ROOT DISTRIBUTION, TENSILE STRENGTH, AND STREAMBANK COHESION OF TAXA USED IN AFFORESTATION ALONG ICELANDIC RIVERS (Invited Presentation)

Soil erosion is one of the largest environmental challenges in Iceland. Approximately 50% of the vegetation and soil present at the time of human settlement in the 9^th century has been lost. Iceland has countered with afforestation, the planting of forest in previously treeless areas. Limited data exist on the channel stabilization characteristics and capabilities of various taxa used in afforestation of riparian zones, however. Here we investigate root properties of taxa that influence streambank cohesion on four rivers within national forests in Iceland. We measured root and bank characteristics and sediment textures at 15 planted or protected treatment sites and 6 unplanted or unprotected control sites along the Sandá (south Iceland), Örnólfsdalsá (west), and Blanda and Fnjóská (north) rivers. We focused on the dominant taxa used in afforestation or which establish naturally in riparian areas including native Downy Birch (Betula pubescens), Wooly and Tea-leaved Willow (Salix lanata, Salix phylicifolia), nonnative Lodgepole Pine (Pinus contorta), Norway Spruce (Picea abies) and Black Cottonwood (Populous trichocarpa). Statistical analysis suggests that there is a significant difference between the mean number of roots per meter squared of birch and willow, birch and conifer, willow and conifer, and conifer and cottonwood. Much of this significant difference is driven by disparities in the number of very fine roots (<1 mm) and indicates that the native birch and willow have greater numbers of smaller roots (<1-3 mm) per area of soil within measured banks than do the nonnative taxa. Using published values for parameters of the tensile strength curves, birch has the greatest tensile strength (MPa) for all root diameters <7 mm; for larger root diameters, birch, pine, and cottonwood exhibit comparable tensile strength. Preliminary results of a fiber bundle model of bank stability indicate that birch and willow produce higher average added cohesion for the dominant sandy loam soil texture comprising most streambanks. Results of this research are immediately useful for ongoing afforestation efforts and support additional planting within riparian areas throughout Iceland.