New research shows encroaching trees and shrubs threaten the biodiversity of grasslands globally

Much of conservation centres on trees rather than the plants beneath them but important new research from scientists at the Royal Botanic Garden Edinburgh (RBGE) shows that increasing numbers of trees and shrubs in open grassy ecosystems critically threaten their unique biodiversity.

Grassy ecosystems stretch across the world from temperate to tropical regions, and are rich in biodiversity adapted to open sunlit conditions. Herbaceous plants are often overlooked although they define grassy ecosystems, make up much of the unique plant diversity, and derive from a range of plant families from grasses to orchids and daisies. 

In a paper published this week*, 11 July 2022, in Global Change Biology, scientists at RBGE demonstrate that herbaceous diversity is threatened by woody plants increasingly encroaching into grasslands and savannas worldwide.

Jakub Wieczorkowski, co-author of the paper said: “Many savannas like the Cerrado of Brazil are global biodiversity hotspots home to rich plant communities. There are around 10,000 herbaceous plant species in Cerrado and many of them can be threatened by the addition of trees and shrubs.

While the encroachment of woody plants in grassy ecosystems has been observed for over 100 years, in recent decades the advance has increased, facilitated by increasing CO₂ emissions, climate change, megafaunal extinctions and changes to land use, such as fire suppression.

“As trees and shrubs encroach these naturally open ecosystems, local environments are altered via increased shade from trees, leaf litter and competition for water, nutrients and space. The cumulative effect causes the loss of the herbaceous plants, including those which are rare and endangered, that typify grasslands and savannas. Too often, this results in ecosystem degradation that is difficult to halt and reverse.”

Little research had been done to show the global trend in the impact of this environmental change on the herbaceous plants, so experts at RBGE analysed 42 field studies from across the planet including Europe, North and South America, Africa, Asia and Australia, to determine how encroachment impacts grassy ecosystems worldwide.

The results showed that the biodiversity of herbaceous plants had reduced in 87 per cent of cases and that high levels of encroachment, on average, led to the loss of more than half of species richness.

Dr Caroline Lehmann, Head of Tropical Diversity at RBGE and co-author of the paper added: “We tend to focus on trees as uniformly beneficial to the environment, but healthy grassy ecosystems house a unique diversity and are themselves effective at storing carbon. It is a case of the right trees in the right numbers in the right ecosystem.

“Once lost, it can take centuries to restore grasslands, so it is vital to implement environmental management policies that will mitigate, halt and reverse severe encroachment before it is too late.”