COS 36-1
Anchor ice controls vegetation in boreal streams
Vegetation in boreal streams is heavily affected by seasonal flow regimes. Traditionally, flow-related processes during the growing season, particularly the spring flood, are seen as the most important, whereas vegetation is believed to be dormant and ‘unaffected’ during winter. We challenged this view by evaluating the effects of ice regimes on riparian and instream vegetation during three years by relating the abundance of anchor ice and the number of winter floods to the cover, composition and biomass of vegetation. Anchor ice forms during super-cooled conditions by tiny ice particles that attach to instream objects and accumulate – growing from the bottom and up.
Results/Conclusions
We found that plant diversity of riparian vegetation was higher in reaches with anchor ice than in reaches without. Anchor ice may cause damming and floods that reduce the cover of riparian dwarf shrubs and remove instream algae as it detaches from the substrate. This disturbance paves the way for colonization of graminoids and forbs in the riparian zone and of bryophytes in the instream channel. During our study period, we encountered a frequency of zero-crossings on the Celsius scale that was as high as the frequency that is projected following future climate change. This suggests large vegetation dynamics in these streams in the near future. Higher temperatures and even more zero-crossings can reduce ice dynamics. This might eliminate some species and change the composition and production of the riparian vegetation, with consequences for the entire stream system. Our study emphasizes the importance of including the ice regime as a fundamental governing factor in future management of stream and river channels in cold regions.