Responses of riparian plants to flooding in free-flowing and regulated boreal rivers: an experimental study

Published source details Johansson M.E. & Nilsson C. (2002) Responses of riparian plants to flooding in free-flowing and regulated boreal rivers: an experimental study. Journal of Applied Ecology, 39, 971-986.
Share Tweet Email

Summary

River regulation has resulted in extensively changes to natural rivers and associated environments. Riparian plant communities in regulated rivers often have lower species richness and reduced cover compared with free-flowing rivers. During recent years the importance of natural flooding for maintaining river ecosystems has become more widely appreciated and artificial flooding has been used as a means of river rehabilitation. However, few studies have been made to determine the influence of contrasting water-level regimes on plant performance in order to develop guidelines for river restoration.

Study area: A study was undertaken in the Ume River system in northern Sweden examining growth and survival of transplanted individuals of four riparian plant species over two years (1993-1994) on four free-flowing and four regulated riverbank sites. Along these riverbanks vegetation is characteristically zoned, with riparian forest at the top of the banks, then willow Salix-dominated shrub vegetation, a herb and grass-dominated zone and finally a graminoid belt dominated by various sedge Carex species.

Study species: The four plant species selected for the study were chosen as dominant plants of each of the different riverbank zones: downy birch Betula pubescens (representative of riparian forest); meadowsweet Filipendula ulmaria (a tall herb common in mid-elevations); autumnal hawkbit Leontodon autumnalis (a rosette forming herb often found in disturbed patches at mid-elevations); and slender tufted sedge Carex acuta (representative of the lowermost part of the riverbank were it often forms dense stands).

Experimental design: Eight riverbank sites within the Ume River system were chosen and situated along:

i) two free-flowing small rivers with a pronounced but short spring flood (hereafter FS1 and FS2)

ii) a large free-flowing river with a longer spring flood (FL1 and FL2)

iii) two run-of-river impoundments with an extremely variable water level (RI1 and RI2)

iv) two storage reservoirs with a flood during the latter part of the summer (SR1 and SR2)

To estimate as best as possible water-level variation, the criteria for selecting a site were that it should: be located close to a daily recording water-level gauge; have no tributaries larger than second order streams entering the stretch between the study site and the gauge; have banks gradually sloping and only moderately exposed to waves and currents.

At all sites, soils were composed of fine sediments and sand except at the storage reservoirs where stone and gravel dominated a 1-2 m band slightly below the maximum damming level. Bank vegetation was generally dense and continuous at free-flowing sites but patchy at regulated sites.

Plant performance: Average growth rates of Betula and Filipendula, were significantly higher at free-flowing sites than at regulated sites; no consistent differences were found in Carex and Leontodon. Differences among species were generally in accordance with natural distribution patterns along riverbank elevation gradients and with experimental evidence on flooding tolerance, although plants of all species survived and even showed positive growth rates on elevations below their natural range of occurrence.

Flood duration and frequency typically reduced performance of all species during all time periods, although to various degrees. Flood events early in the experiment determined the outcome to a high degree at all sites.

Two computer models generated from the experimental data predicted that with an overall reduction in flooding duration and frequency of 50-75%, plant performance of Filipendula at low riverbank elevations increased by about 20-30%, levelling off to zero at the highest elevations. Reductions in summer floods represented about 33-50% of this increase.

Conclusions: For a range of species, individual plant performance is clearly reduced on banks of impoundments and storage reservoirs due to changes in the water-level regime. Furthermore, model simulations suggests that rather substantial reductions of flood duration and frequency are needed to improve plant performance on riverbanks upstream from dams in impounded rivers. Consequently, successful re-regulation schemes in boreal rivers should include both reductions of summer and winter floods as well as re-introduction of spring flooding.

Note: If using or referring to this published study, please read and quote the original paper.