Vegetation patterns in small boreal streams relate to ice and winter floods

Abstract

Summary In‐stream and riparian vegetation are species rich, productive and dynamic. Their patterns in small boreal streams are largely driven by seasonal flow regimes. Traditionally, flow‐related processes during the growing season, particularly the spring flood, have been seen as the most important, whereas vegetation has been viewed as being dormant and ‘less affected' during winter. Riparian and in‐stream vegetation were inventoried during the summers 2011–2013 in eight reaches of northern Swedish streams. Along each reach, the ice formation was surveyed during winter by visual inspections and with permanently placed cameras. We then evaluated the potential effects of ice regimes and winter flooding on riparian and in‐stream vegetation during 3 years by relating the abundance of winter floods caused by anchor ice to the cover, composition and biomass of vegetation. We found that the numbers of winter floods were higher along reaches with anchor‐ice formation than in reaches without. We also found that species diversity of riparian vegetation was higher in the reaches with anchor ice. This resulted from a lower cover of riparian dwarf shrubs and a higher cover of graminoids and forbs along reaches with anchor ice. We also found a lower cover of in‐stream algae but a higher cover of bryophytes in anchor‐ice reaches. These patterns were consistent throughout the study period although there were interannual differences in temperature, water levels and ice cover. During our study period, we encountered an average of 20 shifts per winter between freezing and thawing, while there was an average of 10 shifts per winter during 1960–1990. This indicates a warming climate in high latitudes. Higher temperatures and more shifts between freezing and thawing may initially increase ice dynamics. However, with further increases in mean temperature, ice production should eventually decrease. Synthesis. Ice and winter floods caused by anchor ice appear to be important disturbance agents that allow less competitive species to establish along small boreal streams. If ice dynamics is reduced, the composition and production of riparian and in‐stream vegetation may be changed, with possible consequences for the entire stream ecosystem.