The tree savanna is a unique biome that occupies a transitional zone between the dense canopy of a forest and the open expanse of a grassland. This environment is defined by the persistent coexistence of two life forms: widely spaced woody plants and a continuous layer of herbaceous grasses. The savanna’s open structure allows sunlight to reach the ground, which sustains the dense grass understory. This ecological balance supports a vast diversity of large mammals and specialized flora.
Defining the Tree Savanna
A tree savanna is characterized by scattered trees and shrubs interspersed across a continuous layer of grasses. To be classified as a tree savanna, the woody canopy must remain open, typically ranging from 5% to 80% cover, ensuring sufficient light reaches the ground to maintain the grass cover. This arrangement is governed by a climate regime of warm to hot temperatures year-round with highly seasonal precipitation. Annual rainfall generally falls between 500 and 1,270 millimeters, but it is concentrated in a short wet season.
The climate includes a long, pronounced dry season that can last anywhere from four to eleven months, which imposes a severe seasonal moisture limitation on the vegetation. Savanna soils are typically deep and highly weathered, often classified as Alfisols or Ultisols, and are generally low in fertility. The alternating wet and dry seasons help retain some nutrients compared to consistently wet tropical regions. This combination of seasonal drought and low soil fertility establishes the physical context for the competition and coexistence of trees and grasses.
The Role of Disturbance Regimes
The balance between the woody and herbaceous layers in the tree savanna is maintained by disturbance regimes. The two primary disturbances are fire and herbivory, which interact to regulate vegetation density. Fire is a frequent occurrence, particularly during the dry season, fueled by the continuous, dry grass layer. These fires limit the encroachment of trees by killing vulnerable juvenile trees and saplings before they can grow above the flame zone.
Fires tend to be hottest near the ground, posing a greater threat to short woody plants than to taller, established trees. Herbivory by large mammals also determines the vegetation structure. Browsing animals, such as elephants, directly reduce woody vegetation density by eating leaves, stripping bark, and even toppling mature trees, effectively converting wooded areas into more open savanna.
Conversely, grazing animals, like wildebeest and zebra, influence the woody layer indirectly by consuming the grass layer. This consumption reduces the fuel load available for fires, which in turn lowers the intensity and frequency of burns. Reduced fire intensity can decrease the mortality rate of tree saplings, potentially leading to an increase in woody cover or a phenomenon known as bush encroachment. The combined effect of fire and herbivory suppresses tree growth, preventing the savanna from transitioning into a closed-canopy forest.
Water and Nutrient Dynamics
The coexistence of trees and grasses relies on partitioning limited water resources in the soil, often described by the two-layer hypothesis. Deep-rooted trees develop taproots that access deeper soil moisture and groundwater, especially during the long dry season. This access allows them to survive when surface layers are completely dry. In contrast, the dense, fibrous root systems of grasses are concentrated in the shallow, upper soil layers, where they efficiently absorb water from the seasonal rainfall events.
This spatial separation in rooting depth minimizes direct competition for water, allowing both life forms to thrive in the same landscape. Water is considered the primary factor limiting growth and distribution in this biome, with its availability dictating the overall productivity.
Nutrient cycling also imposes constraints on plant growth, as savanna soils are inherently nutrient-poor, particularly in nitrogen and phosphorus. The breakdown of tree leaf litter concentrates nutrients near the base of the woody plants, creating localized zones of higher soil fertility. These “islands of fertility” surrounding the trees can support richer microbial activity and provide a slight advantage to nearby vegetation. Overall productivity is often limited by the scarcity of these nutrients, especially on the ancient, weathered land surfaces where many savannas are found.
Geographic Distribution and Types
Tree savannas are globally distributed across tropical and subtropical regions, covering vast areas on multiple continents. The largest and most famous expanses of this biome are found in Africa, encompassing significant portions of countries like Kenya, Tanzania, and South Africa. Other major concentrations exist in South America, where the Cerrado region of Brazil represents a significant savanna ecosystem. The biome is also widely represented across Northern Australia and in parts of Asia, including India and the Myanmar-Thailand region.
While tropical and subtropical savannas are the most common, the biome includes several variations defined by climate and hydrology. These types include temperate savannas, which have more moderate climates, and flooded savannas, such as the Pantanal in South America, which experience seasonal inundation.