Light travels from the sun and falls on the trees — a disjointed umbrella of leaves and branches — then through onto grass and other plants below. But not all trees are the same: at times very little light penetrates the gaps between the leaves, and at other times plenty shines through. Understanding the interplay of light between trees and grass may help us to manage the climate-change-fuelled bush encroachment that is threatening the savannahs of southern Africa.
The amount of light being transmitted through trees to the soil surface below, often called the incidence of light, varies and depends on the gaps between individual leaves, their size and number, their angle relative to the sun and whether they are there all year round. For instance, trees with large leaves will block out more light than those with smaller leaves, and leaves that are always flat will create more shade below.
The plants that grow under trees prefer different light conditions and the trees determine those light conditions. These factors determine which plants can, and which cannot, live underneath a tree. This means that certain plants and grasses will flourish under certain trees — trees that allow just the right amount of light through: neither too much, nor too little.
Climate change models suggest that woody cover will increase in savannahs because tree growth is linked to the amount of carbon dioxide available in the atmosphere. When carbon dioxide is plentiful, young trees grow faster and adult trees grow bigger. This means that the light environment under tree canopies, as well as the plants that can grow under them, change. If there is less light, shade-loving grass — which is less nutritious for grazing animals — will dominate.
To understand the relationships between light, trees, grass and fire, I sampled 11 sites across five provinces in South Africa, with each of the sites representing a different tree species. A number of leaf traits — shape, type, angle and quantity — were assessed to determine the extent to which they affect how much light a tree canopy will cut out.
Our research at the University of Witwatersrand found that, if more than 60% of the full sunlight is being transmitted through a tree’s canopy, there is an increase in the quantity of grass under the tree, particularly of grass species such as Setaria sphaselata. With its long, lemony-green blades, it is a favourite food of grazing animals, such as cattle and sheep as well as buffalos and antelopes.
If a tree’s canopy blocks out more light, allowing only about 20% to 40% of sunlight through, grass species such as Panicum maximum dominate. With its broad knee-high blades and fine white hairs from base to tip, it is an important source of nitrogen for animals. But below 20% light transmittance ±— which is like walking into a room with closed doors and drawn curtains on a sunny day — only one, frail dark green grass species, Oplismenus hirtellus, could be recorded. If animals eat this grass, it has not been reported by researchers. Animals aren’t stupid: they would need to eat more than 50 individual plants to have a mouthful to chew and search under more than 50 trees to find that much because it is sparse and scanty.
Bush encroachment limits the amount of grass available to animals, and reduces the nutritious grass species that the animals need. Bush encroachment presents another danger: if there are many trees, during a veld fire these trees can catch fire. Burning trees not only create a lot of smoke but also cause the fire to burn deeper into the topsoil, killing many of the creatures that live there, such as earthworms. This makes the soil partly sterile, because the fire has killed the creatures that break down living material in the soil. Woodland fires are also more difficult to control or stop than grass fires.
At the same time, fires are a hazardous necessity for the savannah ecosystem. They maintain the savanna and grassland, and keep down the number of trees. The major finding of our research was that the light that comes through tree canopies controls both the type and amount of grass that grows under trees and therefore fire behaviour.
But if the climate models are true, and trees flourish, then there will be fewer but larger and hotter fires, and we need to be able to predict and manage these fire conditions. With uncontrolled bush encroachment, we run the risk of losing the main food source of grazing animals: grass. Understanding the relationship between trees, light and grass can help us understand what climate change will do to our savannahs.
The iconic African savannas will become forests, a continuous tree layer with no grass on the soil surface below.
But by using this tree density information to monitor grass availability and its growth potential, we can monitor fires and predict their behaviour, an important tool to protect our savannas in a changing climate.
Basanda Nondlazi is a MSc cadidate at the University of the Witwatersrand