By Michele Hofmeyr
A slow silent war rages between trees and grass in the veld and what determines who gets the upper hand has prompted further investigation. Dr Ed February from the Department department of Botany botany at the University of Cape Town is leading a team of researchers who is tackling the mystery of how trees and grass manage to live together in the face of competition for limited water and nutrient resources.
This work is an extension of the previously extensive Tree-Grass Program, which has been running for the last five years in the Kruger National Park (KNP). This work brought to light the fact that the competition between trees and grasses is more complex than researchers initially thought.
"Savannas are a tropical vegetation type" explained Dr. February, "and form a wide band across the continents of the world. It is estimated that 12-14 percent of the world's plant biomass falls within the savanna system.
Yet despite this huge vegetation type, very little is known about the actual dynamics of the trees and grass. I have seen grass grow right up underneath trees and it doesn't appear that there is any apparent competition and that trees don't over shadow grasses". So what are the dynamics that are involved here? This is the question that the research team is looking into.
It would seem from previous research that grasses do have the ability to out-compete trees in some instances, especially for water, as they have a shallow root system that can take advantage of water as soon as it becomes available. However, trees and grass can still live together. It would seem that other factors are at play here and in certain parts of the Kruger National Park it would seem that trees are the victors and are increasing in the system.
"We are now thinking that trees are doing something different," says Dr February. "and Iit appears as if trees are producing leaves and greening up earlier than grasses at the end of winter in anticipation of the coming spring rais" he explained.
This is adding a new dimension to the conflict as it means trees have a time advantage over grasses as they are ready to start growing and using resources as soon as the welcome rains appear. "Trees will be ready to take up the nutrients that come in a rapid pulse after the first rains and start growing before the grasses get going" said Dr February.
To see how the conflict is unfolding on the ground, it takes lots of equipment and a dedicated team of skilled researchers. Team members Corli Coetsee and Ben Wigley are leading the research on the ground. They have been setting up a series of eight weather stations between Skukuza and Pretoriuskop.
There is a documented rainfall gradient between the two, with Pretoriuskop having about 750mm each year compared to Skukuza with an annual rainfall average of about 500mm. These weather stations are situated in specific locations along this gradient to meet the criteria for the tree-grass monitoring experiment. The sites have the three study tree species present, namely marula (Sclerocarya birrea), red bushwillow (Combretum apiculatum) and silver cluster leaf (Terminalia sericea).
The sites have well-drained soils with no seep lines present and are not visible from any tourist roads. The weather stations are constructed to stringent Sanparks regulations to make sure that there is minimal impact or disturbance to the area. Attached to these towers are specialised specialised equipment that monitors rainfall, ambient temperature, relative humidity, windspeed and the strength of solar radiation.
All of this data is downloaded every two weeks and added to the monitoring database. Extra care had to be taken to protect the sensitive equipment from curious animals or from an elephants, who would like a back-scratch on the poles of the weather station. It took time to cover the bases of each structure with mesh to house the equipment and to camouflage it with shade cloth.
So far the weather stations have been up for a month and have not attracted too much unwanted attention from the animal inhabitants. "We did have an aardvark come sniffing around one of the weather stations" says Corli, "but we think it was more interested in digging in the soft sand around the bases of the poles".
The sturdy wooden structures are lower than the trees at each site and will be removed once the project reaches completion in three years' time. It is anticipated that the trees in Pretoriuskop should green up earlier than the trees in Skukuza in response to the higher rainfall in that area. The research hopes to shed some light on why some areas have become bush encroached, often with large areas converted to thicket. This reduces the visibility for tourists who come to see Kruger's world-renowned biodiversity.
"This is research with tangible benefits" says Dr February. With all this the data collected over the seasons, the team hopes to unravel what environmental cues, from rainfall to solar radiation, are responsible for how the trees and grass distribute themselves across the veld, which creates the unique savanna patterns that sustain the myriad of life forms in the KNP.