Grasses and trees are the primary producers of the savanna, together generating the vast majority of the biome’s organic matter through photosynthesis. Grasses, particularly those using the C4 photosynthetic pathway, are the single largest contributor, accounting for an estimated 59% of net primary productivity across tropical savannas worldwide. Trees, shrubs, and a surprisingly diverse layer of herbaceous forbs make up the rest. But calling savannas a “grassland with some trees” misses the complexity of how these producers coexist, compete, and respond to fire, drought, and grazing in ways that define the character of the entire ecosystem.
C4 Grasses and Why They Dominate
The grasslands that stretch across African, South American, and Australian savannas are not ordinary lawns. Most savanna grasses use a specialized form of photosynthesis called the C4 pathway, which gives them a decisive edge in the hot, sun-drenched, and often water-limited conditions that define the biome. C4 grasses absorb carbon dioxide and convert it to sugar more efficiently than the C3 pathway used by most trees, and they lose less water in the process. Research comparing native and invasive grasses in the Brazilian cerrado confirmed the superior carbon assimilation and water use efficiency of C4 species, along with better performance under excess light.1Flora. Do invasive and native grasses in Brazilian Savanna differ in their photosynthetic performance under natural light radiation?
In an African savanna-wetland mosaic, researchers found that C4 grasses at rain-fed sites showed exceptionally high leaf gas exchange rates and carboxylation efficiency, reflecting a growth strategy built around squeezing maximum productivity out of a brief window of water availability.2Journal of Experimental Botany. Photosynthetic properties of C4 plants growing in an African savanna/wetland mosaic The ability to control water loss through highly responsive stomata appears to underpin the dominance of certain C4 grass groups in arid savanna environments.3Ecosphere. Responses of C4 grasses to aridity reflect species‐specific strategies in a semiarid savanna
Common genera include Themeda, Hyparrhenia, and Andropogon in African savannas, and Heteropogon and Sorghum in Australian ones. In South American cerrado, grasses like Paspalum and Axonopus fill the same role. These are not passive background plants. They produce the bulk of the fuel that carries fire across the landscape, they feed enormous populations of grazers, and their roots cycle carbon and nitrogen into the soil at rates disproportionate to their biomass.
Savanna Trees Are Not Just Bystanders
Trees may contribute less total productivity than grasses in many savannas, but they are far from marginal. Across African savannas, woody plants actually contribute a larger share of total productivity than in the Neotropics, reflecting differences in soil fertility and rainfall patterns between continents.4PubMed. Contributions of woody and herbaceous vegetation to tropical savanna ecosystem productivity: a quasi-global estimate The genus Vachellia (formerly classified under Acacia) is among the most emblematic savanna trees, and its success across vastly different environments tells a story about adaptation. A common-garden experiment on Acacia tortilis (now Vachellia tortilis) found that populations from deserts and tropical savannas had diverged in both their physical traits and their ability to adjust those traits in response to conditions, helping explain the species’ remarkably wide distribution.5Journal of Ecology. Divergent evolution of traits and plasticity across climate gradients in a widespread tree species
Different Acacia species cope with drought in distinct ways. A study comparing Acacia senegal and Acacia seyal from Ethiopian savanna woodlands found that A. seyal shed nearly all its leaves during the hottest month and maintained a relatively mild internal water deficit, while A. senegal used a different drought-tolerance strategy altogether. These mechanisms operated independently of soil fertility.6Trees. Effects of water stress and substrate fertility on the early growth of Acacia senegal and Acacia seyal from Ethiopian Savanna woodlands Beyond drought tolerance, savanna Acacias deploy physical and chemical defenses against herbivory, which is significant given the pressure from elephants and other large browsers. Some West African Acacia species showed limited ant-based defense but appeared to compensate with strong physical and chemical defenses against elephant damage.7PubMed Central. Limited ant co-occurrence and defensive mutualism in Acacia plants in a West African savanna
Other important tree genera vary by continent. In African savannas, Combretum, Terminalia, and Brachystegia (the dominant tree of the miombo woodlands across southern and eastern Africa) are widespread. Australian savannas are dominated by Eucalyptus. South American cerrado supports an entirely different flora, including Qualea, Caryocar, and Curatella.
The Forb Layer That Most People Miss
Ask someone to picture a savanna and they will imagine grass and scattered trees. Almost nobody pictures the forbs: the non-grass herbaceous plants that include wildflowers, herbs, ground-creeping species, and small shrubs. Yet in some savannas, forbs are astonishingly diverse. A study of a semi-arid granitic savanna found that forbs contributed 78% of the total herbaceous layer species richness.8South African Journal of Botany. Browsing intensity of herbaceous forbs across a semi-arid savanna catenal sequence That means for every grass species recorded, there were roughly three or four forb species sharing the same space.
These forbs are not just ecologically interesting. About 43% of the forbs recorded in that study showed signs of being browsed by herbivores at varying intensities, with some forming prostrate, spreading mats in nutrient-rich bottomland soils that created lawn-like patches dominated by forbs rather than grasses. So while forbs contribute less total biomass than the grasses, they form a significant part of the food web and represent most of the plant diversity on the ground.
How Trees and Grasses Manage to Coexist
A defining puzzle of savanna ecology is why trees do not simply shade out the grasses, or why grasses do not outcompete trees for water and take over. A recent synthesis of decades of research proposed that functional rooting separation is the key to the whole system. Grasses are comprehensively superior at capturing soil moisture in the upper soil layers, which limits tree growth and keeps young trees vulnerable. But trees persist because their roots reach deeper water sources that grasses cannot access. In dry savannas, water limitations prevent trees from ever growing dense enough to shade out the grass. In wet savannas, fire and large herbivores serve the same role, repeatedly knocking trees back before they can close the canopy.9PubMed. Linking resource- and disturbance-based models to explain tree-grass coexistence in savannas
Root data from an Australian savanna added texture to this picture. Both tree and grass roots concentrated in the top 20 centimeters of soil, but grass roots contributed a disproportionate amount of nitrogen and carbon to the soil relative to their total biomass. The researchers hypothesized that grasses maintain soil nutrient pools and provide the fuel for regular fires that prevent forest trees from establishing, while savanna trees are important for increasing soil nitrogen content and cycling rates.10Austral Ecology. Root dynamics influence tree–grass coexistence in an Australian savanna In other words, each group of producers inadvertently helps maintain the conditions the other depends on.
Fire as a Force That Shapes Producers
Fire is not something that happens to savannas. It is part of how savannas work. Grasses grow, cure in the dry season, and burn, and the cycle repeats. The producers that thrive here have evolved strategies built around this reality. Most savanna grasses are resprouters, regenerating from belowground buds after fire scorches their aboveground tissue. Interestingly, research found that resprouting grasses were associated with less frequent fire regimes than seeder grasses. Seeders were restricted to regions with more frequent fire, likely because long-lived resprouters outcompete them when fire is rare.11PubMed Central. Resprouting grasses are associated with less frequent fire than seeders
Woody plants face a different fire calculus. In a wet savanna in southern Africa, researchers compared areas burned at different frequencies and found that trees in frequently burned areas were shorter and less dense but had relatively thicker bark, a classic fire-survival trait. Species associated with high fire frequency included Protea gaguedi and Brachystegia utilis, while species like Vachellia and Cassia were more strongly associated with areas that burned less often.12International Journal of Ecology. Effects of Fire Frequency on Woody Plant Composition and Functional Traits in a Wet Savanna Ecosystem Fire frequency effectively sorts which tree species dominate any given patch, creating a mosaic of woody vegetation across the landscape.
The relationship between fire and producers also extends underground. Frequent fires shape how much carbon plants allocate to their root systems for post-fire recovery, a dynamic that makes belowground carbon storage in fire-prone savannas difficult to predict from aboveground vegetation alone.13PubMed Central. Root traits in response to frequent fires: Implications for belowground carbon dynamics in fire-prone savannas
What Happens Underground
Savanna producers invest heavily in roots, often more than their aboveground appearance suggests. A study of oak savanna in California found that fine root biomass and soil carbon were spatially patchy and increased dramatically with tree size. The oaks possessed a dimorphic root architecture, with both shallow and deep root systems, that enabled them to maintain metabolism through the dry summer and rapidly acquire water when the rainy season began.14Ecohydrology. Spatial heterogeneity of fine root biomass and soil carbon in a California oak savanna illuminates plant functional strategy across periods of high and low resource supply
The carbon stored in root biomass varies enormously across savanna types. In Cameroonian savannas, root carbon stocks ranged from about 1.6 tonnes of carbon per hectare in some areas to over 11 tonnes per hectare in wooded savannas.15International Journal of Low-Carbon Technologies. Carbon management for savannah ecosystems in Central Africa: a case study from Cameroon That belowground pool is significant for global carbon accounting and is one of the reasons savanna conversion and degradation can release large amounts of stored carbon.
The Seasonal Pulse of Productivity
Savanna producers do not photosynthesize at a steady rate year-round. Productivity surges during the wet season and drops sharply in the dry season. In mesic (wetter) Australian savannas, the wet season accounted for roughly 75-80% of total annual productivity, with C4 grasses converting light into biomass more efficiently than the trees during this period.16Global Change Biology. Is productivity of mesic savannas light limited or water limited? Results of a simulation study On a seasonal basis, solar radiation drives tree and grass productivity during the wet season, while soil moisture becomes the limiting factor during the dry season.17PubMed. Seasonal, interannual and decadal drivers of tree and grass productivity in an Australian tropical savanna
Trees and grasses respond to different seasonal triggers. Research across African savannas found that trees begin flushing their leaves before the rains arrive, apparently tapping stored water reserves rather than waiting for new rainfall.18PubMed. Will trees or grasses profit from changing rainfall regimes in savannas? A study in Zambian dry forest showed that large trees began rehydrating their stems two to three months before the rainy season began, using water drawn from deep lateral roots. Smaller trees and shrubs, with less stored water and shallower roots, only rehydrated after sufficient rain had fallen.19PubMed. Stem water monitoring reveals an association with rainfall and leaf flush timing in a tropical dry forest of Zambia This staggered timing means that large trees get a head start on the growing season, photosynthesizing for weeks before the grasses green up.
How Herbivores Reshape the Producers
Savannas support some of the densest populations of large herbivores on Earth, and these animals profoundly influence which producers thrive and where. A synthesis of herbivore-exclusion experiments across African savannas found that herbivory reduced grass abundance by about 57% and tree abundance by about 31%. The effects scaled with herbivore density: more grazers meant larger impacts on grasses, and more browsers meant larger impacts on trees. The observed impacts were roughly twice as large as existing estimates based on consumption rates alone, suggesting that trampling, soil compaction, and other indirect effects amplify the direct effects of eating.20Journal of Ecology. The past, present, and future of herbivore impacts on savanna vegetation
Herbivores do not just reduce plant abundance; they alter community composition. Heavily grazed areas tend to shift toward short, grazing-tolerant grass species and prostrate forbs that can survive being cropped repeatedly. Browsed woodlands tend to favor thorny or chemically defended tree species. The producers you see in any given savanna are the ones that have survived the combined filter of fire, drought, and being eaten.
Nutrient Cycling and the Nitrogen Question
Many savanna soils are nutrient-poor, which raises the question of how producers sustain themselves. Nitrogen-fixing trees, particularly in the Acacia-Vachellia group, have long been assumed to be the answer. These trees host root bacteria that convert atmospheric nitrogen into forms plants can use. Research confirmed that nitrogen-fixing tree seedlings had high tissue nitrogen and high water use efficiency, making them strong competitors alongside C4 grasses. However, the seedlings’ growth was additionally constrained by competition with grasses for phosphorus, not just water or nitrogen.21Journal of Ecology. Growth of N2-fixing African savanna Acacia species is constrained by below-ground competition with grass
Surprisingly, a study of Acacia senegal plantations in Sudan found that nitrogen fixation was not actually an important contributor to soil nitrogen content. Instead, the accumulation of nitrogen appeared to come from animals: grazing and browsing herbivores that deposited nitrogen through their excrement as they moved through the landscape. The soil chemistry also indicated that the plantations were limited by phosphorus, while nearby grasslands were limited by nitrogen.22PubMed Central. Linkages between soil carbon, soil fertility and nitrogen fixation in Acacia senegal plantations of varying age in Sudan This complicates the simple story of “trees fix nitrogen, grasses benefit.” The actual nutrient economy of a savanna involves animals, fire, and soil chemistry in ways that differ from site to site.
Savanna Producers Differ by Continent
Not all savannas are alike. A global analysis that classified vegetation as savanna (open habitat with a C4 grass layer) or non-savanna found three major continental divergences in where savannas occur that could not be explained by climate or soil alone.23PubMed. Deciphering the distribution of the savanna biome The relative contributions of grasses versus trees also shift across continents. C4 grasses make their greatest relative contribution to savanna productivity in the Neotropics (South American savannas like the cerrado and the llanos), while African savannas have a proportionally larger contribution from woody plants. Australian savannas fall in between.4PubMed. Contributions of woody and herbaceous vegetation to tropical savanna ecosystem productivity: a quasi-global estimate
These differences trace largely to soil fertility and rainfall patterns. African savannas often have richer soils that support more tree growth, while Neotropical cerrado soils tend to be ancient, deeply weathered, and nutrient-poor, conditions that favor grass dominance. The species lists are almost entirely non-overlapping: you will not find Eucalyptus in a natural African savanna or Brachystegia in Australia. But the functional template — C4 grasses, fire-tolerant trees, belowground storage organs, a seasonal pulse of productivity — repeats across all of them.
Rising CO2 and the Bush Encroachment Problem
Savanna producers are not in a static equilibrium. Over recent decades, many savannas have experienced woody thickening, sometimes called bush encroachment, in which trees and shrubs expand at the expense of the grass layer. There is theoretical, experimental, and modeling evidence that rising atmospheric carbon dioxide is a significant driver of this shift.24Environmental and Experimental Botany. CO2 enrichment does not entirely ameliorate Vachellia karroo drought inhibition: A missing mechanism explaining savanna bush encroachment The logic is straightforward: C3 trees benefit more from elevated CO2 than C4 grasses do, because the C4 pathway already concentrates CO2 internally. As atmospheric CO2 rises, trees gain a relative advantage.
Modeling work from a high-rainfall Australian savanna showed that rising CO2 alone accounted for about 63% of the trend in tree productivity, while grass productivity was driven almost entirely by rainfall, with CO2 contributing only an additional 6%.25PubMed Central. Gross primary productivity and water use efficiency are increasing in a high rainfall tropical savanna This asymmetry means that as CO2 continues to climb, the balance of producers in savannas could tilt further toward trees. Bush encroachment has practical consequences: it reduces grazing land, alters fire regimes, and changes the habitat available to wildlife adapted to open grassland. One hypothesis proposes that CO2 provides an important undercurrent to global woody plant encroachment into grassy systems, operating alongside land-use changes and fire suppression.26Global Change Biology. A proposed CO2-controlled mechanism of woody plant invasion in grasslands and savannas
When Savanna Producers Became Savannas
The savanna biome is geologically young. The C4 grasses that define it expanded rapidly around 8 million years ago, in a near-synchronous event across multiple continents. One hypothesis links this expansion to declining atmospheric CO2, which disadvantaged C3 plants and favored C4 grasses, combined with positive feedback loops involving fire. As C4 grasses spread, they increased the flammability of landscapes, which promoted drought and further fire, which opened more space for grasses at the expense of forests.27Global Change Biology. The origin of the savanna biome Phylogenetic analysis of African savanna tree species suggests an initial tropical or subtropical expansion of savanna between 10 and 15 million years ago, with the biome later extending to higher latitudes and reaching southern Africa around 3 million years ago.28PubMed Central. Savanna tree evolutionary ages inform the reconstruction of the paleoenvironment of our hominin ancestors
This timeline encompasses the entire hominin fossil record, which is why the “savanna hypothesis” of human evolution — the idea that early hominins evolved bipedalism, tool use, and other traits in response to open savanna environments — has remained a durable, if debated, framework. The producers of the savanna were not just the backdrop for human origins. They were the food web that our earliest ancestors depended on.
Savanna Plants in Human Medicine and Daily Life
People living in and around savannas have used these producers for food, medicine, fiber, and construction materials for thousands of years. An ethnobotanical survey of Brazilian savanna (cerrado) communities in Pernambuco documented 78 plant species used for 87 different therapeutic purposes, with 11 species showing particularly versatile medicinal applications. These included Copaifera langsdorffii, used widely in traditional medicine for its resinous oil, and Myracrodruon urundeuva, a tree whose bark is commonly prepared for anti-inflammatory uses.29PubMed. Plant species as a therapeutic resource in areas of the savanna in the state of Pernambuco, Northeast Brazil
African savannas have equally deep ethnobotanical traditions. Acacia species provide gum arabic, one of the most commercially important plant products from the Sahel, used in food processing, pharmaceuticals, and printing. Shea trees (Vitellaria paradoxa), a classic savanna species of West African woodlands, produce the nuts from which shea butter is extracted, supporting livelihoods for millions of women across the region. Baobabs (Adansenia digitata) provide fruit, leaves, bark fiber, and water storage — essentially serving as a general store rooted in the soil. The producers of the savanna are not abstract ecological categories. They are resources that human communities have shaped, managed, and depended upon for millennia, and that interdependence continues today.