Plant recolonization following dam removal : a phytometer experiment

Abstract: In the spring of 2007 the Kuba Dam in Nätra Stream was removed for restoration purposes, mainly to give free passage to migrating fish. This action gave an opportunity to study the effects of dam removal on organisms both on land and in water, a type of study which is in demand for planning purposes of future dam removals. This study focuses on the effects upon the vegetation along the stream where the former reservoir was situated. Willow cuttings and sunflower seeds were used as phytometers to assess the growth conditions one year after removal at three different elevation levels at four different sites along the Nätra Stream. The reaches studied were the former reservoir of Kuba Dam (Site A), the area just downstream of this former dam (Site B), further upstream in the reservoir of the existing Nyfors Dam (Site C), and even further upstream at Myreforsen, a reference area not affected by dams (Site D). Willow cuttings and sunflower seeds were planted at three different riparian levels at the sites in June and left to grow until August when growth results were collected. Many of the cuttings had died but statistically significant differences between sites could still be detected. They showed that the former reservoir shorelines in Kuba (Site A) had a plant growth significantly higher than the other regulated site. In other words, it was no longer equal to an existing reservoir but not yet similar to a free-flowing reach. The western side of the stream at this site had a higher growth than the eastern side and also higher than all the other sites. As for the three levels, the results showed a higher growth at the lowest elevation level at site A than all at the other levels and sites. This suggests that a combination of good access to sunlight and nutrients and fine-grade moist substrate results in a good growing conditions nearer the water in this former reservoir. The results for the sunflower seeds showed that the reference reach had a higher colonization success than the other sites.