Root System Morphology of Oregon White Oak on a Glacial Outwash Soil

Oregon white oak is reportedly a deeply rooted species, but its rooting habit on coarse-textured soils is undocumented. In the Puget Trough of western Washington, Oregon white oak grows in coarse-textured glacial outwash soils on lowland sites. Our objective was to quantify the gross root system morphology of Oregon white oak in these soils, thereby improving our understanding of its belowground resource acquisition on these sites. Study trees were located on a Spanaway gravelly sandy loam soil near Olympia, Washington. Root systems of 27 oak trees (age 3-95 yr) were excavated and measured. Root systems of seedlings and small trees had prominent taproots, but root systems of larger trees were structurally dominated by shallow lateral roots. Vertical penetration of roots, including the taproot, was restricted by gravelly and cobbly layers within the C horizon, which began at a depth of about 70 cm. Although most roots were located in the finer-textured A horizon, some small roots penetrated to depths greater than 150 cm, where soils remain moist in summer. The predominance of shallow roots suggests that management of understory vegetation or overstory conifers, which both have a similar rooting zone, will likely influence growth and survival of oak. Activities, such as vehicular traffic, that significantly disturb surface soils may adversely affect oak trees.