Positive feedback between grass and woody vegetation

The photo above is of a prairie and savanna remnant developing on a steep south-facing slope with a thin, stony soil. Even without doing any quantitative measurements, it is easy to see that the prairie grasses (primarily little bluestem and Indian grass) are sparse or absent under the shade of the woody plants. At one time, this whole hillside was wooded, and in the first few years after it was cleared the prairie grasses were sparse or nonexistent. Cutting with herbicide treatment and annual controlled burns eliminated most of the woody plants and eventually the grasses recovered. Today they are well established, but only a few years in the absence of burns and they would be struggling again.

Prairie grasses interact with woody vegetation in complex ways, but one of the principal effects is on available light. Prairie grasses require close to full sunlight to develop to their fullest extent and even a little bit of shade will have an effect. Since the principal fuel in this hillside is grass, if the grasses are inhibited woody plants can get started. On the other hand, if burns are skipped for a few years woody plants will start to return, and once they start to shade out the grasses, even more woodies can grow, leading to even less grass, which leads to more woodies. This is a classical positive feedback loop. Since the grasses are the principal fuel, when they are inhibited there is progressively less fuel. Eventually a stage is reached where grasses are gone and only woodies are present.

Research has shown that this positive feedback mechanism operates all over. It has been reported in the Chihuahua desert, Texas range country, Kansas prairie, and African savannas. All of these habitats depend on fire for maintenance.