Savannas are characterised by the co-existence of two very different types of plants – trees and grasses. They may be open, with large swathes of grass and an occasional tree dotting the landscape, or closed with a near complete cover of trees and a sparse grass layer beneath. In drier parts of the world, drought may play an important role in determining the balance between the trees and grasses in savannas. Extreme droughts, which are likely to become more common with climate change, could permanently shift a closed savanna to an open one. Such changes would have significant consequences for the functioning of these ecosystems and the animals they support. Dr Anthony Swemmer of the South African Environmental Observation Network explored the impact of an unusually severe drought on trees in South Africa. His team’s research shows that the response of trees to drought depends on a suite of local factors.

Invasive plants can permanently alter ecosystems to promote conditions that support their own persistence. For example, certain invasive grasses can make areas prone to more frequent and larger wildfires, which negatively impact native species but favour fire-resistant invaders. This self-perpetuating process, termed a grass-fire cycle, can be impossible to reverse. Dr Jeanne Chambers of the United States Department of Agriculture’s Rocky Mountain Research Station and her colleagues – Matt Brooks, Matt Germino, Jeremy Maestas, David Board, Matt Jones, and Brady Allred – recently examined how an ecosystem’s resilience to fire and resistance to invasive grasses influence whether a grass-fire cycle will establish. In their paper, the scientists introduced a geospatial tool and decision matrix that incorporate measures of ecological resilience and resistance to invasive grasses for designing management strategies to combat grass-fire cycles.