Thinning out forests for more oaks

Oak regrowth increases dramatically when carrying out thinning in oak-rich mixed forests, according to a new research thesis from the University of Gothenburg. Thinning, forest clearing and protection against grazing all help to protect oaks.

Doctoral student Jenny Leonardsson has studied nature reserves with oak forests.

Doctoral student Jenny Leonardsson has studied nature reserves with oak forests.

In her research studies of nature reserves with oak forests, doctoral student Jenny Leonardsson has found that oak forests that are thinned experience a 600 percent increase in rejuvenation following ecological thinning compared with unrestricted development.
“Ecological thinning can benefit large oaks and promote oak rejuvenation if combined with protecting young oak trees against grazing animals,” says Jenny, who is studying at the Department of Biological and Environmental Sciences. “Repeated clearing can also reduce the number of competing species and benefit oaks.”

The oak is highly significant when it comes to biological diversity, and provides a habitat for a large number of organisms. In Sweden alone, 1,500 species are estimated to be dependent on oaks to some degree. These include beetles, wood fungi, lichens and birds, most of which are endangered.

“But oak rejuvenation is too low for new oaks to grow and replace the old oaks when they die. This is partly due to high grazing pressure from animals such as elk and roe deer, but it’s also a consequence of so many open oak-rich pastures having regrown. Instead, dense mixed deciduous forests have appeared. This means that not enough light penetrates to the undergrowth for oaks to be able to grow.”

The Oak Project at the University of Gothenburg, in which Jenny is involved, is evaluating ecological thinning as a maintenance method for oak-rich mixed deciduous forests with high natural value. The aim is to benefit large oaks and species linked to oaks, as well as promoting oak rejuvenation. Ecological thinning involves clearing around a quarter of the large trees and undergrowth. Jenny’s research shows that ecological thinning results in significantly higher growth of all trees and bushes compared with unrestricted development. However, both spruces and oaks benefit to a lesser degree than other deciduous trees and bushes.