Around the globe, the reintroduction of large woody debris / material is a common tool for river restoration schemes in an effort to promote biodiversity and enhance natural flood protection. Several reviews of the scientific literature have concluded that it is generally considered as good practice, yet results do vary (e.g. it is often difficult to demonstrate an increase in fish which is what most anglers want), and it is difficult to compare across studies because of the various ways restorations have been carried out. A new study by Murray Thompson and colleagues provides valuable new insights, critically using a ‘multiple before-after control-impact’ study design to allow such comparisons across different rivers.
They carried out biological, physical and chemical surveys of five UK rivers in the months before and after the addition of large woody debris: on the Bure, Loddon, Lyde, Test and Wensum. Three stretches were sampled on each river: a ‘restored’ stretch where a large willow or alder tree was felled and tethered to the river bed; a ‘control’ stretch which resembled the ‘restored’ stretch before tree-felling; and a ‘target’ stretch which contained a substantial tree which had fallen 3 to 5 years earlier.
Murray says: "Restoration of woody debris has been used to enhance in-river habitat throughout the world for over a century in tens of thousands of projects. Woody debris is increasingly used to reinstate natural processes, restore biodiversity and thus recover degraded river ecosystems. Yet, there is a striking lack of causal evidence to support this approach.
In the first experiment of its kind conducted across multiple rivers, we set out to test if, by felling trees in-river, biodiversity and food web metrics were restored relative to 'control' (i.e. unrestored) and ‘target’ conditions where naturally fallen trees were already in place. We were able to demonstrate causal links between habitat restoration, biodiversity restoration and food-web responses. For instance, elevated species richness in restored areas relative to controls was primarily driven by the repopulation of rare invertebrate taxa which also had many potential predators.