Ice increases the richness of streamside plant communities

Posted on September 24, 2012

Climate change models predict that at high latitudes temperatures will increase disproportionately in winter, with more frequent transitions between freezing and melting.  Flooding of streams at times of freezing creates ice cover in the riparian zone,  while flooding at times of break-up leads to scouring of vegetation and soil by ice. The impacts of winter ice formation on riparian vegetation and the success of stream restoration projects have received almost no attention.  To help fill this void, 24 streams in northern Sweden were monitored through two successive winters.  The study sites, in areas of pine and spruce forest, were separated into two categories depending on the prevalence of anchor ice.  Anchor ice (submerged ice attached to the stream bottom) is created when turbulent water in small upland streams is supercooled below 0oC.  Tiny ice crystals are formed, and these adhere to substrate particles and habitat structures in the channel.  Anchor ice increases bottom roughness, displaces stream fauna and flora, changes sedimentation patterns and impedes vertical connections with groundwater.  Large dams of anchor ice restrict flows but can then break, allowing water and ice to burst over banks and cover the riparian zone.  The Swedish researchers found that riparian plant diversity was higher at the sites with anchor ice than those without, which led them to suspect that the extra disturbance imposed by the ice had changed the competitive balance in favour of plant species that were normally excluded by more dominant forms.  In parallel experiments, in which ice was introduced by adding water to bankside plots and buckets containing plants, evergreen shrubs (species of Vaccinium and Lycopodium) were visibly damaged by ice but herbs and grasses weren’t. 

Reference:  Engstrom, J., Jansson, R., Nilsson, C. & Weber, C.  2011.   Effects of river ice on riparian vegetation.  Freshwater Biology 56, 1095–1105.