Chapter 2
Imperata Grassland Ecology

2.1 Grassland development and forest succession

Forests are the natural, original "climax" vegetation in nearly all of Southeast Asia, but Imperata grasslands are now widespread. When forests are disturbed by logging, shifting agriculture, or burning, Imperata often takes over. Imperata's seeds are blown far and wide, and it is able to grow on wet or dry, fertile or infertile soils. Once established, the grass is a very flammable fuel. Even three days without rain can dry out the grass enough to carry a fire, burning both the grass and nearby forest vegetation. Fire stimulates both flowering and immediate regrowth of Imperata's, rhizomes. At the same time, fire damages or kills forest vegetation. If fires are frequent, Imperata will gradually become more dominant. It often forms monocultures except for a few scattered fire-resistant trees and shrubs, or it is part of mixed grasslands with other fire-adapted grasses. This is called a "fire climax."

Imperata grasslands also persist because many other species have difficulty competing with Imperata for water, nutrients, and light. Some species are also affected by the toxic ("allelopathic") substances produced in and leaking from Imperata's roots and rhizomes.

If an Imperata grassland does not burn, it will naturally and gradually return to forest ("forest succession"). Slowly, pioneer trees and shrubs sprout or grow from, seed, and eventually some grow above Imperata and outcompete it for light and water. Once Imperata is suppressed, additional species can grow more easily.

The Transition from Imperata grassland

Assisted natural regeneration (ANR, Chapter 5) imitates this natural process. Both ANR and agroforestry speed up the conversion of grasslands by:

• protecting the whole area from fire,

• planting trees,

• suppressing Imperata so that it does not compete with trees for light and water, and/or

• speeding tree growth by fertilizing, liming, or adding organic material.

It is often assumed that grasslands are the final, irreversible result of deforestation and the abandonment of farmlands, and that therefore grasslands are increasing in area. While it is true that grasslands are expanding in many places, Imperata is only a "climax" if fire or other disturbances continue. Many grasslands have been converted into agriculture or agroforestry during this century. In some countries and provinces, the total area of Imperata grasslands has decreased as human populations and the demand for land have increased.