where are riverine forests special and what role do they play in the savanna ecosystem? The first special thing that's immediately obvious is that they're green, when no-where else is. Ti understand what makes riverine forest special we're going to think again about nutrients, fire, water and grazing.browsing, the big four of the savanna ecology world. Lots of riverine trees are evergreen, or nearly so. And the reason they can keep green is that they typically have incredibly deep roots, penetrating right down into the water-table. With a few exceptions, the surrounding Acacia and Commiphoras tend to have shallow roots, spreading horizontally just below the surface of the soil, spread out and designed to catch as much of the water that falls on the surface as possible, very different from the tap roots of the evergreen riverine forests. Tap roots only work if the water table below ground isn't too deep, and alongside rivers that's fairly likely, so water-availability is the main factor altering the ecology of riverine forest.
Look closer and you'll notice some other things – trees of the riverine forest are often not thorny, in complete contrast to the surrounding vegetation. Now why is this? Obviously thorns are to do with defence against herbivory, so something about these trees means they somehow escape herbivory in a way that the surrounding habitats can't. That's odd, given that rivers are generally excellent places to find animals. So the key here isn't that the herbivory is much lower than in the surrounding woodlands, but that the tree growth rates are much higher and the trees themselves are larger, all thatks to that ready availability of water. So once established, riverine trees can grow all year around, escaping from even the tallest giraffe in a fraction of the time water-limited plants in other habitats need. So although the animal densities can be high, the impact of herbivory is less, at least on mature trees. Now that is a problem for seedlings, of course – although they grow fast, they still need to escape heavy herbivory whilst they're short, and this the achieve by heavy reliance on nurse plants – plants of other species (bushes and shrubs) that are well defended against herbivores, but that allow the seedlings of the riverine species to sneak in among them and also escape herbivory. Once the forest is dense, of course, the mature riverine forest can be so dense (especially if there's a dense palm understory that nothing can get into) that herbivores are basically precluded anyway, making regeneration of the forest fairly straight forward. So herbivory also plays a part in the riverine system.
And finally, fire is a big issue in riverine areas. Typically, trees of the riverine forest (like other forest – and riverine really can be a forest habitat attracting many species typical of more montane habitats, particularly in our current cold season) are fairly sensitive to fire. That's not much of a problem for them in general though, as they're usually green and moist, dense and with little grass in the understory (especially if there are lots of buffalo around keeping it short around their preferred river habitats), so don't have lots of dry fuels waiting to burn. In fact, riverine forests can play an important role in fire suppression, allowing different fire regimes on one side of a forest strip to the other. This sensitivity, however, causes a problem if you burn an area too much so the nurse plants are burnt off (one of the reason we think many of Serengeti's riverine forests are currently on the way out). And once recruitment is lost in this system, it's really difficult to re-establish riverine habitats, as the natural fire suppression is lost, and the herbivores are also given much better access. A savanna with healthy riverine forest is therefore likely to be well managed savanna.
|Otolemur garnettii, common in Riverine Forest. Maji Moto July 2011|
Figs are a valuable year-round food resource in riverine forest
|Fallen ones are eaten by everything!|
And riverine forest is, of course, a vital habitat for many species. I've already mentioned the fact that buffalo like to hang out in these areas, and they have their own set of animals too. They're great places in the shady spots to find leopards, or you might look for some of the more specialised riverine species too – lots of good birds, loads of bush-babies and maybe tree hyrax, etc. Obviously many game drives take in riverine forest, particularly in the dry-season when they're the only green areas around, especially as they're one of the only habitats at this time that has fruit on offer – figs are everywhere in the riverine forest, and there's always some ripe somewhere nearby. Figs are on all rivers, but up here Tamarind is a pretty specialised riverine tree too, and you'll often find Marulas on the rivers too, all excellent food sources for monkeys, baboons and birds, plus the fallen fruit are loved by bushbuck, elephants and just about anything really! At this time of year when food is hard to find in the open savanna, riverine forest really comes into it's own and lots of animals will make extra use of the habitat during the dry season. All in all, a very important part of the savanna ecosystem, both through proving food resources, but also from it's role as a fire break, etc. And you can have lots of fun in the river too!
Many special birds are easiest to find in riverine forest – a Pygmy Kingfisher, July 2011
|In the cold season|
higher elevation forest species like these Black Saw-wings follow rivers into the lowlands