Bush Fires: Part 2 – Feedback Loops that Shape Grasslands

This is the second of four articles on fire, its’ impact on the environment and its’ use as an environmental management tool. The others are:

• Bush Fires: Part 1 – Effects and Management
• Bush Fires: Part 3 – Management of Fire in Conservation Areas
• Bush Fires: Part 4 – Fire as a Bush Management Tool

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Grasslands Worldwide

A grassland is dominated by a nearly continuous cover of grasses in an open, often rolling, landscape with little or no cover of trees and shrubs.

Grasslands are known by different names in different regions of the world, for example, pampas in South America, steppes in Europe and Asia, prairie in North America, veldt in South Africa, and downs in Australia.

Grasslands come in two main forms, temperate and tropical. Tropical grasslands remain warm all the time but have dry and wet seasons. Temperate grasslands have cold winters and warm summers with some rain.

Tropical grasslands are also called savanna (also spelt savannah) especially in Africa. Savanna is a mixed woodland-grassland ecosystem characterised by the trees being sufficiently widely spaced so that the canopy does not close (the tops of the trees do not touch each other). The open canopy allows sufficient light to reach the ground to support an unbroken herbaceous layer consisting primarily of grasses.

There are four types of savanna:
– Savanna woodland where trees and shrubs form a light canopy,
– Tree savanna with scattered trees and shrubs,
– Shrub savanna with distributed shrubs,
– Grass savanna where trees and shrubs are mostly non-existent.
 
A grassland ecosystem is also called a “transitional landscape” because grassland ecosystems occur in areas where rainfall is about 15-75 cm per year which is not enough rainfall to support a forest but more rainfall than you would find in a true desert.

Importance of Grasses and Grasslands

Grasses are the most important plant family on Earth. The reasons for this importance include:
– Huge number of species – over 12,000 species of grasses worldwide (about 10% of them occur in southern Africa)
– Area covered – over 30% of the Earth’s land area is covered with natural grassland and savanna areas that are dominated by grasses.
– Diversity of habitats – they occur in every type of environment, from the tropics through to arctic areas and from seashores to mountain tops.
–Value to humans – the three major crops planted by humans are grasses i.e. maize, wheat and rice. Oats, barley, rye and millets are other important grass crops.

Why are grasses so widespread and successful?

Grasses occupy almost every type of habitat because they have the following physical characteristics:-
– Herbaceous stems and leaves
– Most grasses are herbaceous (green and leafy as opposed to stick-like). During favourable seasons grasses flourish, completing their annual growth and reproduction while the weather is warm and wet. During unfavourable seasons of cold or drought they die back, leaving their growing parts under or just under the surface of the soil. Annual species survive unfavourable seasons as seeds whilst perennials survive dormant as rootstocks and rhizomes.
– Growing points at the bases of internodes and leaves. ln most plants the growing points are located at the tips of stems and branches. In grasses however, there are two growing points:
–At the tip of each shoot
–Most important, at the base of each internode and each leaf.
This is easily noted when one chews a grass stem. The base of the stem always the juiciest because it is the area of newest cell growth. As a result, when grass leaves are grazed or burnt, they have not usually lost their growth point because they do not regenerate from the tips as other plants do. This allows for continued development throughout the season.
– Open structure allowing maximum sun exposure. The leaves of grasses are generally long, vertical-growing and very thin. The whole plant has an open design allowing for maximum penetration of sunlight onto every leaf surface, allowing for maximum photosynthetic efficiency. Grasses are thus able to produce a large biomass (quantity of plant material) per volume of area occupied.
– Thrive on being heavily utilized. Many other plant families put out unpalatable secondary chemical compounds to discourage animals from eating them (and some grasses, such as the turpentine grasses, do the same). These compounds are lacking in the majority of grasses so grasses are highly sought after by grazing animals. Grasses can thrive whilst being heavily utilised (grazed), and in fact need to be utilised if they are not to become moribund and eventually die.

Grasslands are exceedingly important for the wildlife of Zimbabwe as a huge proportion of our game lives by grazing grass, e.g. zebra, buffalo, wildebeest, white rhino, impala.

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How grasslands work in Southern Africa

Our semi-arid savannas are landscapes where grasses form a continuous layer, with trees and shrubs scattered among them. This mix is not accidental. It is shaped by a combination of rainfall, soils, fire, and animals, all of which interact in complex ways.
Water availability is the basic determining factor
Grasses mainly use water from the topsoil.
Trees can use water from both the topsoil and deeper layers.
 
Soils matter too:
Sandy soils let water drain down more easily, helping trees.
Clay soils hold water near the surface, favouring grasses.
 
As rainfall increases across a large area, landscapes often shift naturally:

grassland → savanna (trees + grass) → woodland
 
Fire and animals change everything
Rainfall alone does not explain what we actually see on the ground. Fire and herbivores strongly influence whether trees or grasses dominate and there are several mechanisms which cause or maintain a grassland or encourage the development of woodland. The core mechanism in the maintenance of savanna grasslands in Africa south of the Sahara is the Grass-Fire-Tree feedback loop.

1. Grass–Fire-Tree Feedback Loop

When grass is abundant it provides the abundant fuel for fires that burn very hot and kill young trees. With fewer trees, grass has more light and space and thrives. This loop helps keep savannas open.
 
Result: Open, grass-rich savanna maintained by frequent, intense fires.

2. Grazing-Driven Bush Encroachment Loop

If grasslands are too heavily grazed, so that the grass does not have enough time to regrow between bouts of being grazed, then the grass cover becomes low and scanty. Fires that sweep through the grassland are therefore weaker and cooler. As a result, more trees survive. The growth of trees increases the competition for light and water that causes the further weakening of the grass cover.  Bush encroachment accelerates.
 
Result: Woodland expands as fires weaken.
 
N.B. This loop is triggered by heavy grazing by many animals over a long period. See below for the alternative results of the Holistic Planned Grazing approach which involves heavy grazing by many animals over a short period, followed by a long rest period.

3. Browsing-Fire Control Loop (Tree Suppression)

 
Browsers and fire work together to control and discourage trees. This loop supports grass-rich savannas. BUT serious over-population of browsers such as elephants can result in a collapse of the woodland environment and serious loss of biodiversity. This feedback mechanism can cause woodlands to become grasslands with the relatively rapid destruction of large, mature trees.

Result: Trees suppressed, grassland or savanna maintained.
 
This is the summary of the grazing and browsing feedback loops:
Grazing weakens fire → trees increase
Browsing strengthens fire → trees decrease
 
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Threshold-driven Tipping Points

The most important consideration in relation to the above three feedback loops is the following: savannas are not gently balanced systems. They are threshold-driven, i.e. changes in the balance between grass, fire, trees or animals (especially through the feedback between grass fuel and fire intensity) can carry an area over a threshold and trigger big, lasting changes in vegetation.
Important: Once the threshold is crossed, the system may not easily recover, even if conditions improve. This is why bush encroachment can be hard to reverse. Understanding this helps explain why fire and herbivores are central to savanna conservation and land use.

Why this matters for management

Fire, grazing, and browsing must be managed together
Simply resting land from grazing may not reverse bush encroachment
In many cases, intense fires combined with browsing are needed to reduce woody plants.
 
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Holistic Planned Grazing

This brings us to another feedback loop that could be responsible for the recovery of degraded grasslands that are on their way to desertification (as opposed to undergoing bush encroachment). This loop is best represented by Allan Savoury’s Holistic Planned Grazing approach which is based on a simple ecological insight: most of the world’s grasslands evolved with large herds of grazing animals that moved constantly under pressure from predators. When modern land use removed these movement patterns (either by overgrazing of too many animals in one place or by excluding animals altogether) many grasslands began to degrade, leading to desertification.
 
He argues that in “brittle environments” (where moisture is seasonal and decay is slow, like in Zimbabwe) grazing animals are essential for breaking down plant material and keeping nutrient cycles moving.
Without animals:
– Grass dies standing
– Litter oxidises rather than decomposes
– Soils lose life and carbon

Holistic planned grazing seeks to recreate the ecological effects of those ancient migrating herds, using livestock as a tool and a solution rather than seeing them as the problem. Below is a summary of the stages of the argument:
1. How natural grazing systems once worked
In healthy savannas and grasslands large herds (e.g. wildebeest, buffalo, bison) grazed tightly bunched together, looking out for predators
The herds:
– Grazed in a certain area briefly
– Trampled old, dead plant material into the soil
– Deposited dung and urine evenly
– Moved on frequently, driven by predators
The land then received a long recovery period before being grazed again. This pattern kept grasslands productive, soils covered, and water cycles functioning.
 
2. The problem with modern grazing
Savory argues that land degradation often comes not from too many animals, but from poor grazing timing and distribution.
Common damaging patterns include:
– Continuous grazing: animals stay too long in one area, repeatedly grazing new shoots before plants have had time to recover from the previous grazing
– Over-resting: removing animals entirely allows dead grass to accumulate, shading out new growth and reducing soil biological activity
– Bare soil: leads to water runoff, erosion, and loss of soil carbon
N.B. Both over-grazing and under-grazing can degrade grasslands.
 
3. What holistic planned grazing does differently
Holistic planned grazing is a decision-making framework, not a rigid formula. Savory calls his system “holistic” because he emphasises that land decisions must consider ecological health, economic viability and social and cultural realities.
 
The system’s key elements are:
a) High animal impact for a short time
Livestock are concentrated in a small area
They graze plants once, trample litter, and fertilise the soil
They are then moved on quickly
b) Adequate recovery time
Plants are given enough time to fully recover before being grazed again
Recovery time varies with season, rainfall, and grass species
c) Planned movement
Movements are deliberate and scheduled, not random
Grazing plans are adjusted continually in response to rainfall, plant growth, and animal condition
 
4. How this can reverse grassland degradation
When done correctly, holistic planned grazing can:
– Restore ground cover, reducing erosion
– Increase soil organic matter, improving water infiltration and storage because rainwater soaks in rather than running off (therefore great drought resistance)
– Stimulate grass growth by removing old material at the right time
– Suppress bush encroachment by favouring grasses over woody plants
– Improve biodiversity, especially insects and soil organisms
 
Scientific debate continues about the scale and consistency of its’ impacts, but many land managers report practical improvements in ground cover, water retention, and resilience when the principles are applied carefully.
 
To summarise, Alan Savory’s holistic planned grazing proposes that by mimicking the natural movement of grazing herds with short, intense grazing followed by long recovery livestock can help restore degraded grasslands, rebuild soils, and slow or reverse desertification, especially in dry savanna landscapes.
 
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Conclusion

Grasslands and savannas are not static backdrops to wildlife and livestock production; they are living, responsive systems shaped by water, soil, fire, and animals interacting through powerful feedback loops. Across the world — from the pampas to the prairie, from the veld to the savanna — grasses dominate because they are uniquely adapted to disturbance, seasonal stress, and grazing pressure. Their biology allows them not merely to survive fire and herbivory, but to depend on them.

In southern Africa’s semi-arid landscapes, the balance between grass and trees is constantly shifting. Rainfall sets the stage, but fire, grazing, and browsing determine the outcome. When grass fuels strong fires, savannas remain open. When grazing weakens fire, woody plants advance. When browsers and fire combine, trees can be suppressed — or even eliminated if pressure becomes excessive. These systems are governed by thresholds, and once crossed, they can move into new states that are difficult and costly to reverse.

For land managers, the central lesson is clear: grasslands must be actively understood and deliberately managed. Fire, livestock, wildlife numbers, and recovery periods cannot be treated in isolation. Healthy grasslands underpin Zimbabwe’s wildlife populations, livestock industries, water cycles, and rural livelihoods. Understanding how grasslands work — and how easily they can tip into decline — is the foundation of sustainable conservation and land use in our savanna landscapes.