What are Grasslands, Anyway?

Grasslands are ecosystems mostly made up of grasses, which belong to a group of plants called Poaceae [1]. Many, but not all, grasslands have nutrient-rich soils, which are great for growing crops. Grasslands are also easier to convert to housing developments because they have few or no trees. Because of this, over time, people have converted many grasslands into farmlands and housing communities [2]. While the increase in farmland and housing is important for sustaining a growing human population, losing grasslands comes with big costs. Grasslands provide valuable services, which include keeping soils healthy, preventing erosion, cleaning water and air, and supporting a wide variety of organisms [3]. For example, grasslands are home to many unique insects, prairie birds, and grazing mammals [2].

Grazed grasslands are one of the largest land uses on Earth, covering one quarter of the planet’s land! There are many different ways to manage grasslands, and figuring out the best ways to do so for different benefits is important for the health of the planet. For instance, some subtropical humid grasslands in Florida (United States) currently exist as seminatural pastures or improved pastures (Figure 1). Seminatural pastures have a mix of native grasses and non-native grasses, and they support fewer cattle than improved pastures [4]. Improved pastures, on the other hand, have mostly non-native grasses and can support more cattle [4]. In pastures like these, land management strategies like patch-burn grazing are used (Figure 2). Patch-burn grazing copies the natural fire and grazing patterns seen in natural grasslands. Understanding how fire and grazing work together can help us strengthen strategies for maintaining healthy grasslands. Successful land-management strategies will allow us to balance human needs while protecting this vital ecosystem and maintaining the benefits grasslands offer.

Fire and Grazing—The Dynamic Duo!

Grazing is the process in which herbivores (plant-eating animals), like cattle, feed on grasses and other small plants. Grazing can benefit grasslands in many ways. When animals graze plants, they can help keep certain plants from growing too fast or taking over the area. Grazing also helps with nutrient cycling by breaking down plants and putting these nutrients back into the soil. Also, as cattle move around, they can spread plant seeds, allowing new plants to grow in different places.

Fire is another important tool used to keep grasslands healthy. Fire has been used for a long time in many ecosystems to clear dead plants and help new ones grow. Fire can be started naturally, like by lightning, or it can be set by people as a tool to manage the land. Fire needs dry plants as fuel, just like how a campfire needs dry wood to burn. The amount of fuel can change depending on how the land is managed.

Some methods of fire management use full-burns, where the entire area is burned (Figure 2B). Full-burns leave fewer plants behind, and when they regrow, cattle graze the whole area. In contrast, patch-burns only burn certain patches of land each year (Figure 2A). Patch-burns leave parts of the land with more dry plants that can act as fuel for future burns. Cows help to maintain the fuel by mostly grazing in the burned patches and leaving the unburned patches alone. This in turn can affect how the fire burns the following year and how plants grow afterward.

But how does fire work with grazing animals to keep grasslands healthy? Fire and grazing work together to encourage new plant growth, prevent certain plants from taking over, and maintain the plant diversity in the area. When fire influences animals’ grazing patterns, this is known as pyric herbivory. Pyric herbivory creates an environment full of different types of plants, rather than one that looks the same all the time. This, for example, in turn benefits many birds by providing them with a variety of habitats and food sources [4].

Exploring Subtropical Grasslands Through an Experiment

Scientists have studied how temperate grasslands benefit from pyric herbivory. However, little research has been done to understand how fire and grazing work in subtropical grasslands. These grasslands have higher rainfall and longer growing seasons, and they support more animals [4]. These differences could change how fire and grazing work together. To explore this, we conducted a study in south-central Florida (United States), home to unique subtropical grasslands. We studied two types of subtropical grasslands with different land-management practices: seminatural and improved pastures [4]. Our study included 16 pastures and lasted three years. Each pasture was about the size of 30 American football fields (one field is 110 m × 49 m, or about 0.53 hectares).

In 2016, we counted how many different kinds of plants there were in the pastures, how they were spread across the land, how tall the plants were, and the soil’s health. Then, in 2017, we used two different fire plans in the pastures. In eight pastures (four seminatural and four improved), we used a full-burn plan: the entire pastures were burned at the same time once every 3 years. Thirty days after the fire, cows were allowed to graze the fresh vegetation. In the other eight pastures (four seminatural and four improved), we used a patch-burn grazing plan. Here, we burned a different one-third of each pasture (about 10 American football fields, or about 5 hectares) every year. Cows were allowed to graze in these pastures 1–2 weeks after the fire. Each year until 2019, we repeated the plant measurements, and we checked soil health at the end of the experiment. Finally, we compared all the measurements to understand how fire and grazing, under the two fire plans, worked together to affect the pastures [4].

What did We Discover?

When we compared entire pastures to each other, we did not find any big differences between the two fire plans. But when we looked at smaller patches within the pastures, the story was different! In patch-burn grazing pastures, the burned patches had more native plant species than the unburned patches. This was true for both seminatural and improved pastures.

In 2017, burned patches in patch-burn grazing pastures had even more native plant species than burned patches in full-burn pastures. But this was only true for 1 year. In seminatural pastures, patch-burn grazing also increased the variety of sedges (grass-like plants) and forbs (wildflowers) in burned patches compared to full-burn pastures. These results only happened in some years. We also found that vegetation height varied more in patch-burn grazing pastures during the spring but not the fall. These differences were stronger in seminatural pastures.

Finally, when we looked at soil health, the fire plans did not change organic matter, carbon or nitrogen levels. Moreover, the plans did not affect how tightly packed the soil was. However, patch-burn grazing increased phosphorus in improved pastures, with only small changes in seminatural pastures [4].

What do Our Results Mean for Grassland Management?

The positive effects of patch-burn grazing on grasslands in our study were noticeable. However, understanding them can be somewhat complicated. For one, seminatural pastures supported fewer cattle, which left more fuel (dry plants) for fires. As a result, patch-burn fires in these pastures had stronger effects on the number of plant species than in improved pastures. Diversity was also driven by plants like sedges and forbs. This is great for conservation, but not for cattle food. However, other studies showed that patch-burn grazing may actually make these plants tastier for cattle [4].

Patch-burn grazing also increased variation in vegetation height. However, much of this variation was observed in spring and disappeared by fall. This may be because, in seminatural pastures, the native grasses bounced back quickly after burns to match the height of grasses in unburned patches.

Additionally, the effects of patch-burn grazing also depended on the year. In years with super-low rainfall, non-native grasses that would normally outcompete other plants may not have done so because they are not as used to drier conditions. Thus, patch-burn grazing in those years led to lasting variation in vegetation height in pastures with those grasses. Our findings for soil nutrients are a bit easier to understand because they were not affected by patch-burn grazing. This is good, because it means that patch-burn grazing did not make the soil less healthy!

Ultimately, patch-burn grazing improved how nutritious the plants were for cattle, and it increased variation in vegetation height. This is beneficial for birds, insects, and mammals! Since these effects vary by year due to differences in rainfall, further study is required to understand the relationship between rainfall and patch-burn grazing. In particular, the significant effects observed in 2017 may have been affected by that year’s unusually high rainfall. In the future, it would be interesting to look at past rainfall amounts to understand how it changes the effects of patch-burn grazing. As our growing population and surrounding environment changes, it is important that we understand these changes to support and protect our valuable grassland ecosystems!

Glossary

Grazing: ↑ To feed on grasses and small plants.

Seminatural Pastures: ↑ Areas in subtropical grasslands with a mix of native and non-native grasses. They support fewer cattle than improved pastures in the same system.

Improved Pastures: ↑ Areas in subtropical grasslands with mostly non-native grasses. They support more cattle than seminatural pastures in the same system.

Patch-Burn Grazing: ↑ When a part of a pasture is burned each year, and livestock, like cattle, can move between the burned and unburned areas.

Nutrient Cycling: ↑ The movement and reuse of nutrients like carbon and nitrogen through air, water, soil, plants, and animals.

Pyric Herbivory: ↑ When fire changes where animals graze, often causing them to prefer to graze recently burned areas.

Temperate: ↑ Regions with warm and cold seasons each year, not extremely hot or extremely cold all the time.

Organic Matter: ↑ Material made by living things, like plants, animals, and microbes, often in different stages of decay.

Conflict of Interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

We thank and acknowledge Grégory Sonnier, Nuria Gomez-Casanovas, Carl Bernacchi, Evan DeLucia, Jed Sparks, Hilary Swain, Emily Anderson, Keith Brinkso, Luca W. Kuziel, Lydia Landau, Haoyu Li, Nicholas A. McMillian, Alana Rivero, Amartya Saha, Julie Sorfleet, Amanda West, and Maya Zambrano-Lee for their help and intellectual input into the original manuscript. We acknowledge NIFA 2016-67019-24988 awarded to E. H. Boughton, N. Gomez-Casanovas, C. Bernacchi, E. DeLucia, J. Sparks, and H. Swain and ARS NACA 58-0202-7-001. This research was a contribution from the Long-Term Agroecosystem Research (LTAR) network. LTAR is supported by the United States Department of Agriculture. Any opinions, findings, conclusion, or recommendations expressed in this publication are those of the authors and do not necessarily reflect the view of the U.S. Department of Agriculture.

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