By Melissa Wray
From Kruger to Kansas to KwaZulu-Natal, from buffalo to bison, the search is on to find out what makes the world's savannas tick. An ambitious new project is currently underway on two continents to find out if there are any universal rules that govern how the savanna landscape, made up of grasslands dotted about with trees, responds to fire and grazing.
Scientists generally agree that wherever there is a savanna, how it changes over time is usually driven by climate, browsing and grazing by animals and how often fires have swept through the landscape. However, there has been some debate ongoing in scientific circles as to whether or not fire and grazing have the same effects on different continents, and whether a lesson learnt on the effects of fire and grazing on an American savanna can be applied to its African equivalent.
The Kruger National Park (KNP) is ideally suited to help fan the flames of the debate, because for the last 50-odd years certain areas of the park have been designated as Experimental Burn Plots. Religiously protected from ordinary veld fires, any burning done in the plots has been carried out according to a strict regime dictated by researchers. In the hundreds of sub-plots, fires are started at different time intervals and different times of the year to see what changes occur to the trees and grasses growing there.
There are not too many other places in the world where similar experiments have been carried out for so long, but the University of KwaZulu-Natal's Ukulinga burn plots are a couple of years older than Kruger's. Further afield, the state of Kansas in the United States of America has for more than 20 years been home to the Konza Prairie Biological Station, where similar burning experiments are ongoing. Now researchers are linking up between Africa and America to test the fire and grazing theories.
Prof Alan Knapp of Colorado State University and Prof Melinda Smith of Yale University are driving the current threeyear project, roping in the assistance of Dr Richard Fynn and Prof Kevin Kirkman from the University of KwaZulu-Natal as well as Kruger Park's fire ecology manager Navashni Govender.
Professors Scott Collins (University of New Mexico) and John Blair (Kansas State University) are also important US collaborators on the project. In Kruger, the researchers are focussing on the burn plots near Satara, which have the closest rainfall and temperatures to Ukulinga and Konza.
With all three sets of burn plots having similar fire regimes, observing the long-term effect of fire across continents is relatively easy. However, observing the effect of grazing is not so easy. All of Kruger's burn plots have always been open to the park's animals, while Ukulinga's plots have never been grazed. Konza has both grazed and ungrazed areas.
In Kruger, there are 14 animals that are considered as indigenous large herbivores. In America, only deer and bison remain of the continent's indigenous large herbivores. The Konza prairie burn plots have their own introduced bison, placed there some years ago to specifically study the effects of native grazers.
Given the fussiness of scientists when it comes to what is considered comparable data, the researchers were delighted to find a place in Kruger where there is both controlled burning and a single grass eater similar to bison. A fenced veterinary research area is located close to the Satara burn plots, and here a population of buffalo grazes in splendid isolation from Kruger's other herbivores, providing Kruger's equivalent of the bison at Konza.
With the fire regimes in place on the two continents, and the herbivores ready to munch their way through their respective habitats, the scientists can get down to the nitty-gritty of how the savanna is responding to these two influences.
The experiment is focussing on changes that occur in areas that are never burnt, burnt each year, and burnt every third or fourth year. For direct comparisons of areas that are never subject to grazing but are affected by fire, Ukulinga and Konza plots can be used.
To look at the influences of grazing and fire together, Kruger and Konza are compared, with the added bonus of seeing if there are any differences if lots of herbivores graze, or just one species eats the grass. To look at the seasonal effects of grazing in Kruger and Konza, exclosures have been erected in all of the different experimental areas where herbivores are present, with several repeats in each area to allow for statistical accuracy.
In Kruger, this means that there are currently over 100 octagonal diamond mesh structures, each seven metres across, keeping the gnashing teeth of buffalo, wildebeest, zebra and other hungry animals away. These have been shored up at the corners to withstand the rigours of rhinos with itchy hides, who rubbed some of the posts over in the beginning.
To check how things are changing with time, points are permanently marked inside and outside each experimental area. The researchers visit these points three times each year, twice in the growing season and once later in the year, with the US scientists poring over Konza's grasses and Dr Fynn inspecting Ukulinga and Kruger.
At each vegetation monitoring point, the scientists record all the grass species that occur, as well as the herb-like plants (forbs). Also recorded are the abundance of each species, and how much area they cover. Using a small frame, a standard amount of plant material is also clipped out at all the vegetation monitoring sites.
The grasses and the forbs are separated, and the grasses are sorted into this year's growth, last year's growth and dead material. Everything is then weighed to provide a measure of how productive the savanna is. As some of the arguments for why American and African savannas can't be compared come from the nature of the soils, resin bags are also buried at each experiment.
The resin beads inside the bags can provide information on how nitrogen, vital to plant growth, is cycling through the soil and the plants that grow in it. The experiment is set to run for three years, but hopefully can be extended for a further three years to give better insight into how the savannas behave on two different continents.
Is there a universal mechanism that causes the world's savannas to change, or must each area be considered differently? Do buffalo and bison produce the same changes in species diversity in the savannas on two continents? Or do you need all the other herbivores to tweak the African system? Watch the scientific literature for more details.