Between grassland and forest
Savannas and open woodlands occupy the ground between true grassland and closed forest. Trees are present, sometimes widely spaced and sometimes in clumps, but the canopy does not close, and enough light reaches the ground for a continuous layer of grass or herbs. The picture most people have is of African acacias over golden grass, but similar landscapes occur in many places, including northern Australia, parts of South America such as the Brazilian cerrado, India, and, in cooler form, in oak savannas and pine woodlands of North America and parts of Europe.
The terms are used loosely. Some scientists reserve “savanna” for tropical and subtropical systems with a seasonal climate, while “open woodland” or “open forest” covers a wider range, including temperate ones. How much tree cover counts as forest rather than savanna also varies, a problem touched on in the article on forest definition.
What keeps them open
If trees can grow, why do these places not fill in? Several forces act together, and their balance differs from place to place.
Rainfall and its timing. Many savannas have a pronounced dry season. Moisture is enough to grow grass and scattered trees, but often not enough to support a dense closed canopy. In general, drier climates have sparser tree cover. However, in intermediate rainfall zones, either savanna or closed forest could grow, and other factors decide which one occurs.
Soil. Shallow, sandy, nutrient-poor or seasonally waterlogged soils favor grass over dense forest. Some savannas sit on soils too poor or too wet for a closed canopy.
Fire. Dry grass is excellent fuel, and fires in savannas often burn every one to a few years. Grass recovers quickly because its growing points are protected near or under the ground, while young trees that are repeatedly burned back may never grow tall enough to escape the flames. Mature savanna trees often have thick bark and the ability to resprout. This is a classic example of the principles described in fire ecology. Fire frequency, season and intensity all matter, and removing fire altogether can allow shrubs and trees to thicken.
Grazing and browsing. Large herbivores such as elephants, antelope, cattle and giraffe eat grass, browse young trees and sometimes push over mature ones. Their impact varies. In some cases, they open up woodland, while grazing that removes grass reduces fuel and lets woody plants spread. Heavy and sustained livestock overgrazing can also degrade soil and vegetation.
Often these forces interact: grazers reduce fuel, which reduces fire, which allows more trees; fire in turn kills seedlings, which keeps trees few. It is a dynamic balance, not a fixed state.
Trees in an open setting
Trees in savannas tend to be adapted to drought, fire and herbivores. Typical features include:
- Deep taproots that reach water far below the surface, along with extensive shallow roots that capture rain quickly.
- Thick or corky bark to protect against fire.
- Thorns or chemical defenses against browsing.
- Seasonal leaf drop in the dry season in many species, an echo of the pattern described in the article on deciduous forests, but triggered by drought rather than cold.
- Wide, flat-topped or spreading crowns in open light, since there is no neighbor to compete for sunlight.
Open-grown trees tend to be branchier and shorter than those in dense forests, which affects their value as timber but increases their value as shade and habitat.
Life in open woodland
Open woodlands often hold a rich mix of life. Grass-eaters and their predators are the best-known inhabitants, but birds, insects, reptiles and small mammals also thrive in the combination of trees, grass and bare ground. Scattered trees function as perches, nest sites, shade and food sources in an otherwise open landscape, and are sometimes called keystone structures. Some temperate oak savannas and pine woodlands support wildflowers and insects that need sunny conditions and decline when woods become dense and shady.
This is a good reminder that an open habitat is not a degraded forest. Many species depend on open conditions, and a landscape that closes up can lose them.
People and open woodland
People have lived with and shaped savannas for a very long time. Traditional burning, grazing of livestock, collecting firewood, hunting and gathering of fruits, nuts and gums are common. Some systems combine trees, crops and animals deliberately, resembling the practices covered under silvopasture.
Many open woodlands in other regions, such as oak savannas and ponderosa-type pine woodlands in North America, were historically maintained by frequent low-intensity fires, some set by Indigenous peoples and some natural. When fire was excluded for decades, many thickened with young trees and shrubs, which changed the habitat and made later fires more severe. Restoration there often involves thinning and prescribed burning to return to a more open structure.
Pressures and misconceptions
Savannas face several pressures: conversion to farmland and plantations, overgrazing, changes in fire patterns, invasive plants, and climate change. One common mistake is to treat savannas as empty or degraded land suitable for tree planting. Planting dense tree cover on a naturally open ecosystem can harm its grassland species, alter water use and make fires harder to manage, and the trees may not survive drought or fire. Whether tree planting is appropriate depends on whether the site was historically forest, and this should be checked.
At the same time, in some savannas, woody plants are spreading into grassland, a process called woody encroachment, thought to be driven by changes in fire, grazing and atmospheric conditions. How much of it is natural and how much is a problem varies by region and is debated.
Carbon and climate
Savannas store less carbon per hectare in vegetation than dense forests, but they cover large areas, and much of their carbon is in soil and roots. Because fires are frequent, carbon moves quickly in and out of the system. The balance is complex, and projects that claim large carbon gains from changing savanna should be viewed with care, as covered in the discussion of carbon permanence.
Savannas and open forests show that “more trees” is not always the goal. The best outcome depends on what the land naturally supports and what people and wildlife need from it.