Effects of river ice on riparian vegetation

Abstract

1. Many rivers and streams experience pronounced ice dynamics caused by the formation of anchor and frazil ice, leading to flooding and disturbance of riparian and aquatic communities. However, the effects of dynamic ice conditions on riverine biota are little known. 2. We studied the formation of anchor ice in natural streams over 2 years and assessed the effects of anchor ice on riparian vegetation by comparing sites with frequent or abundant and little or no anchor ice formation. We also studied the direct impact of ice on riparian plants by experimentally creating ice in the riparian zone over three winters and by exposing plants of different life forms to )18°C cold ice in the laboratory. 3. Riparian species richness per 1-m 2 plot was higher at sites affected by anchor ice than at sites where anchor ice was absent or rare, whereas dominance was lower, suggesting that disturbance by ice enhances species richness. Species composition was more homogenous among plots at anchor ice sites. By experimentally creating riparian ice, we corroborated the comparative results, with species richness increasing in ice-treated plots compared to controls, irrespective of whether the sites showed natural anchor ice. 4. Because of human alterations of running waters, the natural effects of river ice on stream hydrology, geomorphology and ecology are little known. Global warming in northern streams is expected to lead to more dynamic ice conditions, offering new challenges for aquatic organisms and river management. Our results should stimulate new research, contributing to a better understanding of ecosystem function during winter.

The role of ice dynamics in shaping vegetation in flowing waters

“…A number of frosttolerant plant species can survive under an ice cover or endure total enclosure in ice for several months, thereby giving these species an advantage over annuals, as they already occupy the habitats when the ice melts (Renman, 1989a). Engström et al (2011) showed that plants with budding parts overwintering above the ice sheet had less chance of survival than plants with overwintering organs close to or in the ground. They tested the survival of Vaccinium vitis-idaea (a dwarf shrub with overwintering budding parts aboveground), Filipendula ulmaria (a forb with budding parts underground) and Dechampsia cespitosa (a tussock-forming grass with budding parts at the soil surface).…”

“…The composition and distribution of riparian (Auble & Scott, 1998;Ettema, 2002) and aquatic vegetation (Dayton, Robilliard & Devries, 1969;Erixon, 1981) have been linked to ice disturbance and stress (Uunila, 1997;Engström et al, 2011). The literature describes two major ways in which ice formation and break-up affect vegetation: physical disturbance and physiological stress.…”

“…They tested the survival of Vaccinium vitis-idaea (a dwarf shrub with overwintering budding parts aboveground), Filipendula ulmaria (a forb with budding parts underground) and Dechampsia cespitosa (a tussock-forming grass with budding parts at the soil surface). After controlled freezing of these three species, the forb and grass did not show any signs of damage, whereas the evergreen dwarf shrub turned black and died (Engström et al, 2011). Evergreens, such as Vaccinium vitis-idaea, can start photosynthesis after only a few days of sun exposure (Molau, 1997).…”

“…Riparian ice, such as aufeis formed by floods, can cover riparian vegetation, protecting it from physical damage caused by moving ice (Engström et al, 2011). One specific example of such protection is the ice that forms from mist around big waterfalls in cold regions.…”

“…Recolonisation success also depends on the possibilities for species to spread from undisturbed areas and on the survival of perennating organs (Scrimgeour et al, 1994;Prowse & Culp, 2003). For example, tall, woody plants are rare along ice-scoured banks as they are preferentially removed by ice (Jansson et al, 2000), whereas species with their budding parts below ground are favoured (Engström et al, 2011).…”