Influenced by flooding
Riparian forests develop in locations that are regularly flooded. Hardwood riparian forests only occur in large river floodplains. They grow in areas situated some distance from the river, which are inundated only rarely and for short periods. The most common tree species of these forests are Common Oak, Ash and Sycamore. In areas where floods occur more frequently and last longer, so-called softwood riparian forests develop. These usually consist of willow and alder species. In South Westphalia, only remnants of near-natural hardwood and softwood riparian forests exist today, particularly in the Lippe floodplain in the District of Soest.
Smaller watercourses are fringed by Alder-Ash riparian forest. This forest type has been preserved along many natural streams in the South Westphalian low mountain ranges. Black Alders and Ash trees form a belt along the banks of the stream, occasionally joined by Willows, Poplars and Elms. The Ash and especially the Black Alder are able to survive longer-lasting floods without suffering any harm.
The sturdy roots, especially those of the Black Alder, reinforce the stream banks and protect them against serious erosion during floods. When the water levels are average, the trees slow down the rate of flow, causing suspended solids to settle out. This is why the soils of stream and river floodplains are very fertile.
Where tree roots protrude into the water they cause turbulence which brings additional oxygen from the air into the stream. With its lush canopy, riparian forests shade the streams, ensuring that the water remains cool and rich in oxygen even in summer. This is of great importance for some residents of the streams, such as stonefly larvae, flatworms and Brown Trout. At the banks, the tree roots also provide hiding places for aquatic creatures and breeding sites for birds such as Dipper and Grey Wagtail.