Putting down roots: afforestation and bank cohesion of Icelandic rivers

Riparian vegetation is widely recognized as a critical component of functioning fluvial systems. Human pressures on woody vegetation including riparian areas, especially at high latitude, have had lasting effects. In Iceland, prior to human settlement, native downy birch woodlands covered approximately 15–40% of the land area compared to 1–2% today. Afforestation efforts include planting seedlings, protecting native forest remnants, and acquiring land areas as national forests. The planted and protected nature of vegetation along rivers within some forests provides a unique opportunity to evaluate the various taxa within riparian zones and the channel stabilizing characteristics of the vegetation used in afforestation. We investigated bank properties, sediment textures, and root characteristics within riparian zones along four rivers in forests in Iceland. Bank sediment textures are dominantly sandy loam overlying coarser textures. Undercut banks are common because of erosion of the less cohesive subsurface layer. Quantitative root data indicate that the woody taxa have greater root densities, rooting depths, and more complex root structures than forbs or graminoids. The native downy birch has the highest root densities, with <1 mm roots most abundant. Modeling of added bank cohesion indicates that willow provides up to 6 times and birch up to 4 times more added cohesion to the coarse sediment textures comprising stream banks compared to no vegetation. We conclude that planting and protecting the native birch and willow helps to reduce bank erosion, especially where long-term grazing exclusion can be maintained.

河岸植被（riparian vegetation）被广泛认定为正常运转的河流系统（fluvial systems）的关键组成部分。人类活动对包括河岸区域在内的木本植被（woody vegetation）所施加的压力，尤其是在高纬度地区，已产生了持久影响。在冰岛，人类定居之前，本土绒毛桦（downy birch）林的覆盖面积约占陆地总面积的15%至40%，而如今这一比例仅为1%至2%。当前的造林（afforestation）工作包括栽植幼苗、保护原生森林残迹以及收购土地用作国家森林。部分林区内沿河植被的人工栽植与保护状态，为评估河岸带（riparian zones）内的各类分类群（taxa）以及造林所用植被的固岸（channel stabilizing）特性提供了独特契机。本研究针对冰岛林区内四条河流的河岸带（riparian zones）展开调查，分析了岸坡特性、沉积物质地（sediment textures）与根系特征（root characteristics）。岸坡沉积物质地以砂壤土为主，其下伏层为更粗的沉积物质地。由于黏聚力较弱的地下层遭受侵蚀，凹岸岸坡十分常见。定量根系数据显示，木本分类群的根系密度、根系深度以及根系结构复杂度均高于草本双子叶植物（forbs）与禾本科草本植物（graminoids）。本土绒毛桦的根系密度最高，其中直径小于1毫米的根系占比最多。岸坡附加黏聚力（bank cohesion）模型结果显示，相较于无植被覆盖的情况，柳树可为构成河岸的粗质地沉积物提供最高达6倍的附加黏聚力，桦树则可提供最高达4倍的附加黏聚力。本研究得出结论：栽植并保护本土桦树与柳树有助于降低岸坡侵蚀，尤其在能够长期实施禁牧的区域效果更为显著。