In the natural process of forest development, for example after a fire, fast-growing, sun-loving pioneer trees initially colonise the open areas until they are gradually replaced by slow-growing, more shade-tolerant species. This creates a resilient forest that provides habitat for a variety of flora and fauna. "A major debate in forest restoration is whether to introduce late-successional tree species sequentially once the early-successional trees have grown, or both at the same time. Planting them simultaneously has the advantage of shortening the process and may even stabilise the emerging ecosystem and promotes its biodiversity," says Sarah Bürli. As part of the Living Laboratories programme at Auckland University of Technology in Aotearoa/New Zealand, the plant scientist from the University of Graz investigated what increases the chances of survival for late-successional trees when they are planted simultaneously with pioneer species.
To find out more about optimal conditions, three former agricultural areas were reforested and restored in cooperation with Māori communities. "We observed and analysed the development of more than 1,800 trees," says Bürli, reporting on the experiment. The researchers were able to show that a distance of two metres between early-successional trees significantly improves the survival rate of subsequent species compared to one, three or four metres. "This distance seemed to effectively balance the advantages of protection from the fast-growing trees and the disadvantages of shading and competition," explains the scientist. In addition, larger amounts of dead and living ground vegetation also increased the survival rate of the seedlings.
With regard to the question of simultaneous planting, the study revealed: "Four of the six late-successional tree species in the experiment established well when planted at the same time as the early species," says Bürli. Until now, the opposite was assumed. The fact that planting them together achieves high survival will simplify the work in the future.
Although the trees found in near-natural forests in Europe are different from those in New Zealand, there are also early- and late-successional species in this part of the world. Therefore, the results of the study can be used to conduct European restoration projects.
Publication:
Differential seedling survival of early-planted, late-successional trees: Results from three forest restoration experiments
Sarah Bürli, Luke A. McClean, Bradley S. Case, David Hall, Hannah L. Buckley
Ecological Solutions and Evidence
DOI:10.1002/2688-8319.70207