Restoration effects on germination and survival of plants in the riparian zone: a phytometer study

Abstract

Many streams that were channelized to facilitate timber floating in northern Sweden, have in recent years been restored by returning coarse sediment (cobbles and boulders) to the channel and reconnecting riparian with instream habitats. We asked if such restoration measures affect germination and survival of plants in the riparian zone, and if such potential effects depend on location in the catchment. We used a paired site approach, comparing the performance of Helianthus annuus (sunflower) phytometers (seeds and seedlings) in the riparian zone in channelized versus restored river reaches along climate and stream size gradients in the Vindel River catchment in northern Sweden. Phytometer survival, substrate availability, and soil nutrient content in large streams were enhanced by restoration, but overall, phytometer performance was negatively related to the length of the growing season, i.e. phytometers grew best at high altitudes and short growing seasons. This result may have been caused by less competition from the shorter and sparser neighbouring vegetation at these sites or to more frequent flooding events, enhancing retention of organic matter. Soil nutrient levels were lowest close to the coast and in large streams, probably due to deposition of mineral sediment. The higher availability of riparian habitat at restored than at channelized sites suggests that plant species richness and abundance may potentially increase after restoration. Seedling transplantation seems to be a preferable revegetation measure, because phytometer seedlings established better than seeds and survival was significantly higher at restored sites. The good plant performance at sites with short growing seasons and high altitudes suggests that, with limited resources, restoration measures should first be located to such sites.

Recruitment of riparian plants after restoration of geomorphic complexity in northern Sweden

“…This finding is in line with the differences between simple, channelized reaches and basic restored reaches reported in previous studies (Dietrich et al. , ; Hasselquist et al. ; Kuglerová et al.…”

“…Restoration of channelized reaches, for instance, has been shown to increase sunflower survival, although germination was not affected (Dietrich et al. ). Additionally, Jansson et al.…”

“…Previous research has shown that restoration from channelization increases the survival of sunflower seeds (Dietrich et al. ). Recruitment of sown seeds is most likely affected by restoration due to its effect on habitat characteristics such as soil moisture and soil quality (e.g.…”

Responses of riparian plants to habitat changes following restoration of channelized streams

“…On the other hand, Helfield, Engström, Michel, Nilsson, and Jansson (2012) found little effect of river restoration on riparian plant communities 5 years after restoration of sediment-starved streams. Dietrich, Nilsson, and Jansson (2015) reported higher seedling survival but not germination along restored than channelized streams, and Hasselquist et al (2015) argued that full recovery of riparian vegetation in northern Sweden requires at least 20 years.…”

Recovery of nitrogen cycling in riparian zones after stream restoration using δ 15N along a 25-year chronosequence in northern Sweden

“…There was no seeding or planting of riparian plant species at our restoration sites and it could be that seeding or planting (planting preferred, see Dietrich et al 2015) could accelerate both the recovery of riparian species composition and N-processing. However, given that the arrival order of plants plays an important role for which type of vegetation will develop at restored sites (Sarneel et al 2016;Young et al 2016), the selection of species to be introduced has to be done with care.…”