How does it help biodiversity?

As grasslands get abandoned, controlled burning is discussed as a labor-saving method of keeping forests at bay. A Kobe University research team found that this method results in higher biodiversity and a higher prevalence of endangered plant species in some grasslands compared to others, depending on what soils they grow on.

Humans have been keeping grasslands for millennia by grazing, mowing and controlled burning. All of these are means to keep forests from overgrowing the grasslands.

Grazing and mowing are, however, labor intensive and as rural areas become increasingly depopulated, grasslands have been disappearing worldwide. One consequence of this is the loss of the habitat of plant and insect species, including many endangered ones, that depend on grasslands.

To maintain grasslands, nevertheless, “prescribed” burning, which is what land managers call controlled burning, is being discussed as a labor-efficient option. But not all grasslands are equal.

The Kobe University ecologist Ushimaru Atushi says, “Many studies have focused on examining the effects of different management measures on plant diversity, but few researchers have paid attention to those of soil differences.”

Therefore, it has remained unclear which soils are better candidates for being managed by prescribed burning in terms of allowing a high diversity of endangered plants or plant species in general.

To settle this question, Ushimaru’s team, led by master’s student Asada Airi, turned to Mount Fuji. There, on a training site of the Japan Self-Defense Forces, grasslands covering different soil types are kept by prescribed burning once a year in April.

“We noticed that there were some areas, especially on young lava flows, where endangered species were concentrated,” Ushimaru explains the choice of the research site, allowing them to study which soil factors influence species abundance.

On site, they set up 100 one-square-meter plots across the grasslands on four different soil types and in each identified all plants as well as measured a range of physical and chemical factors.

Their results, published in the journal Plants, People, Planet, show that grasslands on young lava flows exhibit higher total species richness, native species richness and prevalence of red-listed plant species than grasslands on both young and old scoria bedrock.

They also found that the soils were different in their acidity, depth and coverage by rocks and stones, which allowed them to infer that these factors are important to the persistence of rare plants.

Previous studies showed that acidic soils make it difficult for plants to take up nutrients and shallow soils favor slow-growing plants. Asada thus argues that, as prescribed burning resets plant growth on the grasslands, these conditions suppress the growth of otherwise fast-growing and thus dominant species and so give those that are usually outcompeted and that are therefore rare a better chance of gaining a foothold.

Ushimaru sums up, “Our study was able to identify environments where grasslands managed by burning alone can harbor a significant variety of endangered species.”

The team concludes their paper saying, “Identifying environments where high plant diversity can be maintained through prescribed burning not only elucidates the survival conditions of grassland plants in Japan but also proves essential for the conservation and restoration of grassland environments and grassland species in the future.”