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

1. In-stream and riparian vegetation is species-rich, productive and dynamic. Its 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. 2. Riparian and in-stream vegetation was 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. 3. 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. 4. 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. 5. 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.

1. 河道内与河岸带植被（in-stream and riparian vegetation）物种丰富、生产力高且动态性强。小型北方溪流（boreal streams）中的该类植被格局主要受季节性水流情势调控。传统研究普遍认为，生长季内与水流相关的过程——尤其是春汛（spring flood）——是影响植被的核心驱动因子，而冬季植被处于休眠状态，受水文干扰的影响相对较弱。
2. 研究团队于2011至2013年夏季，对瑞典北部溪流的8个河段开展了河道内与河岸带植被的清查采样。同时，在冬季通过目视巡查与固定架设的摄像头，对各河段的结冰情况进行监测。随后，基于锚冰（anchor-ice）引发的冬季洪水发生频次与植被盖度、群落组成及生物量之间的关联，评估了3年间冰情与冬季洪水对河道内及河岸带植被的潜在影响。
3. 调查结果显示，形成锚冰的河段冬季洪水发生频次显著高于无锚冰河段。同时，锚冰河段的河岸带植被物种多样性更高，这源于该类河段内河岸矮灌丛（dwarf shrubs）盖度更低，而禾草类（graminoids）与非禾本科草本植物（forbs）的盖度更高。此外，锚冰河段的河道内藻类盖度更低，但苔藓植物（bryophytes）盖度更高。尽管研究期间气温、水位与冰盖覆盖情况存在年际差异，上述植被格局在整个研究周期内均保持一致。
4. 本研究期间，冬季平均发生20次冻融循环（freezing and thawing），而1960至1990年期间冬季平均仅发生10次。这一结果表明高纬度地区正经历气候变暖。气温升高与冻融循环频次增加可能在初期增强冰体动态（ice dynamics），但随着平均气温持续升高，冰体生成量最终会出现下降。
5. 综合分析：锚冰引发的冰体与冬季洪水，是小型北方溪流生境中帮助竞争力较弱的物种成功定植的关键干扰因子（disturbance agents）。若冰体动态减弱，河道内与河岸带植被的群落组成及生产力可能发生改变，进而对整个溪流生态系统产生潜在影响。